The myth of high compression in 2-strokes

[Dennbb]

It is not the pumping losses that limit power. Second, increasing compression does not reduce power...within the bounds of keeping the ignition advance due to detonation. Some of y'all are rolling fuel octane, and durability into this, and a few have even mentioned heat transfer out of the piston...
cheers,
Douglas

Increasing compression in a 2 stroke limits RPM.

 

[Slotracer577]

Many things interrelate, generally more compression is more hp, but it comes at the cost of heat and increased likelihood of detonation. It also tends to decrease power above peak rpm, ie it signs off harder. Your fuel quality will be one of the major limits. Also squish velocity will be a limit as well. Squish velocity is a function of squish band width and height, if too high you can make the squish band narrower. This was a common change to 70’s and 80’s motocross engines which tended to have wide squish bands which lead to bad detonation. Fix is to make it narrower.
squish height tends to get lumped in with increasing compression. IMHO squish height is more important than the actual compression. It should be set so that the piston has just .001-.005” clearance when running. This pushes any mixture into the center of the combustion chamber and increases turbulence in the combustion chamber. This increases flame speed allowing less timing to be used and gives a more complete burn both increasing efficiency. really a gain with no down side. I think the only reason it’s not done from the factory is it takes too much time on assembly to get it right. Test after test show this helps any engine.

 

[Maintenance supervisor]

Increasing fuel limits rpm in a 2 stroke.

 

[Termite52]

There are systems to prevent the problems you describe. For chain saw use I mix about two half gallon clear bottles for an average day. almost all the time I will use one bottle up in two days. If the second bottle sits around more than a few days then I just dump it. Its the easiest cheapest thing to do. If I have a heavy week of cutting coming I will mix several bottles of mixed gas to start out the day. Clear bottles tell you if there is contaminates in the fuel or if moisture is present. For other equipment the situation is different. Here three out of every household here has a generator, at least half are programmed to start up every few days and run for fifteen minutes. For all others a petcock is installed with a drain hose. About once every week or two the owner starts up generator and runs it for fifteen minutes. The owner never lets the fuel go below a half a tank. After the fuel is three months old fuel is drained with new fuel added. Here we have fifty outages a year with the longest being about three weeks at a time. Then we will go five years with only one or two outages knowing the loss of power will happen. Thanks

If you use a top grade gas like Sunoco gtx 260 you don’t have to drain the tank every three months. It comes from the factory with a preservative in it for storage. Same for small engines.

 

[weimedog]

Many things interrelate, .... This was a common change to 70’s and 80’s motocross engines which tended to have wide squish bands which lead to bad detonation. Fix is to make it narrower.
squish height tends to get lumped in with increasing compression. IMHO squish height is more important than the actual compression. It should be set so that the piston has just .001-.005” clearance when running. This pushes any mixture into the center of the combustion chamber and increases turbulence in the combustion chamber. This increases flame speed allowing less timing to be used and gives a more complete burn both increasing efficiency. really a gain with no down side. I think the only reason it’s not done from the factory is it takes too much time on assembly to get it right. Test after test show this helps any engine.

Yup, an important concept in this post. I hate generalizations as like all things, they often are miss leading and focus on trying to make some point somewhat not related. As with all the physical characteristics of a motor, there is a blend, a combination of "sweet spots' based on a design & desired characteristic and as a result there is a concept of "diminishing returns" on all those things as well. Squish bands and moving the mix to the combustion chamber is not a trivial thing and is often the goal of reducing the squish number. TO Compression, it too can be described with math and effects efficiency, but there is a point of diminishing returns based on a whole host of other issues. Conversely if compression didn't matter...50psi would be fine in your hot saw. ALL the factors of these motors working in concert define the power characteristics. From primary compression to port timing, from air box to muffler. X-sectional area of the ports. All of it. Not many are old enough to have experienced the curse of the old "open" class bikes from the 70's and 80's. Especially the air cooled ones.. Just got rid of my last , a 1979 KTM 420 MC80

 

[Ted Jenkins]

Yup, an important concept in this post. I hate generalizations as like all things, they often are miss leading and focus on trying to make some point somewhat not related. As with all the physical characteristics of a motor, there is a blend, a combination of "sweet spots' based on a design & desired characteristic and as a result there is a concept of "diminishing returns" on all those things as well. Squish bands and moving the mix to the combustion chamber is not a trivial thing and is often the goal of reducing the squish number. TO Compression, it too can be described with math and effects efficiency, but there is a point of diminishing returns based on a whole host of other issues. Conversely if compression didn't matter...50psi would be fine in your hot saw. ALL the factors of these motors working in concert define the power characteristics. From primary compression to port timing, from air box to muffler. X-sectional area of the ports. All of it. Not many are old enough to have experienced the curse of the old "open" class bikes from the 70's and 80's. Especially the air cooled ones.. Just got rid of my last , a 1979 KTM 420 MC80

Chain saws as we know them today are in their own class and maybe only compared to a leaf blower. They are low tech low performance low cost and contain little technology. Because of these issues raising the compression on many chain saw motors yield an improvement. On two stroke race motors such as dirt bike motors compression can harm or explode literally. The reasons for most part are that race motors with sophisticated tuning can not tolerate very high compression. As a typical two stroke race motor increases RPM several things start to change like the expansion chamber will push air back into the cylinder more and more causing higher compression in the combustion chamber. If the compression becomes too high detonation will be the first sign of problems then maybe burn the piston and so on. I think that some one with a sense of engineering could improve the exhaust systems on many chain saws, but why. Simple has worked well for many decades. Yes am quite familiar with open class dirt bikes as I started playing with them in the sixties and still have a few, but do not ride them with the same enthusiasm that I once did. Sailing at 10 to 15 knots seems quite fast enough. Thanks

 

[bwalker]

FWIW many modern dirtbike and snowmobile engines have more than 180psi static compression.

 

[Huskybill]

More compression puts more of a load on the crank pin, crankbearings, piston, wristpin. It may cut down on longevity. I think the stock compression is enough. Port it more. More easier flow equals free hp.

I took a ‘68/396/375hp 11:1 compression engine and installed the valve train in a ‘68/396/325hp 10:25 compression engine, ported the heads, matched the ports, advanced the timing, she had 400 hp at the rear wheels on the dyno. The porting matters.

 

[bwalker]

More compression puts more of a load on the crank pin, crankbearings, piston, wristpin. It may cut down on longevity. I think the stock compression is enough. Port it more. More easier flow equals free hp.

I took a ‘68/396/375hp 11:1 compression engine and installed the valve train in a ‘68/396/325hp 10:25 compression engine, ported the heads, matched the ports, advanced the timing, she had 400 hp at the rear wheels on the dyno. The porting matters.

Most of the gains from porting a saw come from the boost in compression. Hogging out the ports for "more flow" is often very counterproductive in a two stroke.

 

[Ted Jenkins]

Most of the gains from porting a saw come from the boost in compression. Hogging out the ports for "more flow" is often very counterproductive in a two stroke.

I am pretty sure that I am correct on this. Increasing two stroke flow can increase power at a dramatic difference. Increasing compression will also increase power dramatically. So why not increase ports by 200% and compression to 20:1 well any tuner will tell you it may not even run. There is a balance on what works well and what is too much. Often with chainsaw motors just a little will be just right but on race motors some time excessive is called for. Chain saws are not race motors needing excessive, Thanks

 

[bwalker]

I am pretty sure that I am correct on this. Increasing two stroke flow can increase power at a dramatic difference. Increasing compression will also increase power dramatically. So why not increase ports by 200% and compression to 20:1 well any tuner will tell you it may not even run. There is a balance on what works well and what is too much. Often with chainsaw motors just a little will be just right but on race motors some time excessive is called for. Chain saws are not race motors needing excessive, Thanks

Hogging out the ports kills velocity. You might flow more at a certain high RPM but the motor will have a very narrow power curve and that's not what you want in a saw. Excessive compression can hurt too via pumping losses, but the compression ratio of saws is such that all the compression you can through at them is beneficial.

 

[Ketchup]

Hogging out the ports kills velocity. You might flow more at a certain high RPM but the motor will have a very narrow power curve and that's not what you want in a saw. Excessive compression can hurt too via pumping losses, but the compression ratio of saws is such that all the compression you can through at them is beneficial.

Hogging out transfers can kill velocity but it seems less significant in larger displacement saws. Intake and exhaust can benefit quite a bit from enlargement. I see no gains in a chainsaw with compression over 225. I like around 190 in a work saw. Race saws probably want less to hit max rpm. 175 seems good.

Even porting a cookie cutter I would set a pretty tight squish to start with, then lower compression with exhaust height and width. It seems RPM is king in the larger race classes.

 

[bwalker]

Hogging out transfers can kill velocity but it seems less significant in larger displacement saws. Intake and exhaust can benefit quite a bit from enlargement. I see no gains in a chainsaw with compression over 225. I like around 190 in a work saw. Race saws probably want less to hit max rpm. 175 seems good.

Even porting a cookie cutter I would set a pretty tight squish to start with, then lower compression with exhaust height and width. It seems RPM is king in the larger race classes.

With respect to the intake my initial comment in regards to velocity stands. Thats why some builders are applying epoxy to the intake.
In fact hogging out the intake is probably the most detrimental of the three ports.
Also keep in mind that 225psi is still a pretty low compression ratio in a saw.

 

[Ketchup]

I haven’t seen it all, but typically the epoxy I see in intakes is to raise the floor. I agree you can make the intake too large. You can make any port too large. And you can grind in the wrong places. On an intake I mostly see widening at the cylinder wall and sometimes lowering of the intake floor. If swapping in a larger boot and carb, general enlargement may be called for.

As far as hogging, it increases volume, so you have to compensate for that. I don’t typically hog out any port, but I know good porters that do.


Curious to hear about saws that run above 225psi. I see very few of those, but I’m not a racer.

 

[bwalker]

I haven’t seen it all, but typically the epoxy I see in intakes is to raise the floor. I agree you can make the intake too large. You can make any port too large. And you can grind in the wrong places. On an intake I mostly see widening at the cylinder wall and sometimes lowering of the intake floor. If swapping in a larger boot and carb, general enlargement may be called for.

As far as hogging, it increases volume, so you have to compensate for that. I don’t typically hog out any port, but I know good porters that do.


Curious to hear about saws that run above 225psi. I see very few of those, but I’m not a racer.

Without cutting the head off and building one it might be hard to get there.

 

[Piotr Pakuła]

I noticed that the engine with low compression has problems with starting when it is cold and the second thing, when I put on the engine too large carburetor after a year of use, the crankshaft broke to my surprise, not the piston

 

[machv5]

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

120psi is the average compression for 2 stroke engines. They need higher compression to function properly. However keep in mind that on 2 strokes there is a compression release which lowers the starting compression to around 60psi. If they didn't do that then you would have a hellish time pull starting them. Most saws have a button to engage the release and if you don't engage it you will notice it's near impossible to pull start the engine.

 

[machv5]

I noticed that the engine with low compression has problems with starting when it is cold and the second thing, when I put on the engine too large carburetor after a year of use, the crankshaft broke to my surprise, not the piston

I recently bought a 33cc stand up scooter that ran the first day but it hasn't run since. I bought a new carb too. It will not start. I am going to try squirting a small amount of 10w30 into the cylinder to give it more compression in order to start it. I think the previous owner did something like that to start it before selling it to me. And the ********* blocked me on facebook when I tried to get them to be honest and tell me the trick to starting it if they wouldn't refund me the money and take it back.