The myth of high compression in 2-strokes

[trappermike]

In fact I have never heard anyone tell me what the stock timing is in any saw,they have never checked it. No one in the rest of the performance world(cars,motorcycles,etc.) does not know where stock timing is or what it is when they alter it.

 

[trappermike]

For instance when I worked with bigblock Chrysler engines ,I knew the timing should be 10 degrees BTDC at idle,advancing to 20 degrees at 2000 rpm and be at 38 degrees with full vacuum advance,which I confirmed with a timing light. No guesswork will do.
When you are dealing with a stock low output chainsaw timing isn't too critical,but as you raise performance it's best to know where it is.

 

[SmellyPirateHooker]

Hmm. Everything I've ever bumped the compression up on (2 stroke) showed gains everywhere. Every chainsaw, even thinking back to my 91'kx500 bike. So I guess maybe if we are talking way over the comps I'm running or rpms.

 

[trappermike]

What I have been saying is that increasing compression can give a nice boost in low and mid range power,but when looking for more top end power compression cannot be too high because at some rpm the extra compression will start taking more power(from the hp taken by cranking against comp.) than the extra comp. is making,and from that rpm and on the motor is losing power.
Of course it all depends on how much you are increasing comp. and what the stock engine has.
I myself like to have 150-180 in a work saw and 140 -160 in a competition saw.
High compression got it's fame from the older V-8 race engines that had comp. as high as 14-1,but 4-strokes only make compression every 2 revolutions of the crank compared to 2-strokes which make comp. every revolution of the crank,so 4-strokes only use half the power cranking against compression, at any rpm.
If you take any very high output factory racing 2-stroke and bumped it's compression up it would lose a considerable amount of top speed power and probably cook the piston.
In chainsaws that are not highly modified extra compression will help power,but in competition engines you don't want to sacrifice too much high rpm power to the compression Gods...

 

[trappermike]

In the days when the Japanese actually listed the comp. ratios of their engines,street to factory race engines listed the comp. ratio from 6.7 to 7.5 to 1 -"Actual" compression. For a slower speed engine bump it up,for a high speed motor keep it down.

 

[Ted Jenkins]

Trapper very well said. Thanks

 

[trappermike]

Well I have been documenting 2-stroke specs since 1972 when they used to publish data,now they don't. I know those ratios sound incredibly low,but that's what factory race engines use. I know I'm not smarter than Yamaha,Suzuki and Kawasaki engineers and all out racers.
Thanks Ted,I'm glad someone believes me.

 

[trappermike]

Believe or not,6.7-1 was the race engine,7.5-1 was the street engine!

 

[trappermike]

Long ago I made a mistake and my hotsaw engine had 220 psi compression. The ignition broke down and misfired badly about 12,000 rpm. I reconfigured it to be about 170 psi and then it ran perfectly to 14,500 plus.

 

[logger450]

Long ago I made a mistake and my hotsaw engine had 220 psi compression. The ignition broke down and misfired badly about 12,000 rpm. I reconfigured it to be about 170 psi and then it ran perfectly to 14,500 plus.

Trappermike Big Woods had a discussion about his 372s. He would take a 268 piston and machine it down to make a popup. The piston skirt is already shorter than the stock 372 piston and actually a lot lighter. The shorter skirt would obviously give it a bigger intake duration. He would keep the squish at .020-.024 and the compression no higher than 180. He would use the base gasket so the exhaust timing would stay the same. I have bought a few modded saws where the exhaust port is raised up and makes for a fast saw but loses much needed torque when your in big hardwoods. He also suggested putting a 385-390 carb on. I'm not interested in cookie cutters for my self just strong pulling saws. Although I do like watching saws fall right thru the wood at the meets. Everybody is looking for something different and that's ok. I do like reading your posts and getting what I can out of them. Thanks

 

[trappermike]

As for the 372 piston mod I was the one who invented it back in 1998,see my thread-"372 piston swap/mod" in this section. It is true smaller cc saws will suffer more loss of low to mid power if port duration is increased much.
If you want strong midrange I would like 165-170 comp.,any more will loose some high rpm power.
The 385/390 carb is a good mod,however use the entire 385/390 intake from carb boot to choke horn,it's well worth it.

 

[Huskybill]

I used my race setup in my 2100 cutting in the woods. I used a 16” bar, the rakers filed down to .040”, a 8th tooth rim driver, plug the govenor in the carb, I squared the bar rails, then chamfered the inside and outside corners on the bar rails. Move the timing a hair more advanced. . She turns into a beast.

On the 2100 they had apart they enlarged the ports, changed the port timing.

Even plugging the govenor on the 266se/xp & the early 240sg when they were all metal makes a difference. I like the older saws.

 

[mixxer]

I don't have time to type out the details on my phone keypad right now , but there is a crucial component completely missing in the conversation about compression....

Dynamic compression takes into account the actual pressures of a captured and compressed charge...

This can and will be a different amount based on the efficiency of actual delivered and trapped charge at functioning rpm ranges. which the rpm speeds while pull starting to check psi doesn't emulate in any real fashion....

For example.... Let's say your setup isn't delivering a lot of efficient charge volume under an operating rpm band... And for illustration we will exaggerate... If you have no trapped charge, it doesn't matter how high your static compression ratio measures at.. 7:1... 9.1... if there is no charge , squeezing nothing is still going to produce no psi pressure.... Next up... If the engine IS volumetrically efficient at this point and a LOT of charge is being trapped in the combustion chamber... You will see a LOT of pressure in real operating conditions....

Pull starting speeds are nice for reference changes in psi , but it isn't telling you how things are happening under dynsmic cylinder fill high rpm conditions... Pull start speeds give plenty of time for intake, transfer, and exhausting of an un burnt charge... Running conditions are entirely different...

One engine might be filling with charge, and another may not doing as well under operating conditions...
Carb size.. intake duration... Transfer efficiency... Exhaust efficiency... All that and more come into play while running an engine...

One that is less volumetrically efficient may indeed benefit from more static compression ratio while running as is compensates somewhat for less trapped volume... Another may be quite efficient with cylinder charge fill and find that a lot of static compression ratio is making for too much running psi. The multiple compression events of high pressure actually adding a "load" to the engine...

Now... Adding a tuned exhaust system... An expansion chamber/ Sonic pulse charger style exhaust changes EVERYTHING... And that's an understatement...

It does a similar thing to a 2 stroke that a turbocharger does to a 4 stroke.... Higher static comp is not so much wanted or needed because pressure is now more an end result of how MUCH charge is being squeezed.. actual pressures are no longer as dependant on static ratios , but are greatly affected by the much bigger charge being compressed.... Put a bigger dome on a 4 stroke piston and at bdc you are taking up and wasting space with aluminum ...that could be extra open space for the turbo to cram more charge into...

The expansion chamber works from the other end of the engine... And uses the Sonic pulses of the exhaust to create vacuum... Enough to pull down the pressure of not only the cylinder, but to actually reach down through the transfer ports and evacuate/ pull charge up from the crankcase...

It actually pulls fresh charge into the exhaust system headpipe area and when the Sonic pulse meets the converging cone, it acts like sound hitting a wall and reflects back an echo pressure wave... The magic of the exhaust tuning of the 2 stroke is to time this to push the escaped fresh charge back into the cylinder just as the piston is closing off the exhaust port in it's way up... Massive cylinder fill... turbocharger in reverse.. higher pressure via cylinder fill and not from static high comp/ small combustion chamber volume... Too small would induce a compressive load and negatively effect top rpm run out potential...

Also why when you are using are using an expansion chamber exhaust you are not concerned with small crankcase volumes pushing charge through transfer ports, but since the exhaust system doing the vacuuming work, a bigger crankcase volume can be used to move more charge ....

The whole engine is one dynamic/ harmonic system... Static measurements are a window of information to track with... But it is far from the whole story of running conditions...

 

[Thomas Lilli]

So, what octane fuel should you run in a tuned saw? is There any reason not to run higher octane fuels on stock modern pro saws?

 

[buzz sawyer]

So, what octane fuel should you run in a tuned saw? is There any reason not to run higher octane fuels on stock modern pro saws?

Use what the mfg recommends - most say 89 ron. Rec 90 is good. The higher the octane, the slower it burns - to prevent pre-ignition.

 

[Bobby Kirbos]

@trappermike
I just did the top end on a 660 - 0.5mm base gasket, used OEM jug, Hyway pop-up piston. What are your thoughts on this piston (increased compression without fkng with port timing)?

Freshly installed (lightly oiled and before 1st start), it pulled 140psi on my gauge. I have 4 tanks through it but have not remeasured the compression yet.

 

[Thomas Lilli]

Use what the mfg recommends - most say 89 ron. Rec 90 is good. The higher the octane, the slower it burns - to prevent pre-ignition.

How about a tuned saw? Also, if you run Something a 110 octane in a stock saw, is there any draw back? I understand the function of octane in 4 stroke engines.

 

[MacAttack]

This is a great thread! I remember once reading about (Maico?) 2 stroke dirtbikes from the 1970's that had insane compression and wondering what they were thinking? I mean like 12:1!

When I was more into snowmobile performance stuff, I remember the thunderdome heads were a big thing to up compression.

 

[Ted Jenkins]

Short answer NO it does not matter what octane one uses for two stroke chain saws motors. On two stroke race engines they start out at relatively low compression because as RPM increases the efficiency increases. With tuned exhaust and well tuned porting the compression increases dramatically as RPM increases because of the supercharging effect. On chain saw motors which is at the other end of spectrum the compression stays much more constant so high compression most often does not become a factor. When one deletes the base gasket the compresion jumps a very small amount and stays there. Even though a chain saw motor runs from 10,000 RPM to 15,000 RPM the compression does not change much. At higher RPM less octane is needed because the fuel is ignited while staying in the combustion for a shorter period of time thus long burning time is not needed as much. At ellevations near sea level on cool days is when octanes are most likely to become a factor in chain saws. Now if a saw is modified with porting and muffler mods then the efficiency could be come a factor under some conditions. In most cases when one runs 87 octane at 1,000 feet above sea level on warm days octane would not be an issue. With modified saws being used near sea level 90 octane will provide all necessary requirements. If one suspects that they have preignition conditions all they have to do is take the key out and file it a bit to see if the vibration goes away. Thanks

 

[Huskybill]

My husky dealer always told me to use high test gas in all my power equipment wether it’s a lawn mower to chainsaws. I never had one problem. But I always use seafoam when I store equipment and use husky 2 t oil