The myth of high compression in 2-strokes

[trappermike]

I want to put to bed the myth of high compression in chainsaws,having worked on 2 stroke racing engines since the 70's I want to lay some straight facts down. High compression was first made popular during the 60's with big v-8 racing engines(4-stroke) and compression ratios went up to 13-1 or more. But the best 2-stroke racing engines actually reduced the compression ratio from the stock engines. Why,because the more the compression ratio the more horsepower it takes from the engine to crank the engine against that high compression,at some rpm the HP taken to crank that high compression takes more HP than it makes,and then the engine starts to lose HP. Thats why a factory motor making 60 HP at 8.5-1 compression,makes 100HP at 7.5-1 comp. ratio.
Now higher compression does make more power at low and mid range power for sure,but not many saws run in that range. so if you want more low end or mid range power compression is good,but if you need high rpm race power high compression will simply cost you Hp. As my instructor told me in 1975 "high compression fights high rpms",he was right.
So more more compression can help you,but not at high rpm's.
When you yank the cyl. gasket out of a saw you do gain some compression,BUT you also lower the exhaust and transfer ports down,reducing their duration,and reducing higher rpm power. I would much prefer to raise ports than compression for power,much more can be gained.
In many engines I would much prefer to raise the exhaust port and lose compression,I know I will make much more top end power.
Removing the cyl. gasket and lowering the transfer ports is really bad,you reduce their duration and reduce their open time from tens of thousanths of a second to even less!
I checked a new Poulan 3400 compression and was surprised to see it was barely 110 psi,so yes I yanked the cyl. gasket and it was better.
So in my mind port timing is far more important than high compression. All 2-stroke race engine designers agree,port timing makes big power,not high compression.

 

[wcorey]

What amount of comp, in psi, do you consider as ideal, as well as too much, in a chainsaw engine?

 

[trappermike]

Well it depends on what your motor is for. Since chainsaws mostly run at high rpms,bottom end or mid-range power is not much of a consideration,I would say (roughly) 130-145 for a race engine,150=160 for a work engine wanting good realistic power.
Go for better port timing,not high compression.
Of course you can run more compression,I wish I had a formula to show how much HP it takes for comp.at a certain rpm etc.,but it doesn't exist. Lotsa people are gonna call me wrong,I can't wait...
I would go for as much ex. duration as I can while still maintaining 150-160 compression. That's the key.
If you go over 160-170 midrange power will increase but higher rpm will suffer.It may be a good woods saw but won't win races.

 

[trappermike]

It is simple math that no BS can change,as rpms increase horsepower taken by cranking against compression loses power at a given rpm.

 

[buzz sawyer]

Just found this thread - very interesting! So how does widening the ports affect power? Does widening them effectively increase duration?

 

[Huskybill]

I measured the husky 2100 race saw we had. The exhaust port was widened, raised and polished. The transfer port ribs were a knife edge, the intake port was lowered.
The cut some off the bottom skirt of the piston too.

 

[Leafy]

I believe how you change compression is what matters. Using 4 stroke terms you have dynamic and static compression. Moving the height of the exhaust port only changes what I'm calling dynamic compression, reducing the volume of the combustion chamber at TDC changes both static and dynamic. Increasing compression by moving the exhaust port down increases dynamic compression only and will increase the octane requirement on the motor and lower the peak power rpm due to the increased pumping loss at low rpm. Reducing the size of the combustion chamber will also increase the octane requirement but only because of the increase of dynamic compression it will also increase power across the whole rpm range.

Ideally you would determine the dynamic compression that your fuel can withstand without detonating with your spark advance at mbt, choose your exhaust timing for the type of powerband you want and then size the combustion chamber to get that dynamic compression.

 

[trappermike]

Widening a port increases the area of a port which will increase power,without affecting the duration of a port which can be advantageous sometimes. You can widen a port to max. width for competition when you are not concerned with engine life,for a work saw you must not widen an ex. or intake too far or rapid piston and ring wear may occur. Widen ports too far and engine destruction will occur.

 

[trappermike]

So as always I hear the same myth over and over...Yank out the cylinder gasket and get more compression. A cyl. gasket is usually around .5 mm thick,pull it out and dispose of it and you gained 20-30 psi compression. But you have also lowered the ex. port .5 mm which will cause a loss of high speed power,also you have lowered the transfer ports,and .5 mm is a big power loss to them,so you may have increased low speed power,but definitely lost high rpm power,which most chainsaws run at. Port timing in a 2-stroke definitely makes more power than compression. I have seen some saws with stock compression of 100-110 psi,of course improving that will help All internal combustion gasoline engines make more power when more fuel and air are put into them,(bigger carb,turbocharging,supercharging),and that's what all race engine designers try to do. Anyone who makes big horsepower claims to me with a tiny stock carb an a saw makes me smile,no matter what mods you do to a motor-compression,port timing,exhaust etc,.if you are not getting anymore fuel and air INTO the motor you wil not make much more power.
Now for all the so-called experts who will refute me (because they are afraid of carbs) let us compare experience and training-
I have more than a dozen certificates from training since 1974,and have studied and built 2-stroke performance engines since then. You know more? -Show me your credentials. The fact is I'm retired and not trying to make money off of anyone,but anyone who insists that a too-small stock carb can make more power is not correct,no one in the rest of the 2-stroke world would believe it," I find a 17mm carb to be best power on a 95cc motor" and you would be laughed out of the room...
It's too bad there is not a carb between the size of the WJ and the HT,thats what the saw world really needs.

 

[Little Al]

Can I ask if you are referring to compression pressure or compression ratio as a good 7/1will have more pressure than a worn 9/1 job Iv'e always understood the squish band is a contributing factor so if I have you right you maintain the higher the compression pressure Ie a 140 psi will rev better than a motor reading 180psi if all else is left as is I'm not arguing /knocking tying to understand your thinking Agree with you on the carb size thoughts what are your thoughts on more % age of oil in the mix with regard to obtaining more power ?

 

[Ketchup]

Here's a hypothetical question to help me understand.
Let's say you have two stihl 066's with succeeding serial numbers (same batch of production).
Saw 1 has stock timing and compression, (155 psi), but exhaust and Intake ports have been widened to 65%.
Saw 2 has extended durations in Exhaust, Transfer and Intake, and has an increased compression (190 psi), but the ports are left stock width.
Which one wins a race?

 

[Leafy]

So as always I hear the same myth over and over...Yank out the cylinder gasket and get more compression. A cyl. gasket is usually around .5 mm thick,pull it out and dispose of it and you gained 20-30 psi compression. But you have also lowered the ex. port .5 mm which will cause a loss of high speed power,also you have lowered the transfer ports,and .5 mm is a big power loss to them,so you may have increased low speed power,but definitely lost high rpm power,which most chainsaws run at. Port timing in a 2-stroke definitely makes more power than compression. I have seen some saws with stock compression of 100-110 psi,of course improving that will help All internal combustion gasoline engines make more power when more fuel and air are put into them,(bigger carb,turbocharging,supercharging),and that's what all race engine designers try to do. Anyone who makes big horsepower claims to me with a tiny stock carb an a saw makes me smile,no matter what mods you do to a motor-compression,port timing,exhaust etc,.if you are not getting anymore fuel and air INTO the motor you wil not make much more power.
Now for all the so-called experts who will refute me (because they are afraid of carbs) let us compare experience and training-
I have more than a dozen certificates from training since 1974,and have studied and built 2-stroke performance engines since then. You know more? -Show me your credentials. The fact is I'm retired and not trying to make money off of anyone,but anyone who insists that a too-small stock carb can make more power is not correct,no one in the rest of the 2-stroke world would believe it," I find a 17mm carb to be best power on a 95cc motor" and you would be laughed out of the room...
It's too bad there is not a carb between the size of the WJ and the HT,thats what the saw world really needs.

I don't understand the tiny carb thing either. A 17mm carb is a good size for a 62cc for like 9-10k peak power. Heck I put a 16.5mm carb on a 52cc saw with no draw backs. The only time it seems hard to go up in carb size is when you're already using the largest carb with that bolt spacing like if you have an HDA or c3 carb, next size up would require a custom intake because the bolt spacing is different But putting an HDA where a wt was is pretty easy.

 

[Leafy]

I guess I get to test this out. The huztl 372 big bore cylinder was very screwed up in the squish area after cutting it to be flat the 272 piston had 24 thou of squish with the base gasket installed. I cced the head after and it was 6ml, add in the 1.2ml from the squish area gets 7.2 on a 77cc saw. I'm glad it has a compression release. Exhaust timing is at 98 without porting, hopefully that's early enough that it won't knock on 93 with 40:1 mix.

 

[rngrchad]

I like what your saying trappermike. Right on.

 

[trappermike]

There is obviously a balance between CR,carb size etc.port timing when getting into high-output 2-stroke technology,but what I know is that no really high output 2-stroke has high-compression,at least not more than 160 psi generally.

 

[trappermike]

Now there is a good case for high compression in 2-strokes,that's in low to mid range motors (like a dirt or trials bike),it will help a lot,but not in a high speed 2-stroke.

 

[trappermike]

Squish gap does not mean a lot to me,port timing and compression do,I like to tweak squish gap to about .020" in the end when I get more important things right.
Wheras someone might yank the cyl. gasket out of their saw,I may actually like to add one and gain port duration,if original compression was ok.

 

[Philbert]

I appreciate your perspective; I can follow the build threads, but really don't understand the technical implications of each mod. How do you account for the reported performance gains of guys who do the compression mods you mention? Are these limited to certain types of cutting?

Thanks.

Philbert

 

[trappermike]

Compression mods can help in some circumstances,but fall short of other correctly done mods for more power.Compression mods will only gain low to midrange power,which may benefit some smaller cc saws, to prevent loss of low speed power.

 

[Beetlejuice]

Compression mods can help in some circumstances,but fall short of other correctly done mods for more power.Compression mods will only gain low to midrange power,which may benefit some smaller cc saws, to prevent loss of low speed power.

I always wondered why my old MACs liked to growl when eating big timber.. Seems like throttle wide open runs and cuts best in mid to upper mid range..curious if the porting was designed that way, or got lucky.. And if a little tweaking would help or hinder an old 80 cc saw