# Wanna Talk About Port Shapes?



## Mastermind (Jul 30, 2014)

We talk a lot about compression, port timing, muffler mods, etc. We don't talk too much about the shape of the port, and how the shape relates to performance.

Here's a few pics of an 064 I just did.......and there's some pics of another 064 jug in here too. I sanded through the plating when I was getting the transfer off the second 064 jug. 

This pic really shows the difference between a "performance shape" and a stock port. Notice how the corners have been opened up to increase time area.........but the port height is unaffected. 





In this pic the rough in of the intake port is nearly complete......I'll tighten the corners a little more and work on out to my marks. Notice that this port is only a little wider than stock, and at the stock height. It should be plain to see that this port will be more conductive to flow than a stock port.





Here is a stock 064 intake port for comparison.





And here they are side by side.


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## watsonr (Jul 30, 2014)

how did you determine the width to widen to and what did you do to draw the lines on the jug? The intake roof is pretty flat, why?


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## Mastermind (Jul 30, 2014)

Now, for the transfer ports. 

Here's a shot of the stock ports. 





And here's a fully finished transfer. 

Notice the corners. They are even tighter than the intake port. When the transfers first crack open, you want to be fully flowing across the entire area of the port. 





Here's another shot that shows how tight the corners are......and that shape is carried back into the port a good ways. 





Here's another......note the lower opening of the transfer. I don't open this down as far as I once did. I just take the lip off to prevent fuel puddling.


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## Mastermind (Jul 30, 2014)

watsonr said:


> how did you determine the width to widen to and what did you do to draw the lines on the jug? The intake roof is pretty flat, why?



I just widen a little past the factory bevels Randy. I see no need to overly widen the ports.....

I use a pencil and a business card to draw the lines. I sit the card against the squish band so my lines will be square with the bore. 

The roof being as flat as it is comes from the corners being tightened up. I didn't lower the floor, or raise the roof. Just tightened the corners.


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## watsonr (Jul 30, 2014)

thank you Sir... great post!


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## Mastermind (Jul 30, 2014)

Let me throw this pic in again. In it you can see the bevels that I put on the ports. Notice that the intake port is heavily beveled....


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## SquareFile (Jul 30, 2014)

When you connect the upper transfer to the lower is that considered a boost port?


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## Mastermind (Jul 30, 2014)

Same treatment on the exhaust port.......just raise it to the height I want, and tighten the corners up a bit. Not as much as on the intake though. The rings ride across the exhaust port so you want a nice arch to the port to keep from beating the rings up. 





Here's another thought. In a two stroke engine, it's all about the time/area of the ports went it comes to performance. You can widen the port to gain T/A, or you can raise it. If the rings travel across the exhaust would it be wiser to get the T/A you want by setting the height, or widening the port?????


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## deaves61 (Jul 30, 2014)

Hey mastermind I have maybe a dumb question, do you know if a 455 husky piston may have a bit more compression distance than a ms 311 stihl thought about trying to do a swap. Not really sure if wrist pin is there same size.


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## CapitaineHaddoc (Jul 30, 2014)

shawn nolder said:


> When you connect the upper transfer to the lower is that considered a boost port?



I was going to ask the same question...


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## Mastermind (Jul 30, 2014)

shawn nolder said:


> When you connect the upper transfer to the lower is that considered a boost port?



No......that's what I call a bridge port. It crosses the bridge in the bore.

It's more for direction than flow. Notice that it meets the very corner of the transfer. The flow from the bridge port changes the direction of the incoming fuel/air charge......forcing it upward and into the combustion chamber.

This is the sort of stuff that makes two saws with the same timing numbers so different. Just because you follow the numbers that one builder would use, does not mean that the unit will perform the same way.


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## Mastermind (Jul 30, 2014)

deaves61 said:


> Hey mastermind I have maybe a dumb question, do you know if a 455 husky piston may have a bit more compression distance than a ms 311 stihl thought about trying to do a swap. Not really sure if wrist pin is there same size.



I've never worked on either of those saws.


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## deaves61 (Jul 30, 2014)

Thank you


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## Mastermind (Jul 30, 2014)

Here's the last pic I have for now.

I don't normally enlarge the exhaust flange area on these saws. It's plenty big enough. I just deburr, and smooth it out. It's more important the match the muffler, heat shield, and gasket to the jug perfectly than it is to hog stuff out and weaken the flange.....

Note the arch across the roof of the exhaust port.........easy on the rings is key here.


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## CapitaineHaddoc (Jul 30, 2014)

Mastermind said:


> No......that's what I call a bridge port. It crosses the bridge in the bore.
> 
> It's more for direction than flow. Notice that it meets the very corner of the transfer. The flow from the bridge port changes the direction of the incoming fuel/air charge......forcing it upward and into the combustion chamber.
> 
> This is the sort of stuff that makes two saws with the same timing numbers so different. Just because you follow the numbers that one builder would use, does not mean that the unit will perform the same way.



So basically, this bridge port is used to accelerate the flow?


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## Mastermind (Jul 30, 2014)

Exactly. It being a small passage does just that. I see in my mind, the mixture squeezing through there at a high rate of speed.


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## CapitaineHaddoc (Jul 30, 2014)

Mastermind said:


> Exactly. It being a small passage does just that. I see in my mind, the mixture squeezing through there at a high rate of speed.



Thank you for the explanation. Just one last question: It's not the first time i see a ported cylinder wit theses "bridge ports", but as fas as i remember, those cylinders have only 2 transferts (i see this often on 288 cylinders). Is this something you will do on a quad transferts cylinder too?


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## cgraham1 (Jul 30, 2014)

Mastermind said:


> We talk a lot about compression, port timing, muffler mods, etc. We don't talk too much about the shape of the port, and how the shape relates to performance.
> 
> ...And here they are side by side.


Well said, and great pictures!
Thanks for sharing, Randy! 

Now, do you have any Husky cylinders to show us?


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## Trx250r180 (Jul 30, 2014)

Them little bridges make a noticeable difference in the one i had .


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## Mastermind (Jul 30, 2014)

No need to add anything to the quad port jugs.


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## rattler362 (Jul 30, 2014)

Sure is some fine looking ports Randy. Thanks for posting the pic's.


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## ELECT6845 (Jul 30, 2014)

I just want to know how you grind so perfect and square. Your work is top notch and what I try to match since I'm always chattering around with the hand piece. Guess it takes awhile to try and smooth it out. Very nice post by the way.


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## redfin (Jul 30, 2014)

Thanks for taking your time to post this Randy. Beautiful work.


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## gulity1 (Jul 30, 2014)

Thanks, Randy


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## treesmith (Jul 30, 2014)

Quality


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## sunfish (Jul 30, 2014)

Thanks for sharing, Randy. Good stuff!


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## fastLeo151 (Jul 30, 2014)

Boy am I glad I sent my 064 to ya. Hopefully its there now. But mine has a flat top 066 jug on it.


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## Mastermind (Jul 30, 2014)

Yeah, it's here Andrew. 

Safe and sound.


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## farmer steve (Jul 30, 2014)

thanks MM for the pics. helps me figure out what the heck you guys are always talking about.


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## Duane(Pa) (Jul 30, 2014)

Mastermind said:


> Exactly. It being a small passage does just that. I see in my mind, the mixture squeezing through there at a high rate of speed.


You mentioned being a chess player. I think you see consequences, and cause & effect relationships of things a lot better than most people I know. This stuff is PFM to me (Pure F----ng Majic)


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## Mastermind (Jul 30, 2014)

I never said I was a good chess player though.


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## Duane(Pa) (Jul 30, 2014)

I withdraw my blipping challenge. I decided to concentrate on Throttle Pinning.


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## Mastermind (Jul 30, 2014)

Challenge accepted. I'm a throttle pinning hero in the lower levels of competition. 

Bring it on.......


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## Duane(Pa) (Jul 30, 2014)

I think I see how this works. Jon fixes saws, and you go around picking fights and threatening the meek. Get to work.


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## Mastermind (Jul 30, 2014)

About done for the day.


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## walexa07 (Jul 30, 2014)

Awesome thread, Randy. I've read no telling how many threads with photos showing modified ports but your explanations here helps to bring the pictures to life and clear things up more. Not that I'll ever try to port a saw, but understanding these basics helps alot.

Thanks again.....

Waylan


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## Nate66n1 (Jul 30, 2014)

What would happen if you widen the upper transfers? Also what do you find the best for the bevels around the ports


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## SquareFile (Jul 30, 2014)

Boring.......... Need more videos in shorts.


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## fastLeo151 (Jul 30, 2014)

Mastermind said:


> Yeah, it's here Andrew.
> 
> Safe and sound.


Sweet. don't scratch the paint. Its muh baby. Feel free to violate the crap out of the I sides though.


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## Mastermind (Jul 30, 2014)

Nate66n1 said:


> What would happen if you widen the upper transfers? Also what do you find the best for the bevels around the ports



It helps little or nothing. I've tried every combination you can imagine. 

Too wide and you kill the scavenging effect of the exhaust drawing on the transfer.......


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## Adirondackstihl (Jul 30, 2014)

Bridge port = finger port?
I think so? Maybe not? It's all relevant.

I think of a boost port as an additional/added port in the lower transfer area through the cylinder wall. There was some pics of what I'm attempting to describe here somewhere......once upon a time.


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## Adirondackstihl (Jul 30, 2014)

Mastermind said:


> It helps little or nothing. I've tried every combination you can imagine.
> 
> Too wide and you kill the scavenging effect of the exhaust drawing on the transfer.......


That's why it's important not to hog the transfer tunnels out right? It sacrifices transfer velocity.....?!


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## Mastermind (Jul 30, 2014)

No.....here's a set of finger ports......with bridges. Done by Dennis Cahoon.


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## Mastermind (Jul 30, 2014)

Adirondackstihl said:


> That's why it's important not to hog the transfer tunnels out right? It sacrifices transfer velocity.....?!




In a work saw anyway. If you're building a saw for racing......on alky.......hog away.


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## wigglesworth (Jul 30, 2014)

I can't believe people pay for such shoddy work...

I've heard Mitch Weber is the guy you want to do your porting. Word is he's gonna clean house at the West KY GTG....


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## sunfish (Jul 30, 2014)

Mastermind said:


> In a work saw anyway. If you're building a saw for racing......on alky.......hog away.


Randy, how about raising the upper transfers, is that done for torque? Thinking 346 here.


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## SquareFile (Jul 30, 2014)

Bridge ports, Finger ports, Can I see some boost ports?

PLZ?


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## Adirondackstihl (Jul 30, 2014)

There's gotta pics of a "boost" ported quad Swede cylinder somewhere around Photobucket land?


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## redfin (Jul 30, 2014)

Mastermind said:


> No.....here's a set of finger ports......with bridges. Done by Dennis Cahoon.



Oh my there's a lot goin on in that one!


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## Nate66n1 (Jul 30, 2014)

Mastermind said:


> No.....here's a set of finger ports......with bridges. Done by Dennis Cahoon.


How do rings even survive In that hot mess


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## SquareFile (Jul 30, 2014)

Nate66n1 said:


> How do rings even survive In that hot mess


 I'm sure DC made it a bad Mo Fo.


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## Mastermind (Jul 30, 2014)

Shut up Wiggs......you ain't a member here anymore. 

That DC jug was sent to me by ol Dennis in hopes that I would learn something by studying it. 

It was on a saw he killed trees with during the week..........and it won races on weekends.


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## Adirondackstihl (Jul 30, 2014)

Here is an example of external boost ports....


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## Adirondackstihl (Jul 30, 2014)




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## Mastermind (Jul 30, 2014)

A feller can spend a bunch of time on that stuff Jeremy.....


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## Deets066 (Jul 30, 2014)

This is some pretty impressive stuff, never seen anything like that DC jug! Hey Randy that wouldn't happen to be mine and cgraham's 064 jugs would it? Or do you just get that many 064's in?


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## zogger (Jul 30, 2014)

Deets066 said:


> This is some pretty impressive stuff, never seen anything like that DC jug! Hey Randy that wouldn't happen to be mine and cgraham's 064 jugs would it? Or do you just get that many 064's in?



That is DC's grade B jug. His grade A if you look at it is a series of HAHAHAHAHA all ground around the insides there....


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## Mastermind (Jul 30, 2014)

No......this one is heading to Washington State. 

You guys jugs are carbon copies though.


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## Mastermind (Jul 30, 2014)

That was the 4th 064 of the week.....


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## bryanr2 (Jul 30, 2014)

i mailed a Mastermind ported 064 to PA today.  May be my biggest mistake of the year.


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## Mastermind (Jul 30, 2014)

You have plenty of saws to excite you.


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## cgraham1 (Jul 31, 2014)

Mastermind said:


> You have plenty of saws to excite you.


Is that what they're for??


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## SquareFile (Jul 31, 2014)

Mastermind said:


> That was the 4th 064 of the week.....



Sounds like a lot of 064's come across your bench.

For a saw that has been discontinued for about twenty years.

I can't wait until mine does.


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## nmurph (Jul 31, 2014)

Mastermind said:


> No......this one is heading to Washington State.
> 
> You guys jugs are carbon copies though.



I believe I recognize a cylinder in the OP???


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## Mastermind (Jul 31, 2014)

No Neal.......I've not used that one yet. The one in the OP is a Mahle jug.


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## Mastermind (Jul 31, 2014)

Here's what great guys hang out on these sites. 

I mentioned needing an 064 jug. Three arrived in a few days.


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## SquareFile (Jul 31, 2014)

Randy, have you worked your magic on many 056 mag II?

If so are the gains good?


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## Mastermind (Jul 31, 2014)

I've ported a couple of those units. They are not as easy to get big gains from as the newer epa, stopped up stuff. Some gains are there though.....just not the big gains will see in newer engines.


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## cgraham1 (Jul 31, 2014)

Mastermind said:


> No Neal.......I've not used that one yet. The one in the OP is a Mahle jug.


The one I sent was a Mahle jug. 

I've heard that the KS jugs are better, but after porting is there any noticeable difference?


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## Mastermind (Jul 31, 2014)

I checked and double checked Clint.......no difference in port timing at all. The KS jug does have a much nicer finish, and that is likely why in stock form they seem to out perform the Mahle. 

After port work.......I doubt there is any difference at all.


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## moosejaw6401 (Jul 31, 2014)

Pp.


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## Mastermind (Jul 31, 2014)

I agree completely.


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## wcorey (Jul 31, 2014)

> When the transfers first crack open, you want to be fully flowing across the entire area of the port.



What’s the benefit of keeping it flat (or am I completely misinterpreting that that's what you mean to say...)? 

I seem to recall the more northern monkey mention angling the upper transfers up a bit toward the exhaust side so as to give that end a head start to help get the entire charge over to the intake side at more somewhat the same time. Or something like that…

Is this only an advantage in particular cylinders and/or circumstances?


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## PA Dan (Jul 31, 2014)

Hey Randy thanks for sharing all this info! You answered questions I didnt even think of asking yet! Can't wait to get my 064 to you! Sure glad you had all those other 064's to practice on!


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## maico490 (Jul 31, 2014)

Mastermind said:


> No.....here's a set of finger ports......with bridges. Done by Dennis Cahoon.


I reckon he was laughing when he ported that. Hahahahaha!


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## Terry Syd (Jul 31, 2014)

If DC sent you that jug for you to learn something, I wonder what it was. First thing I noticed was the rear finger ports were not equidistant from the centreline of the cylinder. If they work, they would be tilting the flow to one side. I would have liked to see the flow pattern in the head before it got machined out.

Assuming they work, then what was he attempting to achieve. Looks like it may have been an attempt to increase the time/area of the transfers without raising them too much. Being as it is a dual port jug, there is a limit to how wide the transfers can be, so he may have gone with the rear boost ports to give the transfers more area. THEN, he may have noticed a disruption of the flow from the angle of the ports, so he put in the ports below them to correct the flow.

That's all conjecture without his R&D input - however, if I've learned anything from modding it's - 'what works is just that, what works'.


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## Chris-PA (Jul 31, 2014)

I don't have that many cylinders around to look at, and some of them are Poulan clamshells - but the only one that has a flat lower edge on the intake is a GZ4000. How common is it on higher end cylinders from the factory? And if it's not, then why not? It certainly would be easy for them to make that shape anything they wanted it to be.


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## bryanr2 (Jul 31, 2014)

Mastermind said:


> You have plenty of saws to excite you.



I know right?


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## dl5205 (Aug 1, 2014)

Thank you Randy, for taking the time to make a great thread.


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## Mastermind (Aug 1, 2014)

No problem my friend. 

I enjoy making these little engines perform better.......I figure I'm not the only one.


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## Brush Ape (Aug 2, 2014)

Mastermind said:


> I'm not the only one.




Imagine no compression
It's easy if you try
No saw performance
Without no PSI
Imagine all the people
Freezing all today...

Imagine there's no firewood
It isn't hard to do
Nothing in your pickup
Or in your woodpile, too
Imagine all the people
Paying for LP...

You may say my saw's a screamer
But I'm not the only one
When Randy made my saw
One runnin' son of a gun

Imagine two stroke obsession
I wonder if you can
No more crappy wildthing
Only three saw plan
Imagine all the people
Splitting all that wood...

You may say my saw's a screamer
But I'm not the only one
I hope someday you'll join us
Piss revving so much fun


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## dl5205 (Aug 2, 2014)

Brush Ape said:


> Imagine no compression
> It's easy if you try
> No saw performance
> Without no PSI
> ...


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## Deets066 (Aug 2, 2014)

Brush Ape said:


> Imagine no compression
> It's easy if you try
> No saw performance
> Without no PSI
> ...



You been drinking?


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## Mastermind (Aug 2, 2014)

Who are y'all talking to?


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## Duane(Pa) (Aug 2, 2014)

Mastermind said:


> Who are y'all talking to?


John Lennon showed up and sang us a song.


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## Adirondackstihl (Aug 2, 2014)

Mastermind said:


> Here's what great guys hang out on these sites.
> 
> I mentioned needing an 064 jug. Three arrived in a few days.


Should I mention I need a 268 carb?


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## Terry Syd (Aug 2, 2014)

Taking a look at DC's jug again, maybe he wasn't trying to correct the flow of the rear 'boost ports' with the addition of the grooves below the transfer ports. I've seen jugs ported like that without the rear groove 'boost ports'.

Perhaps the grooves below the transfer ports are used to alter the flow into the cylinder - like a 4-port Husky might use on a 372. On the 372 the ports closest to the intake are angled up a bit. Of course, the up-angle decreases time/area a bit, but if the up-angle creates a better flow in the loop scavenging then it can be a boost in power.

Back to DC's cylinder - The lower grooves below the transfer port create a situation like on an open-port cylinder. That is, the flow attached to the side of the piston 'lifts' the flow off of the top of the piston and angles it higher in the cylinder. What you would then have with the single port cylinder would be the part of the flow closest to the exhaust port angling across the piston while the part closest to the intake port taking a steeper angle up into the combustion chamber (just like the 372 4-port). The good thing about such a mod would be that you would not reduce the time/area of the port by changing the internal dimensions, you would simply be tweaking the flow as it entered the cylinder.


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## Mastermind (Aug 2, 2014)

Terry that's what I see with the bridge ports too.....more of a change in flow direction than volume. If a straight, or flat roofed transfer is best for peak power, and a angled one increases range, then these bridge ports may help to widen the range of peak power.


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## old-cat (Aug 2, 2014)

Mastermind said:


> Terry that's what I see with the bridge ports too.....more of a change in flow direction than volume. If a straight, or flat roofed transfer is best for peak power, and a angled one increases range, then these bridge ports may help to widen the range of peak power.



I think that's what my little MS260 needs, it loses a lot of rpms when it gets more than a little load on it.


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## scallywag (Aug 2, 2014)

Brush Ape said:


> Imagine no compression
> It's easy if you try
> No saw performance
> Without no PSI
> ...


 Did that take long to work up?..........Or did you spontaneously produce it from your exhaust port?


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## Mastermind (Aug 2, 2014)

I pull this stuff straight outta my ass. 

Was the ever any doubt about that?


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## Stihl working hard (Aug 2, 2014)

Another very informative thread Randy you are the master


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## big t double (Aug 2, 2014)

Mastermind said:


> .........
> Here's another thought. In a two stroke engine, it's all about the time/area of the ports went it comes to performance. You can widen the port to gain T/A, or you can raise it. If the rings travel across the exhaust would it be wiser to get the T/A you want by setting the height, or widening the port?????



was this a rhetorical question? I hope not because id like to take a stab at it...for the sake of discussion. id say it would be wiser to set the height instead of widening. I would think widening the exhaust port would allow more of the ring to drop into the port and beat it up worse and risk it hanging up? but my question is what about the guy that doesn't have the ability to cut a squish band and reset the base...would widening the exhaust port alone net any sort of gains or just be more likely to catch a ring? I know I ask that question without knowing if ive first given a correct answer to your original (possible rhetorical for the more in the know) question....but ive been stewing on this for a couple days now...this is the best I could come up with...flame away!!


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## Mastermind (Aug 2, 2014)

Thank you for adding to the discussion. 

First let me say that I do not have all the answers. Sometimes, the more I think I know.......the less I really understand. 

Here's my take on it though. 

In a work saw that is used to make a family money, or heat their home.........the number one consideration should be reliability. That said, an overly wide exhaust port should be avoided. So getting the height *just right* seems like the way to go. 

Can gains be had from just widening a port? Oh yeah. That was considered a "normal" woods port a few years ago. Just widen the ports, and poke a hole in the muffler somewhere. That was good for a 15 - 20% gain in some cases........but in order to get 35 - 40% gains the builder has to dig deeper than just doing a little Dremel work......


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## nmurph (Aug 2, 2014)

Mastermind said:


> ...but in order to get 35 - 40% gains the builder has to dig deeper than just doing a little Dremel *and Enco* work......



Fixed it for you.


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## big t double (Aug 2, 2014)

Thanks for the response mastermind. Now a question about the intake...I notice that the opening on the bore side gets widened and blended back toward the boot but how does that flow more fuel if the intake boot side stays the same diameter opening...then there's the intake boot itself...it seems to me (a moron by the way) that only so much air/fuel can fit through it...this has always confused me when looking at porting threads. Thanks for taking the time to do threads like this to help idiots like me understand what you guys do a little better.


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## Mastermind (Aug 2, 2014)

It's about how much the port flows when it first opens........that's when velocity is at it's peak. Think about what the port would look like at 20% open.......then notice that the corners really infringe upon open area at that point. 

As far as maximum flow........the venturi in the carb is the smallest point in the intake tract.


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## Chris-PA (Aug 2, 2014)

One of the reasons I asked up thread what the manufacturers do is because there is always some trade off. In the case of a flatter lower edge on the intake, I wonder if it creates a stronger, faster rising edge to the pressure pulse that is caused every time the intake port opens or closes. In turn, those pulses may have negative effects on the carb and fuel/air mixture.


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## Mastermind (Aug 2, 2014)

Every move a person makes on an engine is a compromise of some sort.......that is a fact. The flatter intake floor likely hurts fuel economy.


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## big t double (Aug 2, 2014)

Mastermind said:


> It's about how much the port flows when it first opens........that's when velocity is at it's peak. Think about what the port would look like at 20% open.......then notice that the corners really infringe upon open area at that point.
> 
> As far as maximum flow........the venturi in the carb is the smallest point in the intake tract.


Ok now it's starting to Makes sense thanks for taking the time. So being wider on the bore side allows it to pull fuel in through the carb and boot at a faster rate due to more time/area. I kinda wanna dremel on a jug hahahahaha


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## Mastermind (Aug 2, 2014)

I rarely widen the port much........normally I just reshape it.


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## Terry Syd (Aug 2, 2014)

Mastermind said:


> Terry that's what I see with the bridge ports too.....more of a change in flow direction than volume. If a straight, or flat roofed transfer is best for peak power, and a angled one increases range, then these bridge ports may help to widen the range of peak power.



And I would bet that with a dyno available, one of your projects would be to see how wide and deep those channels need to be to get the effect you want...


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## hillwilliam (Aug 4, 2014)

Thanks Randy! You share a lot of knowledge, and it's always appreciated.


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## Bieber (Oct 27, 2014)

If i have the upper transfers angled like in a ms460 jug, is it recommendable to level them so that front and back of them open simultaneously? Or would this result in a flow decrease?


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## Mastermind (Oct 27, 2014)

That is how I end up. Front and back opening at the same time.


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## Bieber (Oct 28, 2014)

Thank you Mastermind! 

I'm grinding on my 440/460 hybrid project right now 
and I'm scared of having the upper corners of the exhaust done a bit too tight...!? 
your opinion... 
this is the Mahle 460 jug:



just for comparison an older 034S jug from Kolbenschmidt: 
was suprised of these tight edges and flat roof...


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## Mastermind (Oct 28, 2014)

You are just fine. 

Watch the floor of the exhaust.......be careful not to lower it.


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## Bieber (Oct 31, 2014)

Mastermind said:


> You are just fine.
> 
> Watch the floor of the exhaust.......be careful not to lower it.



Thank you Sir! 
I though raised the exhaust by 1-1.5° just to smooth it out a little more and now I'm at 101° atdc.


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## Magnum783 (Nov 2, 2014)

Ok I am intrigued as they get about this port shape ordeal. I have access to a mill and lathe so porting work is not a huge stretch. I work for the Department of defense as the foreman of a job shop. I am putting a new cylinder and piston on my ms260 and would like to open up the intake and exhaust just a bit. I don't know a whole lot about this so I am reaching out to the pro's. I have ported and polished many small block chevy engines and have worked as a mechanic part time or years so I understand the concepts of what you guys are doing just looking or details. I am also not looking to do a full port that takes far more education than I poses at this time just looking to open the intake and exhaust up a bit as I live at 6200 feet and we loose a bunch of power up here due to altitude just trying to make some of that up. Pics or advice appriciated.


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## mdavlee (Nov 2, 2014)

Compression will be your friend at 6200'.


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## weimedog (Nov 2, 2014)

Just want to say thank you yet again for sharing some of your experience. Looks to me like the connected porting concept you show here is a way to get a little more towards the intake side of the combustion chamber. Two things cross my mind when I saw this thread.... one. Looks like a way to push my 2094 to the next level, could be FUN ... that saw is a test hog that is both a beast and fun. Two; this might be a way to move the 272's a step closed to the quad port designs.

Was thinking of trying finger ports on some of those older single open port saws just for chits & grins. Maybe on saws like 51-55 Huskys as well. A way to bring them into the realm of the newer designs? 

Still wondering about having differential heights on the quad port saws where the intake side opens at some point before. Or even having a top edge cut on the two port designs to favor the intake side. Saw this in other worlds and wonder if it translates here. I'm a fan of bearing surface for heat transfer. So if there is a way get similar results without reducing bearing surfaces for the piston (Cylinder wall) I'm going to try it. In the same spirit of dropping intake ports vs. widening them to use timing to effectively get the same thing... just thinking in text.. just wondering... think I asked this before once. Might be time to just hack and see. You ever see anything along those lines?


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## Mastermind (Nov 2, 2014)

Compression increase will give you the best gains.


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## kz1000 (Nov 2, 2014)

If I send you some of the tools that are required to port my saw, will you give me a discount??? I have several pencils and a box of business cards from a closed Pizza joint already boxed.


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## redfin (Nov 2, 2014)

Randy, I faintly recall seeing you post what burs you use in your handpiece and I thought they were in this thread? But I must be mistaken. Would you post these again please?


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## Magnum783 (Nov 2, 2014)

So how do I gain compression? in car world we do that by dropping the deck? In a saw that would mean lowering the cylinder height correct? I had already planned on taking out the base gasket and replacing it with gasket maker. I just am un sure if I need to lathe any off the bottom of the cylinder. Does cleaning up the intake or exhaust help the cause in any way?


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## weimedog (Nov 2, 2014)

Mastermind said:


> Compression increase will give you the best gains.


Yup.. hoping to have the Lathe set up before winter.


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## Mastermind (Nov 2, 2014)

Magnum783 said:


> So how do I gain compression? in car world we do that by dropping the deck? In a saw that would mean lowering the cylinder height correct? I had already planned on taking out the base gasket and replacing it with gasket maker. I just am un sure if I need to lathe any off the bottom of the cylinder. Does cleaning up the intake or exhaust help the cause in any way?



I cut the area around the combustion chamber (cutting the squish band) and then the base to lower the jug, and decrease the size of the combustion chamber.

If the port timing numbers are in the right places......then widening (and cleaning up) the ports does little, if anything to gain power.

Remembering that there is very little time for the air/fuel/exhaust to move per stroke at 10,000 rpm, so keeping flow velocity high is far more important than increasing flow volume. At least in my humble opinion.


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## weimedog (Nov 2, 2014)

Mastermind said:


> If the port timing numbers are in the right places......then widening (and cleaning up) the ports does little, if anything to gain power.


 

 

I agree... the race guys looking for tenth's chasing those extra split seconds might find it worth time to work those ports. The rest of us hacks? Compression, muffler mods, port timing, AND a SHARP chain will be where the most useful gains are made. Assuming the rest of the basics are squared away. That's my humble opinion as a "hack".


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## Magnum783 (Nov 2, 2014)

I think my prob is my lack term knowledge. This squish band? Where is this elusive guy? I understand lower the base of the jug, I can easily do that on the 4jaw with an adapter plate such as you have. The squish band I have no idea where to look or how to adjust this thing.


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## redfin (Nov 2, 2014)

Squish band is the bottom edge of the combustion chamber.


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## Magnum783 (Nov 2, 2014)

Ok but is that the same as the bottom of the piston or where is the located in the upper part of the cylinder that is the part I am confused on?


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## Mastermind (Nov 2, 2014)

This is tough to explain.....

Dig thru this thread....

http://www.arboristsite.com/community/threads/ms461-giveaway-saw-build-thread.246494/


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## mdavlee (Nov 3, 2014)

It's the band at the top of the cylinder that's flat before it drops into the combustion chamber.


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## old-cat (Nov 3, 2014)

Magnum783 said:


> Ok but is that the same as the bottom of the piston or where is the located in the upper part of the cylinder that is the part I am confused on?



The flat top of the piston goes up to a flat ring around the combustion chamber, like a sandwich and the fuel gets squished and pushed out of that area. 
That fuel that is being forced out goes directly into the flame and burns VERY rapidly. The tighter the squish gap the higher velocity of fuel into the fire.
THAT makes a better running engine and lowers the octane requirement.


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## mdavlee (Nov 3, 2014)

Shiny part in the top of the cylinder.


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## Mastermind (Nov 3, 2014)

old-cat said:


> The flat top of the piston goes up to a flat ring around the combustion chamber, like a sandwich and the fuel gets squished and pushed out of that area.
> That fuel that is being forced out goes directly into the flame and burns VERY rapidly. The tighter the squish gap the higher velocity of fuel into the fire.
> THAT makes a better running engine and lowers the octane requirement.



This guy gets it.


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## old-cat (Nov 3, 2014)

Mastermind said:


> This guy gets it.


 What I know came from studying a 1928 R&D book by Ricardo plus many hours of studying SAE tech papers plus some engine building.


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## Mastermind (Nov 3, 2014)

Reading up on modern performance engine design just enforces what those guys were onto back in the day. 

Wanna build strong running engines? 

Study......study......and study some more.


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## wigglesworth (Nov 3, 2014)

Mastermind said:


> Reading up on modern performance engine design just enforces what those guys were onto back in the day.
> 
> Wanna build strong running engines?
> 
> Study......study......and study some more.



You forgot the luck part...


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## old-cat (Nov 3, 2014)

wigglesworth said:


> You forgot the luck part...



2 cycle motors are Black Magic


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## wigglesworth (Nov 3, 2014)

old-cat said:


> 2 cycle motors are Black Magic



I prefer grey magic. It's a bit less sinister...


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## Bilge Rat LT 20 (Nov 3, 2014)

Suck, squeeze, fire and fart. 
Motors are simple
Te devil is in the details though.


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## CJ1 (Nov 4, 2014)

You really want to add a PITA put a expansion chamber on a 2st. Then everything has to work together. The only thing I know for a fact is you will have a sore hand and wrist from grinding! CJ


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## ft. churchill (Jan 11, 2015)

Mastermind said:


> We talk a lot about compression, port timing, muffler mods, etc. We don't talk too much about the shape of the port, and how the shape relates to performance.
> 
> Here's a few pics of an 064 I just did.......and there's some pics of another 064 jug in here too. I sanded through the plating when I was getting the transfer off the second 064 jug.
> 
> ...








Thank you for yet another great thread on porting.


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## jar944 (Jan 11, 2015)

quote
par Frits Overmars le Ven 21 Déc - 7:50
Here is a little technical story for the coming Christmas days. Its official title is Transfer Theory, but I call it

*The leaning tower of Pisa
[...]

Transfer theory part 1

the central column*
We want ample angle.area for our transfer ports while at the same time keeping their height within limits, so we need all the transfer area we can get; we want to use as much as possible of the cylinder circumference.
The best way to utilize the available real estate would be to aim all transfers radially inward; that way the cross section widths of all ports would be equal to their chord widths and you can't get any better than that. All transfer streams would meet in the center of the cylinder, slow each other down and form a central column with only one direction to go: upwards, in the direction of the cylinder head.
But since you can't have a transfer port at the exhaust side of the cylinder, an imbalance would occur and that central column would be inclined (sic) to topple over towards the exhaust side of the cylinder. You don't want that because too much of the fresh charge would take the escape route into the exhaust duct without first scavenging the cylinder.

How do you prevent that central column from leaning towards the exhaust side? If you omit the transfer ports directly opposite the exhaust, you would restore the scavenging balance, but you would sacrifice too much valuable port area. There is a solution, but let me address some other scavenging aspects first.

We want as much transfer port area as possible, so it would make sense to have all transfer ducts enter the cylinder perpendicularly, right? Nope.
To begin with, most pistons are domed, so transfer flow entering the cylinder would collide with the dome. Aiming the transfer ducts axially at about the same angle as the piston dome, usually about 10°, will not cost any effective cross section area and it will noticeably improve the flow coefficient. Larger-than-zero axial angles at the port floors will also enable you to fit larger inner radii in the transfer ducts, another benefit for the flow.

Second: those transfer streams entering the cylinder and colliding in the center will convert kinetic energy into potential energy. In English: their flow velocities will slow each other down in the collision process and the static pressure in the middle of the resulting central column will be higher than the pressure in the transfer ducts.
That static pressure in the central column is a good thing: it will provide for a higher density of the fresh charge in the column and that helps to expel the hot, thin burnt gases from the previous combustion cycle. Think of it as using a jet of water to chase away smoke: that will work a lot better than the other way around (using smoke to chase away the water).

But the static pressure at the foot of the central column can also have adverse effects. Too high a static pressure will impair the flow, because the higher this pressure is, the smaller will be the pressure differential that accelerates the charge through the transfer ducts. Aiming the transfer ducts axially a little will improve the flow, just like it did because of the domed piston. Slightly axially-aimed transfer streams will provide for a less violent, not completely head-on collision. The central pressure can be controlled this way, and the transfer streams will keep the axial component of their velocity, so the central column does not need to begin its journey to the cylinder head with zero velocity. So the axial column speed can be controlled as well by the axial transfer angles.



*Transfer theory part 2

positional & directional scavenging angles*
Most two-stroke people define radial scavenging directions by quoting the distances where the ports would intersect the center line (the leading distance and trailing distance in the drawing below left). Gordon Blair used that notation in his publications, and 95% of us followed suit.
But there is a better, more universally applicable way.
I will explain with an example, not of scavenging directions but of port timing: I might say that a transfer port height of 13 mm is perfect for a racing engine. That may be true for a 125 cc engine but it would be nonsense for a 50 cc or a 500 cc.
But if I say that a transfer port timing of 130° is perfect for a racing engine, then that is valid for any engine, regardless of its cubic capacity. Absolute distance values (millimeters, inches etc.) are not suitable for universal guidelines. Degrees are, as are percentages of bore or stroke. Rpm values are not; mean piston velocities are.

I express transfer duct directions in degrees. Each duct has a leading flank and a trailing flank. Each flank intersects the bore at a point which I can define with a positional angle. And each flank hits the fore-aft center line of the bore with an included angle which I call the directional angle. The drawing below left may clarify what I mean. And the drawing on the right is an example of an existing cylinder.
Now we can express the radial characteristics of the transfer ports with positional and directional angles, regardless of bore and stroke.
And we can express the ports' axial characteristics with axial angles, but that only gives a 'universal value' for engines with identical bore/stroke-ratios.
We may quote a height H in the cylinder where the transfer port's roof would hit the opposite cylinder wall. But we need to express H as a percentage of the stroke. Then we will have a truly universal value.
Then we will also see that short-stroke engines require smaller axial angles.







*Transfer theory part 3
the tower of Pisa*
As we are on the subject of scavenging angles, now would be a good time to say something about the axial angles of the A-transfers.
Surely a duct with an axial angle of over 20° offers a smaller cross-section to the flow than a duct that enters the cylinder perpendicularly?
Yes it does. But there are two good reasons to angle it upward anyway.

First, perpendicular mixture streams coming from the A-ports would collide and slow one another right down. The axial angles provide for less velocity losses and less pressure losses, so despite their smaller cross-section, upward ports may flow as much, if not more, than perpendicular ports.
(Now you may well ask why the B-ports do not get the same treatment. It is because the central scavenging column, resulting from all incoming scavenging streams together, must not have too much axial velocity, or the loop scavenging will result in a loop-loss into the exhaust).

Second, there is a thing called scavenging balance (I invented the word for my personal use, so this may well be the first time you ever saw it).
If you looked closely at the scavenging picture of the MB-cylinder I posted above, you may have noticed that the 'radial scavenging directional resultant' had a value of 101,045°.
90° would have meant 'straight up'; more than 90° indicates that the central scavenging column is leaning towards the exhaust side of the cylinder.
But we don't want that; it is bad for the scavenging of the rear part of the cylinder, and it is risky because it may provoke scavenging losses straight into the exhaust.

[...]

But how can we prevent a scavenging column from toppling over to the exhaust side like the leaning tower of Pisa? Not by pushing against its basis, but by pushing higher up. Hence the axial angle of the A-ports. The pictures will tell the story. (If only the Pisa architect had known a bit more about two-stroke scavenging....)

[...]


*Transfer theory part 4
vectors*
Let us assume that all transfer ports are of the same height. Let's also assume that a port with twice the cross-sectional width will give twice as strong an impulse (that is already doubtful; it presumes equal densities and equal flow velocities in all ducts, and as duct contents can have different inertias, their accelerations may differ, as will their flow velocities at any given moment).

If you accept these assumptions, you can resolve each transfer stream into an axial component, a fore-aft component over the piston, and a left-to-right component over the piston. The axial components all work in the same direction: towards the cylinder head. The left-to-right components will cancel each other out (if they don't the scavenging is asymmetric) while contributing to the pressure creation at the root of the central column (which in turn will accelerate the axial flow and thus enhance the axial vector), and the fore-aft components will result in a vector that may either point towards the rear side of the cylinder, be zero, or point towards the exhaust side.
This fore-aft vector together with the axial vector will give a resultant that will lean towards the rear of the cylinder, or point straight up towards the head, or lean towards the exhaust side.

What we want to achieve, is an axial column that clings to the rear of the cylinder, so it can wash away the spent gases with as little turbulence as possible. Turbulence will result in mixing of fresh charge and burnt gases, and we don't need that. And mixing will heat up the fresh charge, bringing it nearer to the detonation treshold. And we certainly don't need that!

I realize this is a crude way of describing a complicated flow dynamics event, but hopefully it will help you form a mental picture (no pun intended). Quote


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## Chris-PA (Jan 11, 2015)

Is this the ultimate in finger porting?


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## wigglesworth (Jan 11, 2015)

Chris-PA said:


> Is this the ultimate in finger porting?
> 
> View attachment 393637
> 
> View attachment 393638



10-10?


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## Chris-PA (Jan 11, 2015)

wigglesworth said:


> 10-10?


Yeah, from eBay - I was looking for a picture of the PM610 porting, which has two larger ports, but also tilted heavily towards the intake.

Couldn't help but notice the wide flat squish band too. Maybe these guys knew something.


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## kz1000 (Jan 11, 2015)

I saved your post for future re-read Jar-944 thanks.


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## LegDeLimber (Jan 11, 2015)

Is this from that half blocked (non english language) link ?
If so, Any chance of seeing the pics?

Pretty Please, for this barely monolingual, untraveled Yank?


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## jar944 (Jan 11, 2015)

LegDeLimber said:


> Is this from that half blocked (non english language) link ?
> If so, Any chance of seeing the pics?
> 
> Pretty Please, for this barely monolingual, untraveled Yank?



ill see if i can get them to display, but in teh mean time... here is some more nuggets of info from the other half of the dominating Overmars/Thiel team (BTW I didnt put this summary together) 


A few words of wisdom from Mr Thiel


"We did not flow the principal and secondary transfers separately.
In fact flowing the transfers makes little sense!
The important thing is more their direction and how
they influence upon each other."


"No pressure transducers were ever used in our engine development.
And time/area was never calculated.
The port timings remained practically the same during 15 years!
What we did was trying different angles and radiuses, mainly on
the transfer ducts. I think we tried 40 different types of transfer ducts
that did not chanche the time/aerea. It was all about in which direction
the charge entered the cilinder and how the tranfer streams influenced upon
each other! Also about 200 different exhaust pipes were tried. After 2004
nothing much was changed but we improved with different power jet and
ignition mapping. It seemed nearly impossible to improve the transfer ducts
any more. The exhaust ducts were CNC machined, using different programs,
mainly to reduce exhaust duct volume. Also about 100 head designs were tried.
This work was done over a period of 12 years.
In september 2004 the design of the RSA started, at Derbi, and the first dyno
test was in october 2005. In the meantime Aprilia had been bought by Piaggio
which also owned Derbi, so we could use Aprilia cilinders.
The engine went very well almost at once!"

" Very little computer simulation was used, as far as I remember only for the cilinder and head cooling.
We once had an exhaust design software on loan from a very well known company, the results were useless! It was mostly cut and try!
Cilinder development was done without any computing, just logical thinking.
The exhaust ducts were CNC machined.
All improvements were fully understood."

"We never had pistons seize during our steady state tests.
Working on the dyno continuously 5 days a week!
I am 100% convinced our engine could have run for 6 hours at max power without seizing.
The problems arise when you close the throttle, or run part throttle!
The piston is mainly cooled by the transfer flow.
And at part throttle there is less transfer flow, causing detonation (auto ignition)
The entering fresh charge is ignited by the remaining, hot, burned gases!
You can see very severe damage to the piston after maybe 10 seconds at 20% throttle.
This still is an unresolved problem! I was thinking about a way to reduce engine power without closing the throttle. But how can you do this? I did not find a solution before I retired.
And nobody else was really interested.
At 100% throttle the engine was undestructible!
By making the transfer ports as wide as possible we had very good piston cooling"

"The part-throttle detonation was caused by the scavenging pressure dropping soo low that burnt gases entered the transfer ducts, notwithstanding a blowdown time.area that was sufficient for full throttle at high revs. It was not a situation that could have been cured by altering the ignition timing"

"we tested various oils and fuels, there was not much difference.
I think they were all 'copied' from ELF.
1:20 was used, we tried more and less oil: no big difference!
Long ago, at Bultaco, I tested Bell Ray oil 1:50.
The result was 2 seized big-ends in 1 day!
Later we used MOTUL with very good results"


"The most time was spent narrowing the bridges in the exhaust duct.
These could not be CNC machined:the inside was unreachable.
Preparing for plating took about half a day.
And preparing for dyno testing another half day.
A cylinder 'untouched' after plating would give 1,0 to 1,5 Hp less.
All cylinders destined for 'works' riders were dyno tested.
And reworked in case of missing HP.
We could dyno at maximum 3-4 cylinders a day.
Difference between 'best' and 'worst' cylinders was about 0,4 HP.
There were 2 people grinding cylinders daily.
I only did necessary corrections personally.
There were 2 dyno's working every day.
About 300 cylinders were made every year.
And apart of this we also constantly tried to improve HP.
Mostly batches of 30 cylinders were cast, 25 'as before' and 5 with small changes.
Between casting and dynoing took about 3 month's time.
We also dynoed replated cylinders.
They were almost never as good as new one's!"
rsss396
08-04-2013, 11:36 AM
Flow testing thetransfers proved a bit useless.
So we made many different types and tested them.


Why do you want tomake such a short stroke engine?
Square bore and stroke dimensions would be FAR better!
It gives you more port surface, which is power determinating.
And with a short stroke the piston will become very hot.
Personally I would make a slightly long stroke engine.
I cannot see 1 single advantage for a short stroke!

Power is mainlydetermined by blowdown, which is better with a triple port.
A bridged exhaust is wider at the bottom.
Which means the transfers have to be smaller.
This gives less power


When you havesufficient blowdown there is no exhaust flow anymore when the transfers open.
I suppose this happens at max. torque
After max power the blowdown becomes insufficient andexhaust gases penetrate the transfers.
You can see this when you take off the cylinder: thetranser ducts become black.
Some of the fresh charge goes into the exaust, this helpspiston cooling.
But when it becomes too much exhaust temperature becomeslower and you lose power and revs.
Until at a certain no. of revs the engine completelystops.


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## jar944 (Jan 11, 2015)

continued


Normally highcrankcase cv. likes short timings and low crankcase cv. likes longer timings.
I think there is also a limit of how low you can go, butI do not know where this limit is!
The flow from the inlet into the transfer ducts is veryimportant but little known I think.
It is impossible to simulate on a flow bench!
And difficult to understand, with the moving conrod,piston and crankshaft.
One thing I am sure about is: always give priority toflow over crankcase volume!


We sometimesmeasured under-spark plug temperatures, they proved too high so we modified thehead insert,
bringing the water nearer to the plug. This was very succesful, afterwards wenever bothered with the sparkplug temperature anymore!

The best way towork on plated cylinders is by using dental diamond burs.
There were never any problems with the piston rings withunchamfered transfer ports.

Chamfers at thetransfers gave less power.
A radius at the exhaust was better.
But this also depends on your exhaust pipe.
Many years ago we used unchamfered exhaust ports on the50 and 125/2 engines.
And kept the exhaust port as low as possible.
I discovered this did not work on a 125 Aprilia cylinder.
Maybe because the exhaust pipe was already made for ahigh exhaust port!
A radiused exhaust port flows better, but I think a sharpedge may give a stronger pressure wave.
You only arrive at the best compromise by testing.

You should always prepare your cylinders yourself beforeplating.
NEVER permit the plating shop to do this!
Because they have NO idea about the importance ofchamfers etc.
Once Gilardoni destroyed my best 50cc cylinder this way!
30 years ago, I still get angry when I think about it!



First the bottom ofthe C-port was lower than the pistonring at BDC.
Then the C-port was made narrower, to be able to make theB-ports wider.
The idea was that the near horizontal B-ports would flowmore than the steeply upward angled C-port.
This gave a good improvement, and was done in 1997.
Later the bottom of the C-port was raised until nearlyBDC
We made 4 cylinders, just to try.
They gave better power and caused no problems at all.
This was done in 1999, halfway through the season.
After that all cylinders were made like this.
The bottom of the C-port is not quite as important as thebottom of the A and B-ports.
Because of its upward direction it does not cool thepiston much!
A still narrower C-port was tried, (12mm) but this lostsome power.
Also a cylinder without C-port, and still wider, bridged,B-ports was tried, with negative result.
rsss396
08-04-2013, 11:36 AM
a thermostat was never used.
On the dyno the lower the temperature, the more HP!
I do not know the lower limit, if something 'strange' happens if you go too low.
But at a certain piont you might get fuel evaporation problems.
At Aprilia HP still improved going from 45° to 40°.
And at Bultaco I used tap water for cooling, about 20°, with good results and without any problem.
Lots of water moving rapidly was clearly the best.
Some people believe cooling the exhaust duct 'too much' would cost power.
This proved to be nonsense.
The more we cooled the more HP!
Reducing wall thickness also proved to improve HP.
The limit is mechanical reliability.

I just meant to say that reducing wall thickness improved power.
And especially in the hottest places of course.
Like the top of the cylinder, and around the exhaust duct.
The limit, of course, is mechanical strength.
And, in your case, maybe cooling capacity!

You want the aux ports as big as possible to have more blowdown.
You want the most blowdown with a limited exhaust timing in order not to lose power at low revs.
But there are limits.
You have to keep away from the transfer ports to prevent fresh charge going into the aux. ports.
That is why their bottom is inclined.
The same happens if you make the aux. ports too wide.
You loose fresh charge.
This also cools the exhaust gases, so you loose revs.
Symmetry is also important.
We tried a piston with a closed pistonpin hole on one side.
With this piston power was very bad!
Worse than with a 'normal' piston!
Closed piston pins did not improve power at all revs.
Only at some points in the power curve.
Mostly at high revs.
At some points in the power curve you loose something, but not much.

With aux. ports you can use a lower exhaust port, so that you have more power at low revs.


What really counts is blowdown.
But discharging of the burned gases may not be complete until BDC.
In this case you get 'short circuiting' of the fresh charge from the transfer ports into the exhaust.
By using auxiliar exhaust ports you can improve blowdown quite a lot.
So that you might be able to raise the exhaust's 'floor'.
This helps improve blowdown flow, as seen on a flowbench.
In 2007 we started to raise the underside of the exhaust port.
The first results were very promising!
But at the end of 2007 I retired, so I could not finish what I started.
The idea was to raise the exhaust underside as much as possible until power dropped.
And then, with a smaller exhaust port underside it might have been possible to widen the A-ports more
without losing the fresh charge into the exhaust.
The ideal situation would, of course, be that all the burned gases are discharged from the cylinder before
the transfer ports start opening.
So it is difficult to have too much blowdown.
But in trying to achieve enough blowdown you can arrive at a too high exhaust port.
Which first causes power loss at low revs, and if exaggerated still more also a loss of max. power.
Because of shortening the power stroke.


120mm is NOT particularly long, it is quite a normal length.
After much testing 50cc's arrived at a best length of 85mm
85:39,5=2,15
An MBA 125/2 had a 90mm conrod.
90:41=2,19
So a 125 conrod with 120:54,5=2,20 is about the same!
Even calculated in the '50cc way' it would be 54,5X2,15=117!
So the conclusion is that 125's always used way too short conrods.
Why?
Probably to reduce total engine dimensions.
At Aprilia we started with 110, and finally arrived at 120.
Each time the rod was lengthened the power improved.
Also if we kept crankcase volume the same!
Making the crankcase volume bigger increased power even more!
A lot of time and money could have been saved if they had asked, and listened to,
to someone with 50cc experience!
Instead rod lengths of 110, 113, 115, 118 and finally 120 were tried!


The squish band should follow the piston radius.
That was our conclusion after many tests!
rsss396
08-06-2013, 12:55 PM
Flow Testing of flushing channels. (transfer ducts)
I doubt very much to its usefulness. 
We did it for each cylinder, mainly to see if there were strange anomalies. 
They were never there. 
But there were some changes eg cylinders flow more gifts. 
But less power
It turned out in the end that it was best to test the changes. Simply power (dyno test)
Was there ever such Aprilia czech flow bench, Jante principle. 
Also had little effect. 
In all of these tests, flow does not take into account the differences in pressure during the rinse. (scavaging of the cylinder)
The best test for different coil channels was therefore the power test! (coil channels are transfer ducts)
The most useful had the flowen exhaust port and channel
When flowen of the outlet channel, the test was done with the piston in different positions. 

1-2-3 mm open,Blowdown, Completely open. 
We also had buses that you could test the auxiliary ports separately and also the middle gate. 
We searched for the best possible flow with the smallest channel content. 
The blowdown was a lot better with padded channel bottom. 
A cylinder with center arrow had less flow than with auxiliary ports. 
The dams between middle and auxiliary ports gave away less flow. 
Make the exhaust port higher bottom gave slightly less total flow, but did not seem bad for power. 
I was here not quite done with it when I retired
You could say that the outlet channel a flow bench is almost indispensable. 
For the coil channels not. (transfers tunnels)
Bed and think much testing is therefore important. 
And of course, the ability to be able to make. Many cylinders 
I had that opportunity at Aprilia ample. 
If you make little cylinders per year, it is almost impossible to try everything! Sufficient 
What remains a question is how the influx from the crankcase goes into the coil channels. 
It did not work for me as a test system to think. 

Was important for you the total flow or the flow rate in terms flowen?

The total flow was actually the most important. 
Searching for the 'narrowest point' in the engine. 
These are the transfer ports, minimum flow and shortest timing! 
So maximum transfer ports are very important. 
But the timing is not so long, and you can not get too close to the exhaust port. 
Because then bends to the flow, and the exhaust in! 
So it is very important that most, preferably all, the cylinder gas burnt off before the flushing begins. 
As far as the lower edge of the exhaust port than is currently interest for? 
I so can not try! Until the end 
The idea was then to increment to the assets less the bottom was.
And then if necessary
. to be able to make the coil. ports wider


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