Part Four: Compression

[Mastermind]

Now that we've covered the exhaust port.

http://opeforum.com/threads/part-one-the-exhaust-port.7279/

The intake port.

http://opeforum.com/threads/part-two-the-intake-port.7290/

And the transfer ports.

http://opeforum.com/threads/part-three-the-transfer-ports.7306/

Let's talk about compression.

How much is too much?

Pop-up pistons, or squish band mod?

Detonation?

Do we need to think about high octane fuel?

What about alcohol? Nitro Methane?

Dennis Cahoon told me once to make the compression match the fuel used. Thoughts on that?

Now is a good time to talk about lathes and tooling too.

 

[GCJenks204]

Glad I got in to this one before the conversation was over my head...

 

[Mastermind]

I've been fanatical about compression.......I'd say I've been obsessed with it. I had far more experience with small block Chevy engines than I did two-strokes when I first tried to build a saw engine for performance. In that world compression is a big deal.

In a single cylinder, air cooled two stroke engine though........compression can be a whole different thing.

 

[Mastermind]

I'm gonna be at my lathe for awhile........making compression. I'll keep checking in.

 

[GCJenks204]

I've been fanatical about compression.......

Says the guy that doesn't own a compression tester...

 

[Stump Shot]

One thing is for certain, without the squish band detonation would be certain with a high rate of compression. Pressure is used to measure, has anyone actually cc'd one of these engines to figure out the ratio of compression? I have my favorite target, but I know guys here that blow that away, and I'm boggled with how it's gotten away with.

 

[merc_man]

Subbed

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[Moparmyway]

I've heard that too much secondary compression is a bad thing, which I agree with. Where is the too much line ?

I've got 90cc saws (both Husky and Stihl) that are above 240# ......... and they happen to be the strongest saws I've got.

Might they be even stronger with less compression ? Possibly, but I'm too chicked to try finding out. I'd rather keep them as they are ......... for now

 

[merc_man]

What is too high for compression?

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[Sty57]

I've heard that too much secondary compression is a bad thing, which I agree with. Where is the too much line ?

I've got 90cc saws (both Husky and Stihl) that are above 240# ......... snd they happen to be the strongest saws I've got.

Might they be even stronger with less compression ? Possibly, but I'm too chicked to try finding out. I'd rather keep them as they are ......... for now

Could you have a dished piston turned to drop the compression easily with no other changes to the jug or port timing?

 

[merc_man]

For tooling. How snug of a fit are the mandrels for the lathe. Do you turn them down so they are a snug fit or a little loose. I got a friend with a lathe and would love to try turning the base down rather then delete the gasket. Be easyer for taking cylinder on and off.


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[Moparmyway]

Could you have a dished piston turned to drop the compression easily with no other changes to the jug or port timing?

I'd turn it muhself, but I sure could ............ I'm just wondering why would I want to if they're standouts now

 

[Fruecrue]

I like compression. To get more push going down, you have to pay the piper on the way up.

 

[paragonbuilder]

I'd turn it muhself, but I sure could ............ I'm just wondering why would I want to if they're standouts now

Worth it for the price of a piston imho. And if it likes it, it will probably live longer as well...


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[Mastermind]

Says the guy that doesn't own a compression tester...

I have one.......don't really trust it. They have been known to lie.

One thing is for certain, without the squish band detonation would be certain with a high rate of compression. Pressure is used to measure, has anyone actually cc'd one of these engines to figure out the ratio of compression? I have my favorite target, but I know guys here that blow that away, and I'm boggled with how it's gotten away with.

I've never checked actually ratio.

What is too high for compression?

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Good question.

Could you have a dished piston turned to drop the compression easily with no other changes to the jug or port timing?

Sure it could. How would that affect transfer flow?

For tooling. How snug of a fit are the mandrels for the lathe. Do you turn them down so they are a snug fit or a little loose. I got a friend with a lathe and would love to try turning the base down rather then delete the gasket. Be easyer for taking cylinder on and off.


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I like them snug, but no overly tight. A smooth slip fit works well enough.

 

[Moparmyway]

For tooling. How snug of a fit are the mandrels for the lathe. Do you turn them down so they are a snug fit or a little loose

I like it so when the spark plug is in the cylinder, and you drop it on the mandrel, the air in the cylinder slows down the cylinder so it doesnt drop right down ............ more or less, it floats down on the trapped (yet releasing) cushion of air

 

[Moparmyway]

Worth it for the price of a piston imho. And if it likes it, it will probably live longer as well...

Good point, I'll mull that over a little

 

[merc_man]

I like it so when the spark plug is in the cylinder, and you drop it on the mandrel, the air in the cylinder slows down the cylinder so it doesnt drop right down ............ more or less, it floats down on the trapped (yet releasing) cushion of air

Would that be roughly about the diameter of the piston? More? less?

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[Stump Shot]

Most people that talk to me about compression are worried about being able to start their saws, if I increase compression, there had better be a release on the cylinder, or most folks flat out don't want it. So, one saw that I built, did not have a release, I actually decreased compression, with the result of a lower RPM than factory spec. After the saw had been tried out and the resulting discussion, I offered to put a little more pep back into it. The new owner adamantly refused citing "while it does go slow, it's like a tractor motor and won't slow down when it's in a log, and it starts real easy". He was happy as a clam in mud, so just when you think that you know a thing, go figure...

 

[srcarr52]

One thing is for certain, without the squish band detonation would be certain with a high rate of compression. Pressure is used to measure, has anyone actually cc'd one of these engines to figure out the ratio of compression? I have my favorite target, but I know guys here that blow that away, and I'm boggled with how it's gotten away with.

Yes, I've CC'd chambers, but I can check the compression of a saw with good ring seal and know how big the combustion chamber is with a little math. Then with a little more I can figure out how much to take out of the squish to get within 5-10 psi of my target. It's all the ideal gas law, pv=nrt... and nrt is constant.

For tooling. How snug of a fit are the mandrels for the lathe. Do you turn them down so they are a snug fit or a little loose. I got a friend with a lathe and would love to try turning the base down rather then delete the gasket. Be easyer for taking cylinder on and off.

I shoot for 0.001" clearance. I have a few that are right on the money and sometimes I have to heat up the cylinder to get it to slip on the mandrel. I also drill and tap the center of each mandrel to hold sand paper on the end to increase the friction with the squishband, that way you don't need to put a lot of compression on the cylinder with the live center to take heavier cuts.


I usually run around 200-215 psi in most saws. I've even gone as far as to drill out decomp valves to make them easier for people to start.