# Understanding port timing numbers



## Mastermind (Aug 27, 2010)

I've been around here for a little while now and have noticed something. A lot of members (myself included) ask questions about port timing and they often go unanswered. 

Is this because the modders here want to keep the things they have learned (through trial and error no doubt) to themselves. If that's the case who could blame them. I for one am a little apprehensive to alter port timing on an expensive jug without knowing what these changes are going to accomplish.

I have tried different timing numbers on several low dollar saws, and have had a great time attempting to learn all I can. For me, the learning to do a new skill is the best part. 

I reached a point in playing with saws that I have more questions than I had when I started. The basic things I needed to know were easy to find using the search function. It's the details that I wonder about these days. And I'm sure I'm not alone.

For instance, will a longer intake duration raise rpm but cause a lose of torque? If I have amble blowdown time could I give up some of this by raising the transfer roof, in turn increasing torque and rpm? And this is the one I wonder about the most, If I have more exhaust duration than inlet should I increase intake timing to match? If I do this will I lose torque? Do certain numbers work better on a given size saw?

Some of these questions I've got pretty well figured out but know others are wondering about. What I'm hoping for is a good discussion on the pros and cons of altering port timing. Of course I'm trying to learn something here, but I'm also looking for a thread we can all use to trade info in for the good of our members.


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## AUSSIE1 (Aug 27, 2010)

Some don't like parting with hard earned no's. Fair enough.

Personally I don't do racing etc or have the need to keep work private and wouldn't even scratch the surface of the pro's. I try to share as much as I can, but with that said there is always the fear of giving incorrect info. There is a disclaimer attached! 

All saws like different methods/no's etc.

I have put most of my time in the BB kits for the 365 to 372's being cheap if anything goes wrong. A great saw to play with, with the options availible, pistons etc.

Widening the exhaust increases the blowdown and gives increased revs and shifts the power higher. No loss of compression.
Raising the exhaust increases the blowdown and shifts the power higher at the expense of some compression.
Raising the transfers can bring torque down a little and flatter from a high ex duration as long as blowdown allows it.
Increasing the inlet helps balance the need of the transfers and exhaust as long as the duration doesn't excede the crankcase.
Inlet duration usually at or close to exhaust duration.
If revs increase from more inlet duration, it's due to one or more of the other functions needing it.

Remember the disclaimer! :hmm3grin2orange:


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## Hedge Hacker (Aug 27, 2010)

Posts like this make me feel great because i know no matter how long i cut wood or fix saws, i'm never gonna know everything.


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## chopperfreak2k1 (Aug 27, 2010)

TRUELY GREAT THREAD IDEA!!! i'm all ready to learn!


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## mtngun (Aug 28, 2010)

mastermind7864 said:


> A lot of members (myself included) ask questions about port timing and they often go unanswered.


When Timberwolf is around, he has some good suggestions, though you get the impression he only tells you a fraction of what he knows.  But, TW hasn't been around much lately. 

Seriously, other than TW, I'm not aware of any authoritative _*technical *_reference on non-piped two-stroke engines. Most of the reference material like Bell's book is geared toward pipes. Without a pipe, the general tuning trends may still apply, but you won't be able to predict numbers with any certainty.

One of my personal frustrations is that the vast majority of the modding info on the web is geared toward cookie cutting, not milling. It's debatable how much the cookie cutter mods will help a milling saw.

Some people say you shouldn't do any mods to a milling saw. These naysayers invariably seem to be people who are running 120cc saws. You don't encounter many 90cc millers who are satisfied with stock power.  

TW and JJ have given me a few good suggestions specifically for milling, but otherwise, I'm on my own, learning by trial and error.



> For instance, will a longer intake duration raise rpm but cause a lose of torque? If I have amble blowdown time could I give up some of this by raising the transfer roof, in turn increasing torque and rpm? And this is the one I wonder about the most, If I have more exhaust duration than inlet should I increase intake timing to match? If I do this will I lose torque? Do certain numbers work better on a given size saw?


The only person who may be able to answer those types of questions with any precision is TW. He has computer models and a dyno and lots and lots of experience. 

But, all the computer models out there assume you are using a pipe. Lotsa luck finding a model to work with chainsaw mufflers. I assume TW wrote his own program, or else tweaked a program to work with chainsaw mufflers.

One thing we don't see discussed much on AS is the case volume and how that affects optimal porting. I don't know much about that myself, except that it does play a role, and serious modders like TW go to a lot of trouble to measure case volume.

Another bone for thought -- take the Dolmar 7900, which most people consider a best-in-class powerhead. Look at the 7900's port timing and you will find ..... absolutely nothing remarkable. Makes you think, doesn't it ?

My only firm conclusion to date is that I've never had a saw with strong compression that didn't cut well, and I've never had a saw with low compression that impressed me. Dolmars and Solos generally have strong compression. The latest 066BB has strong compression. I like that.


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## AUSSIE1 (Aug 28, 2010)

There is alot of misconseption about cookie cutters, even from some who play with porting. Cookie cutting is basically a way of measuring mods that you have made, whether it's a race saw or a firewood saw.

Some may go to the lengths that TW does, others alter durations etc without even using a degree wheel or port map. What ever works for you as there isn't any law. Some have that many parts at their disposal, if a cut goes wrong, piff it in the bin and grab another.

TW has considerable knowledge, but that doesn't make others redundant.


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## troutfisher (Aug 28, 2010)

mastermind7864 said:


> For instance, will a longer intake duration raise rpm but cause a lose of torque? If I have amble blowdown time could I give up some of this by raising the transfer roof, in turn increasing torque and rpm?



I think of transfers as "bleeding off base pressure" . As the piston moves down, the base compresses, and once the transfers start to open that pressure is released into the cylinder. It takes some time for that air to move. When RPM's increase that time window gets smaller, so I need to raise the transfer a bit to keep the actual time the port is open the same. Just my 2 cents, may be wrong


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## parrisw (Aug 28, 2010)

I'm in for this discussion. 

Where's John with some rat tail file port job discussion?


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## Mastermind (Aug 28, 2010)

We have a great discussion going already. Thanks for everyone's thoughts and comments so far. This is one aspect of the tuning process that we just don't see many threads on. Thanks.


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## mtngun (Aug 28, 2010)

AUSSIE1 said:


> TW has considerable knowledge, but that doesn't make others redundant.


Agreed.

I've learned a lot from you, Brad, Tree Slinger, JJ, and the other experienced modders.

But as near as I can tell, most modders, including myself, are doing what TW calls "guess porting." We make an educated guess, cross our fingers, and hope it works. 

Eventually we find a recipe that works well, and then we tend to stick pretty close to that favorite recipe forever and ever.

But to be able to answer "what if" questions with any certainty, TW is the only one in the chainsaw world who seems to be able to do that. He has the computer model, the theoretical understanding, and the dyno. The rest of us are lacking at least two of those things. 

Troutfisher, thanks for the explanation of transfer timing. 

I would really like to get into the computer modeling someday. If for no other reason, than to be able to play with "what if" scenarios and see how they change things. That should be educational.

That was what I found with computerized reloading software. It wasn't so much that I used the software to find a final load, because the software wasn't _*that*_ reliable, but being able to run many different "what if" scenarios and seeing how it changed the outcome gave me a pretty good feel for what worked and what didn't. I was able to learn more in one year playing with the software than I had learned in the previous 20 years of "guess reloading."


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## Mastermind (Aug 28, 2010)

mtngun said:


> One thing we don't see discussed much on AS is the case volume and how that affects optimal porting. I don't know much about that myself, except that it does play a role, and serious modders like TW go to a lot of trouble to measure case volume.



I'm in complete agreement with this statement. I would truly enjoy learning more on this subject as well.

In the Husqvanra 357xp vs 359 etech comparison I did awhile back, the 357xp had crank stuffers while the 359 did not. After running both saws I found the xp made more rpm stock than the 359, but the 359 (ported) had a lot more torque. I can't say with any certainty how the xp would do ported. (It was not my saw, and was only muff-modded and the squish was lowered to around .020). I would like to be able to say I know what the stuffers did for the performance of this saw but could only parrot what I have heard others say, so a discussion on this subject would be welcome by me for sure.


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## mtngun (Aug 28, 2010)

troutfisher said:


> I think of transfers as "bleeding off base pressure" . As the piston moves down, the base compresses, and once the transfers start to open that pressure is released into the cylinder. It takes some time for that air to move. When RPM's increase that time window gets smaller, so I need to raise the transfer a bit to keep the actual time the port is open the same.


OK, since you did a great job of explaining transfers, I'm going to pick your brain a little more. 

Let's use the current 066BB as a guinea pig. 

And let's say that I this 066BB is used primarily for milling, which means heavy loads at 7000 - 10,000 rpm without bogging. Bog-resistance is very important on a milling saw.

The jug comes from the factory like this, give or take a few degrees:

169 degrees exhaust duration
178 degrees intake duration
32.5 degrees blowdown

Someone suggested raising the transfers to 25 degrees blowdown, and that does sound like a more conventional number than 32 degrees blowdown.

But how is 25 degrees blowdown going to affect the ability to mill at 7000 - 10,000 rpm ? Will it shift the powerband to higher RPMs, and make the saw more prone to bog under heavy load ?

I imagine the answer will be "it just depends."


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## troutfisher (Aug 28, 2010)

169* of exhaust means it must open at 95.5* atdc
95.5 + 32.5* of blowdown = 128, the transfers open at 128* atdc
180-128= 104* of total transfer time, that sounds low to me. 

if you raise the transfers to 25* of blowdown that means the transfers will open at 120.5* atdc. That means you will have 119* of total transfer time. That sounds a lot better to me. I agree with raising the transfers. 

That said, I have never ported a 066BB, or milled with a chainsaw, so I'm not sure about setting up a motor specifically for it.


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## troutfisher (Aug 28, 2010)

mtngun said:


> without bogging. Bog-resistance is very important on a milling saw.




As for Bog-resistance, I would think that would be a function of the chain and gear setup. I have bogged 40 horse motorcyle engines in the cut, and run 346xp's that I could set the dawgs and pull on. Maybe one tooth lower on the sprocket, skip chain, square ground chain, ect. might make it less prone to bogging.


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## Metals406 (Aug 28, 2010)

This is just my own opinion, but I view porting as optimization. . . The path of least resistance if you will.

I like to eyeball a jug, slug, and case and think about ways to get the most amount of charge, through the intake and out the exhaust in the most efficient way possible.

Scientific it is not, but it has worked well for me.

I have yet to degree a saw, or figure out it's numbers. Perhaps I'm leaving some things on the table -- but one can also spend a lot of time behind the grinder (and machining) for gains as small as a few tenths of a second. Unless I plan on racing, it's just not worth the time for me.

When I port, I also want just as much saw longevity as one could expect out of stock.

It's kind of a bean counter concept. . . Do you build an airplane that flies 20 knots faster, but you have to rebuild it 3 times as often?

I also think the 4-stroke concept is lost to a lot of people. That burble you hear in the air, is fuel and air that will be used during work. Ported or not, I always want my saws to cough a little in the air, and clean up in the cut. That to me is optimal tuning, not a certain RPM "target" number. I let atmospheric conditions, altitude, etc dictate where the saw wants to be at that particular time. Maybe one day it's 500 rpm less than the time before, maybe it's 250 rpm more? 

Like I said, just mah two pennies.


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## mtngun (Aug 28, 2010)

troutfisher said:


> As for Bog-resistance, I would think that would be a function of the chain and gear setup. I have bogged 40 horse motorcyle engines in the cut, and run 346xp's that I could set the dawgs and pull on. Maybe one tooth lower on the sprocket, skip chain, square ground chain, ect. might make it less prone to bogging.


Yes, but ...... if it bogs with a 7 tooth, lotsa luck finding a 6 tooth ! ! ! 

The milling equivalent to skip chain is the Granberg grind, where half the teeth score and the other half cut. It doesn't necessarily cut faster, but it does rev faster and makes life easier on the powerhead, similar to skip chain. The downside is the cost if you buy it, or the time consuming grind to make your own.

My limited experience so far with "bog-resistance":

-- a stock 066 was gutless for milling, and seemed to have a narrow powerband. I didn't have a tach at the time, but it had to be fed carefully to keep the revs up.

-- the old, low compression 066BB was prone to bog below 8400, but rarely had enough power to spin more than 9500 in the cut. Talk about a narrow powerband ! 

-- the new, high compression 066BB would lug as low as 6000, or spin as high as 10,500. Despite its questionable port timing, the powerband was awesomely broad and forgiving. As a bonus, it consumed less fuel than any other 066 top end I've run.

-- but then after I widened the 066BB's intake, it would bog below 7000, while not gaining anything on top, and drinking a lot more fuel. 

-- my stock Efcos had a broad, user-friendly power band with conservative port timing and decent compression. A mild woods port shifted the power band to higher RPMs -- cuts a little faster as long as you keep the revs up, but not as forgiving if you lean on it.

So far my favorite recipe is all the compression I can scrounge up and sane port widths and port timing. Seems to give a broad, user-friendly powerband. I'm always looking for something better, though.


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## ronT2 (Aug 28, 2010)

mtngun said:


> OK, since you did a great job of explaining transfers, I'm going to pick your brain a little more.
> 
> Let's use the current 066BB as a guinea pig.
> 
> ...



Those are some interesting timing #s, I’ve never seen a longer intake duration than an exhaust duration. Could the longer intake duration and later transfer opening have something to do with case volume or is it more of a function of where and when Stihl wants the power to come in, probably both. I realize it’s an aftermarket BB kit but I’d imagine they would model it after the OEM jug.

Maybe I’m looking for someone to come along and say something like….because the case volume is greater relative to the 066s swept volume the intake needs to be open a little longer to fill the case and the transfers have to open a little later so intake charge has time to compress?????

I don’t know, I’m just thinkin which usually gets me in trouble. I’m also in the same boat as Troutfisher, I’ve never worked on an 066 but really enjoy this kind of thread.


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## troutfisher (Aug 28, 2010)

mtngun said:


> Yes, but ...... if it bogs with a 7 tooth, lotsa luck finding a 6 tooth ! ! !



I'll bet Danzco would have one, or make one.

http://www.danzcoinc.com/html/pin_drive_sprockets.html


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## chopperfreak2k1 (Aug 28, 2010)

sorry for the thread jack, but i want to give a sincere thanks to you guys for sharing your knowledge here with the rest of us. thanks!


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## mtngun (Aug 28, 2010)

ronT2 said:


> I realize it’s an aftermarket BB kit but I’d imagine they would model it after the OEM jug.


Yes and no.

From what I've read between the lines of various comments Grande Dog has made over the years, the NWP design process originally consisted of something along the lines of sending the Taiwanese manufacturer an OEM P&C, with instructions to "make it like OEM, except 2mm larger," or words to that effect.

Thus in the beginning, the BB timing and compression ratio was nearly identical to the later 660 jugs (159 intake, 173 exhaust, and 21.5 blowdown on the last OEM jug I measured).

Over the years, the BB design has been tweaked. For example, there were problems with the piston skirt hitting the case, so they shortened the skirt -- which increases the intake timing. That seems to be how the 066BB ended up with radical intake timing -- by accident.  I don't think there was a lot of serious thought put into it. 

I'm not sure how the current 066BB ended up with more blowdown than the OEM ? ? ? 

BTW, Stihl seems to follow a "cookbook" approach to saw design. For example, most of the pro Stihls have very aggressive exhaust duration, apparently to compensate for the restrictive muffler. Most of the recent big Stihls have an anemic compression ratio, apparently to compensate for poor gas or poor operators.

Efco also seems to follow a cookbook recipe for all their saws, only their recipe is different than Stihl's. Modest port timing, but decent compression. I like the Efco recipe better.


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## AUSSIE1 (Aug 28, 2010)

mtngun said:


> Agreed.
> 
> I've learned a lot from you, Brad, Tree Slinger, JJ, and the other experienced modders.
> 
> ...



Fair assesment, but I'd say TW has had to make a guess from time to time, probably not as often as most of us. 



mtngun said:


> Someone suggested raising the transfers to 25 degrees blowdown, and that does sound like a more conventional number than 32 degrees blowdown.
> 
> But how is 25 degrees blowdown going to affect the ability to mill at 7000 - 10,000 rpm ? Will it shift the powerband to higher RPMs, and make the saw more prone to bog under heavy load ?
> 
> I imagine the answer will be "it just depends."



When you raise the horsepower to a higher rpm, it norrows it. To help flatten the torque curve raise the transfers to help lower it and make it flatter, It doesn't raise it to a higher rpm.


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## ronT2 (Aug 28, 2010)

Thanks mtngun, sounds like the BB kit has a little history. Has anyone tried welding up the intake floor? I can’t imagine somebody around here doing that.


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## AUSSIE1 (Aug 28, 2010)

mtngun said:


> So far my favorite recipe is all the compression I can scrounge up and sane port widths and port timing. Seems to give a broad, user-friendly powerband. I'm always looking for something better, though.



Your inlet sounds as though it has lost velocity at lower revs, but I would continue to get as much area as you can out of your exhaust as this aids in blowdown.


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## jonsered 535 (Jun 29, 2015)

mtngun said:


> Yes, but ...... if it bogs with a 7 tooth, lotsa luck finding a 6 tooth ! ! !
> 
> The milling equivalent to skip chain is the Granberg grind, where half the teeth score and the other half cut. It doesn't necessarily cut faster, but it does rev faster and makes life easier on the powerhead, similar to skip chain. The downside is the cost if you buy it, or the time consuming grind to make your own.
> 
> ...


In widening exhaust port do I also widen the intake the same amount also or leave the intake alone thanks


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## Rx7man (Jun 29, 2015)

This thread was started before my time here, but I really like the idea... 

Going on pure assumptions based on physics I learned 20 years ago, I would say a few things on intake timing vs case volume..
If you have very aggressive intake timing, your transfers had better be up to the job of flowing it too, otherwise you have excessive residual charge left in the case.. If you have excessive charge in the case, it means your peak intake velocities will suffer, and especially with stuffed/low volume cases, will quickly cause carb reversions, leading to tuning nightmares at lower RPM's... (way rich and boggy at the low end while leaning out on the top).

I've come to this conclusion based on my work with the plurality of 65's I've done.. they have pretty poor transfers, and I tried a setup of about 176* intake and exhaust, and 19* (IIRC) blowdown.. it ran like crap in the cut but sounded wicked if you piss-revved it. it was about 10 seconds slower cutting a 24" fir than my mildly ported saws because it would bog and refuse to recover due to the over rich mixture and intake reversion.

So I tore it apart again, and windowed the piston, did a MAJOR hack job on the transfer runners to open them up, but didn't change the transfer windows or roof. Put it all back together and the thing is a whole different animal now... I will say it certainly isn't a saw with stump-grinding torque, but it's responsive again and once into the powerband it's quite enjoyable to run... Since I raised raised the exhaust port significantly to achieve that duration, even with a base gasket delete it only has about 140 PSI compression.


For milling saws, increased compression leads to increased temps, I'd say leave the compression ratio alone and just make it breath a bit better.. .muffler and intake system mods and better transfers all lead to better airflow through the engine which help cool.. especially if you aren't retaining as much hot exhaust in there.. Increasing your corrected compression ratio (from exhaust port close to TDC) from 5.5:1 to 6.5:1 increases the cylinder temp (just from compression of the gases) from 302C to 346C... With a 4.5:1 ideal compression ratio that temp drops to only 260*C.. All things to take into consideration on a saw that's working long hours.

I'm also a firm believer in a oil mix that.. aww heck, never mind that.!

My ported saw count is only about 8... so take what I say with a grain of salt!


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## Rx7man (Jun 29, 2015)

Here's an idea of the transfer port work required to get the saw running with that kind of intake duration...
Left is bone stock, right is the mild port



And here's the windowed port.. heck, the saw even lost a lot of weight!


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## jonsered 535 (Jul 2, 2015)

Mastermind said:


> I've been around here for a little while now and have noticed something. A lot of members (myself included) ask questions about port timing and they often go unanswered.
> 
> Is this because the modders here want to keep the things they have learned (through trial and error no doubt) to themselves. If that's the case who could blame them. I for one am a little apprehensive to alter port timing on an expensive jug without knowing what these changes are going to accomplish.
> 
> ...


This is the stage i'm at too same questions as you have here mastermind and I'm sure you have figured a lot more since you wrote this.


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## MustangMike (Jul 2, 2015)

Surprising this thread has so few posts.


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## Mastermind (Jul 2, 2015)

This one is from the good old days. 

Since I posted this thread I've found out that I had no idea..........and still don't sometimes.


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## jonsered 535 (Jul 2, 2015)

Mastermind said:


> This one is from the good old days.
> 
> Since I posted this thread I've found out that I had no idea..........and still don't sometimes.


Share some of your knowledge you learned oh wise one. I'll tell you one thing if I could get these 2 359's going good I'll be happy you must have a lot of patience hTs off to you and do a lot of reading


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## Deets066 (Jul 2, 2015)

Is blowdown relative to anything? Should it increase when bore increases? 
And when changing blowdown how do you know whether to change the exhaust or transfers  
Is this all trial and error?


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## weimedog (Jul 2, 2015)

Some times these threads advance the state of the art and sometimes these are fishing trips... From someone on the out side looking in, the 800 lbs gorilla in the room is compression and how you want to get there. Decking the cylinder or pop up or some combination of the two. Some times it feels like the "porting" really ends up being the art of managing the changes created when changing internal geometry to get more compression..... For example. Some one with a lathe and builds 200 plus psi saws decking cylinder's and correcting poor squish band surface conditions now has a bunch of induced issues to work around. The intake duration is increased , exhaust delayed, and a piston overlapping the transfers to the point that ridiculously low blow down numbers correct the reduced x-sectional area and turbulence at the cylinder intersection of the transfer because of the piston interference. The compression changes power characteristics in enough of a positive way to make it worth the other issues. Longer intake duration can be both a blessing or a curse depending on a bunch of other factors such as blow down, case volume, and intake tract characteristic. Delayed exhaust can also be blessing or a curse depending on other factors. Although one very smart guy in here likes that extra time in the power stroke. Another guy without the ability to deck a cylinder might use a tool to fix the lousy squish band and wants to do a pop up to get that compression hit. His porting numbers probably will look a little different. And yet another can make a two piece cylinder/ cylinder head system and has the best possible scenario relative to squish band, compression, and the piston relative to the transfer ports.... So to me I'd like the discussion of porting numbers to be predicated by what did you do to your cylinder?? then start the conversation....just an observation... just an old man.


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## jonsered 535 (Jul 3, 2015)

weimedog said:


> Some times these threads advance the state of the art and sometimes these are fishing trips... From someone on the out side looking in, the 800 lbs gorilla in the room is compression and how you want to get there. Decking the cylinder or pop up or some combination of the two. Some times it feels like the "porting" really ends up being the art of managing the changes created when changing internal geometry to get more compression..... For example. Some one with a lathe and builds 200 plus psi saws decking cylinder's and correcting poor squish band surface conditions now has a bunch of induced issues to work around. The intake duration is increased , exhaust delayed, and a piston overlapping the transfers to the point that ridiculously low blow down numbers correct the reduced x-sectional area and turbulence at the cylinder intersection of the transfer because of the piston interference. The compression changes power characteristics in enough of a positive way to make it worth the other issues. Longer intake duration can be both a blessing or a curse depending on a bunch of other factors such as blow down, case volume, and intake tract characteristic. Delayed exhaust can also be blessing or a curse depending on other factors. Although one very smart guy in here likes that extra time in the power stroke. Another guy without the ability to deck a cylinder might use a tool to fix the lousy squish band and wants to do a pop up to get that compression hit. His porting numbers probably will look a little different. And yet another can make a two piece cylinder/ cylinder head system and has the best possible scenario relative to squish band, compression, and the piston relative to the transfer ports.... So to me I'd like the discussion of porting numbers to be predicated by what did you do to your cylinder?? then start the conversation....just an observation... just an old man.


A smart and wise old man at that who's also seen a lot  and tried everything


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## sawfun (Jul 3, 2015)

Well I dunno how relevant this is, but the full circle crank used in 101's seem to give less torque but more acceleration. I interpretate this as horsepower since it has to do with speed of acceleration. Big saws generally work at full throttle and don't generally need acceleration so much but DO NEED torque. Smaller saws change rpm more, especially limbers. Go karts also need to gain rpm on road courses. So stuffer or case volume may not effect total top end performance as much, but rather, getting to the top end quickly. It would seem two stoke intake duration behaves like increasing the cams duration on a four stroke. More horses due to a higher rpm band. Torque being the ability to do work, and horses the ability to do it in a timed manor.


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## drf255 (Jul 3, 2015)

weimedog said:


> Some times these threads advance the state of the art and sometimes these are fishing trips... From someone on the out side looking in, the 800 lbs gorilla in the room is compression and how you want to get there. Decking the cylinder or pop up or some combination of the two. Some times it feels like the "porting" really ends up being the art of managing the changes created when changing internal geometry to get more compression..... For example. Some one with a lathe and builds 200 plus psi saws decking cylinder's and correcting poor squish band surface conditions now has a bunch of induced issues to work around. The intake duration is increased , exhaust delayed, and a piston overlapping the transfers to the point that ridiculously low blow down numbers correct the reduced x-sectional area and turbulence at the cylinder intersection of the transfer because of the piston interference. The compression changes power characteristics in enough of a positive way to make it worth the other issues. Longer intake duration can be both a blessing or a curse depending on a bunch of other factors such as blow down, case volume, and intake tract characteristic. Delayed exhaust can also be blessing or a curse depending on other factors. Although one very smart guy in here likes that extra time in the power stroke. Another guy without the ability to deck a cylinder might use a tool to fix the lousy squish band and wants to do a pop up to get that compression hit. His porting numbers probably will look a little different. And yet another can make a two piece cylinder/ cylinder head system and has the best possible scenario relative to squish band, compression, and the piston relative to the transfer ports.... So to me I'd like the discussion of porting numbers to be predicated by what did you do to your cylinder?? then start the conversation....just an observation... just an old man.


FWIW, this is exactly how I'm seeing it too.

Everything acts in concert together, so one change may or may not be beneficial without changing another.

The other thing I'm struggling with a bit is the idea that the stroke is happening in one order at a time and making me see it differently than it really is. I may be very wrong here, but in my minds eye, I visualize flow as its occurring, and I see it happening more as a continuum than one singly defined event. At 15,000 RPM, that flow is just flowing, it ain't gonna stop 250 time a second. So changes, like minor intake closing geometry, will have more effect lower in the power band. Having a hard time explaining what I'm thinking here. 

In my now (extremely limited) experience, I'm seeing that the first matter of business is combustion chamber geometry and static compression. I find it interesting (and inexact) to cut the SB with a mandrel type cutter. It gives you a great visual idea of what the chamber looked like before and after the 90* cut. The flatter, wider and more uniform SB will provide a larger quench surface and push more charge into the chamber. The charge in the SB isn't burned in the part of combustion that provides power, so an excessively large squish volume will make any model lazier and effectively act like less CC than it is.

So now you have numbers that have changed and it necessitates other changes.

I'm about to experiment with raising an intake on an assembled saw, from 84 to 75. I'm not sure that I will be able to get the same wood to get the same cut to compare the mod. Not sure if seat of the pants saw feel would be enough to fairly evaluate the difference.

So what would everyone say about the order in which modifications improve saw performance?

I'd say #1 would be muffler opening, #2 compression, #3 ignition timing, #4 power stroke length, etc....

Interested in opinions.


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## weimedog (Jul 3, 2015)

Assuming we are talking about stock performance saws such as Stihl MS460s and Husqvarna 372 (old style vs. new EPA Regulation driven machines) and then working from there to build a better running work saw from that starting point. Compression/ combustion chamber / squish band is number 1. Getting all the available charge in to a controlled volume to burn and burn efficiently. That's my humble opinion

But even in something as straight forward as a premise like this...it depends. work saw vs, race saw. Pro level performance saw vs. home owner weaser as a starting point. Depends on the starting point & what the engineers build into the product for it's intended role. depends on what the intended end points is of the hobbyist / builder modifying the saw. Depends on how much money/effort is "worth" expending vs. the end result. My 440e.... muffler mod is the most bang per resource expended because of its clam shell design, my 372's....compression/squish band.

For me a fun challenge is what can you do with simple tools in your typical garage as an extension of the saw hobby, and not about having the fastest saw on the block, just a better one with smart targeted changes. Not about what you can buy from someone else but where your understanding of physics and mechanics equals a better saw and there for an expression of your interest. Instant gratification is feeling the improvement and knowing "I did that". And the real attraction is being in the woods. The saw is just another reason to spend time out there. For others its having the fastest at a GTG on a given log or cant on a given day. For others its raw racing. And yet other's its a way to enjoy their machine shop tools and skills....all depends

(Depending on your shop you can take up that .020/.030 slop in the typical squish band by...say...removing a base gasket, or decking the cylinder....or why not make a .020 stroker crank! Just depends on what you have for skills, tools, and time. That squish band mod.... lathe & single point tooling, arbor tool, or cut the damn combustion chamber off, weld bossed for head bolts and machine in a shape of your choice!)


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## Chris-PA (Jul 3, 2015)

weimedog said:


> For me a fun challenge is what can you do with simple tools in your typical garage as an extension of the saw hobby, and not about having the fastest saw on the block, just a better one with smart targeted changes. Not about what you can buy from someone else but where your understanding of physics and mechanics equals a better saw and there for an expression of your interest. Instant gratification is feeling the improvement and knowing "I did that". And the real attraction is being in the woods. The saw is just another reason to spend time out there.


Well said! There are types of mods I probably could have done, but I just haven't wanted to get into. I just enjoy experimenting with certain mods using the tools I have access to (at home and at work), but without making it too difficult. And I'm always impatient to get out and try it. 

I had a mod worked out and all ready to go, but found I used the wrong material and now must wait until Monday to do it right.


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## Rx7man (Jul 3, 2015)

Speaking for myself, I agree with weimedog... I like the challenge and knowing I did it myself.. if I went to a GTG and the saw was the slowest there, it would only mean I have things to learn.

All the numbers work together to make a good saw.. Since all the saws we're usually looking at are mass production saws, I think one of the first things to look at is getting rid of the evident flaws in them.. casting flash, rough spots, gasket matching before we even look at the numbers.. after that you can do some reshaping of the ports, widening, beveling... and only after that do you have to look at the numbers.. of course I think we all do all these things at once!

I think on my next husky 65 build, I will make a saw with completely stock numbers, but just work on finessing all the things the factory didn't and run it as a comparison.. would be quite interesting I think.


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## redtractor (Jul 4, 2015)

I'm following these very technical and numbers heavy threads a little better each time but can't figure out what you're calling "blowdown".


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## mdavlee (Jul 4, 2015)

redtractor said:


> I'm following these very technical and numbers heavy threads a little better each time but can't figure out what you're calling "blowdown".


Degrees between exhaust and transfer opening.


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## jonsered 535 (Jul 4, 2015)

I'm new and not really 100 percent but I think it's your port timing minus your transfer timing example if you're exhaust timing is say 102 minus your transfer timing is 120 your blow down is 18 . correct me if I'm wrong here guys I'm just learning too


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## Rx7man (Jul 4, 2015)

Mdavelee is of course right, it's the amount of time in degrees the pressure from combustion can 'blow down' before another part of the cycle (transfer ports opening) happens.. I still don't have a grasp on how it affects the powerband of the saw


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## Rx7man (Jul 4, 2015)

What I'd like someone to explain to me is how blowdown timing affects power... 

Just going from intuition, I'd think that long blowdowns due to low transfers would make a saw that might not be very rev happy due to the lack of time to transfer the mix to the top end, but I'd like to know more about this.. my saws have about 21*, which is probably more than necessary, but that's because I raised the exhaust and lowered the jug, and don't really have the tooling needed to raise the transfers


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## Terry Syd (Jul 5, 2015)

Here's a quick and dirty way to mod your saw. If you want to raise the powerband say 10% (from 10,000 rpm to 11,000 rpm) then you need to have 10% more transfer time/area. (Transfer time/area determines where the maximum delivery ratio from the crankcase occurs, roughly peak torque)

The formula for a 10% increase in time (by raising the transfers) is rather complicated (rod angle and all). However, if you simply WIDEN the transfer ports 10%, that will increase the time/area by 10%.

ALSO, is you widen instead of raising the transfers you retain the 'swept volume' of the piston. - Just because the saw may have 60cc from BDC to TDC, doesn't mean the piston is actually sucking in that volume of mixture. The piston can only start sucking the mixture in when the TRANSFERS CLOSE. So when you raise the transfers you actually reduce the 'displacement' of the engine. The actual 'displacement' is from transfer closing to TDC.

After widening the transfers you need to sort out the blowdown. Start with the stock blowdown and see how the engine is performing. Then take a couple of degrees off the front of the piston at the exhaust port and try it again. You can keep nipping a bit off the edge of the piston until you GO TOO FAR. After you go too far, you can go back to your best exhaust timing and cut that into the jug. Then you install a new piston and ring/s in your saw and you KNOW your blowdown is correct.


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## Rx7man (Jul 5, 2015)

OK... that's some good information, thanks.. I have a spreadsheet with all the rod angle calculations, etc on it, so I can figure out what 1mm in transfer port height comes to in degrees duration, etc and figure the time from there... I think my saws would benefit from getting a little more transfer time (21* right now), especially considering I've already ported them to respond better in the top end... I've nipped off the top of the piston by the transfers for now, otherwise I'd have about 24*... Once I figure out how to get the tooling into the jug to do it and to do it neatly and symmetrically, I think the saw will run better.


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## 056 kid (Jul 5, 2015)

I just had a 660 done. All of the changes, (besides widening intake & exhaust) where timing, and not via cutting the base so no rais in comp. The saw didnt gain any rpm, but it gained much torque. I don't know what the #s are or where the metal was taken from exactly. The builder has been at it for over 30 years...


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## smokey7 (Jul 6, 2015)

Has anyone seen or done this? Remove the tranfer divider completely? They are split at the top the front is pointing strait at each other. The rear is aimed at the intake. It pulls good high revs in cut and tons of torque. Im told transfers facing eachother is a torque thing towards intake and raising them is a rpm rasing mod and scavanging thing. I wonder if that what the reason for the dual upper is for best of both. Buy why remove the divider in the transfer?


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## Cj genever (Nov 8, 2015)

Popcorn


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## jmssaws (Nov 8, 2015)

I think 20 deg of blow down on a 660 is to much, I prefer 17-18.
I use to always use 20 deg but I was leaving power on the table.


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## K-techcowboy (Dec 24, 2021)

Mastermind said:


> I've been around here for a little while now and have noticed something. A lot of members (myself included) ask questions about port timing and they often go unanswered.
> 
> Is this because the modders here want to keep the things they have learned (through trial and error no doubt) to themselves. If that's the case who could blame them. I for one am a little apprehensive to alter port timing on an expensive jug without knowing what these changes are going to accomplish.
> 
> ...


I have a secret thing I bet no one else is doing actually there's a few things I'm surprised aren't brought up more often starting with lighting the piston by taking unless material out of the inside. Then there's knife edging the crank weights but that requires a rebalance which can be a big pain sometimes. What about homemade pistons and coatings, ceramic bearings ext. Any who if you actually see this respond and ill share a neat thing I do to custom shape part of the cylinder to anything you can imagine.


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