The myth of high compression in 2-strokes

[Philbert]

I am wondering if certain types of 'tweaking' help for certain types of cutting, and maybe not so much for others.

It is one thing to try and circumvent imposed environmental limitations, and another, to try and out-think the engineers in Stuttgart and Sweden.

Longevity of a saw, and forgiveness in tuning variation, are also attractive attributes to me.

Philbert

 

[trappermike]

I found in building hot motorcycle 2-strokes that if you have fairly radical port timing substantial low-speed and midrange torque suffered a lot,but by bumping the stock compression from 145 psi to 165 the motor got back an appreciable amount of that lost torque,however the motor became very unwilling to rev like it did before,and at 135 psi the engines wanted to rev forever...
In small cc competition saws(70cc or less) with their lack of torque I would want to keep compression on the high side,160 to 170+.

 

[Leafy]

On my 77cc 372 with the 6cc combustion chamber it doesn't have hard time reving over 13k and I didn't make the port timing so that aggressive. I don't have my notes on hand but it was like E95, T120, i80

 

[trappermike]

So what was your exhaust duration and compression in psi? If it still uses the stock type piston I don't believe it makes enough actual cylinder pressure if your compression is high to reduce peak rpm power. But it all depends on what your compression is in psi. If you have a stock type piston without a dome and you've modified the ex. port,then your comp. is not high and it should rev well.

 

[Leafy]

Duration would be 190. It's not domed but I had to take a crazy amount off the squish band of the Chinese cylinder to clean it up, so much so the 1mm taller 272 piston fit.

 

[trappermike]

Well 190 degrees is a fair number for your size saw,without a domed piston compression wouldn't be too high,so the motor should rev pretty good,nice balance.It should work well.

 

[Leafy]

Sorry I meant 170. 95 atdc means 85 * 2 for duration.

 

[trappermike]

Frankly,squish gap does not mean more to me than duration or compression,it is the final setting after port timing and compression have been achieved.

 

[trappermike]

It must have decent torque at 170 degrees,190 degrees will lose a lot of bottom end. I've seen so many race 70cc saws with too much ex. duration or too big a carb just bog down hopelessly in a cut,focus on midrange torque is the key in those motors.

 

[Leafy]

My squish gap is 24 thou, but I had to take 40 thou off the area to clean it up so my combustion chamber (including squish area) went from 13cc (on the stock saw with 40 thou of squish) to 6cc. Like we're talking 9:1 static compression on a 2 stroke, everything I see says to not make the higher than like 6.5:1 but I ended up here to just make crappily machined chinese parts work. It seems fine, but I think I will consider raising the exhaust and transfers, if only to make the ******* easier to start, you gotta reef on it even with the decomp.

 

[trappermike]

On your saw I would reduce the squish to .020",then maybe go to 176 degrees duration,and do not increase intake duration. I would use a tillotson HS or Walbro WJ for a carb,if it's for competition. Skid the stock intake system and install the entire intake and carb from a 285/390,use my piston and you will really make competitive power.

 

[trappermike]

Well I used to have to start an 090 with 190 psi compression without a decompressor in a cold start competition,you know if you don't start it first pull you've lost...

 

[trappermike]

I would also use the 385/390 carb and intake on a modded work saw if I could,for competition I would bore the carb larger. It would be perfect especially a BB 372.

 

[Leafy]

Is the 390 carb the next body size above HDA? And the 390 intake lets any of that bigger size fit? If so I'd probably go to the biggest of that size range and just swap in the 390 carbshafts. If not the 372 carb I have now already has a 16.5mm venturi and the C3 and HDA sized carbs top out at 17mm.

 

[trappermike]

The HDA size carbs are just absolutely puny for a 372,they will hinder power throughout the rpms. I used to bore them but they are so small it was really futile. Just bolt on the entire 385/390 carb and intake,worth every penny. No matter what you try to do to a 372,if it has the stock carb it will be choked and never really make much more power. The HS carb from the 385 is much better in size(but not best) the Walbro WJ is better,for a modded saw they are good,for more power I would bore them. For a modded work saw I would bore them bigger and not even worry about less low speed power,they will prove great.

 

[trappermike]

I've seen people in competition try to run the big tilltson HT on 70cc hotsaws and they are too big,as soon as the revs drop slightly they bog bigtime and die in the cut,that is an example of a carb too big for the cc's.

 

[Michaelvincentkotowski87]

Im learning alot reading ur guys info...still im far behind. I like the idea of making a saw have or produce more power..so is the easiest way finding a bigger carb...i got , A few 3.7 mcculloch 610 and 3.4 605 plus a couple more pro 10 10..i bought all run down and out of shape...my buddy laughs at my love for these...cause im always working on them..i need rings on this one or carbs on others. Not to mention i just got my first saw 6 or 7 months ago...and im self teaching...id like to make one of these saws to shred his 029...i know sthil is the best but i bet mcculloch was great too. Oh yeah i bought my first stihl 051...i know i should not have a saw like this yet...but u know cant help myself...ill try to be careful...

 

[trappermike]

051 had a long stroke and a lot of torque for it's size,but vibrated itself to pieces too if not a lot of locktite on the screws.

 

[Ted Jenkins]

I found that compression is one thing, but timing and compression is another. Since saws do not have tuned exhaust some of the tuning principles can be thrown out the windows. On certain motors that did not have very high compression needed very high octane because the motor at certain RPM ranges developed very high compression. Saws not so much with there scavenging or lack there of porting. So porting duration and timing that is in balance can be a thing of beauty. On certain motors advancing a few degrees killed RPM and horse power, but brought a lot of torque. Retard the timing and some motors actually come alive with a wide range of available RPM and horse power. Thanks

 

[trappermike]

Please remember,I don't advise altering ignition timing until you really know where it is BTDC stock with a timing light before you start. In a high output motor 1 or 2 degrees too much advance can melt a motor. Don't alter timing blindly... Be professional,document what you are doing,make timing marks and use a strobe light.