Understanding port timing numbers

[AUSSIE1]

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.

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."

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.

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.

 

[ronT2]

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.

 

[AUSSIE1]

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.

 

[jonsered 535]

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.

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

 

[Rx7man]

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!

 

[Rx7man]

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!

 

[jonsered 535]

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.

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.

 

[MustangMike]

Surprising this thread has so few posts.

 

[Mastermind]

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.

 

[jonsered 535]

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

 

[Deets066]

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?

 

[weimedog]

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.

 

[jonsered 535]

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

 

[sawfun]

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.

 

[drf255]

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.

 

[weimedog]

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!)

 

[Chris-PA]

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.

 

[Rx7man]

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.

 

[redtractor]

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".

 

[mdavlee]

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.