The squish factor

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Rep732

Rep732

The alchemist ☠️
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So i was talking to doug at esp today. And I was talking to him about changing cylinder base gaskets for bumping compression. I was told changing it is possible but in turn the engines will run hotter and to also use copper gaskets since they don't crush as much...any feedback? Anyone run a thinner base gasket?
 
I normally set squish to .020" some of the guys have measured and seen it at .014" too close for my tastes. I normally try to go gasket less, or use shim stock, copper, aluminum or whatever other soft metal I have in hand. Actually have a coke can I used for a saw build I did a bit ago. Idk why it would run hotter just from the marginal bump in compression, I'd be more worried about changing the port timing, technically speaking it does change the number a bit. You just lowered the ports by whatever amount you lowered the cylinder. Normally not enough to notice performance issues, but something to think about.
 
And thats what doug said he sets them at .020"... i was just curious for compression sake to try and get a little more out of it. If it won't yield any performance increase I guess there's no point to it
 
I talked top Doug about using a Zenoah ported head on a Rovan bottom end he thought it would work but also recommended a 0.020" Squish test to see how it would fit.
 
I've always been told .020" is as tight as you want to go for an engine to last. (At least in saw world) now dropping down to say .015" most likely isnt going to kill anything, I don't think you'll see the big bump in compression your looking for. Now if were starting off with say .047" and dropping down to .020" and a pop up piston, or cutting the base and squish band, then yeah you can bump it up pretty high. But, I typically don't think that's necessary, unless your planning on at least raising your port roofs a bit to help compensate for lowering them.
 
Seandonato73 said:
I've always been told .020" is as tight as you want to go for an engine to last. (At least in saw world) now dropping down to say .015" most likely isnt going to kill anything, I don't think you'll see the big bump in compression your looking for. Now if were starting off with say .047" and dropping down to .020" and a pop up piston, or cutting the base and squish band, then yeah you can bump it up pretty high. But, I typically don't think that's necessary, unless your planning on at least raising your port roofs a bit to help compensate for lowering them.
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How much difference is there with a double ring piston ? in compression ? just curious
 
Compression should stay the same i would think just adding another ring means it just has another ring to push past..redundant really at this level
 
Well an engine would produce blow by at any rpm with a compromised ring or cylinder wall. I would think blow by would be more noticeable under more cylinder pressure
 
Rep732 said:
Well an engine would produce blow by at any rpm with a compromised ring or cylinder wall. I would think blow by would be more noticeable under more cylinder pressure
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you would need to test leak down of the two compared , at higher RPM less air can blow by I think ?? need a engine builder for this one !
 
I suppose a double ring would help it seal better, but I haven't really messed with double ring sets (in 2 strokes) to say it's better or worse. I think leak down or blow by would be a bit tough to figure out as well.
 
I have a leak tester, well it's for kohler engines. The only issue I see is theres such a narrow window where the cylinder is sealed to check. Not like on a 4 choke where you can test (basically) the entire area where the ring rides. And in 2 stroke it's much more important for the ring to seal everywhere imo.
buzzkill said:
leak down is easy given the correct tester , would be interesting to know. read this from WISECO pistons http://blog.wiseco.com/single-ring-vs.-two-ring-2-stroke-pistons-which-one-is-better
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That's a good read. I wonder if displacment plays a function as well for ring selection. I honestly haven't seen many sub 100cc engines with 2 rings, although my field of view is very narrow, compared to the over all 2 stroke engine class. (Mainly saws, ope, and our 1/5 engines)
 
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Seandonato73 said:
I have a leak tester, well it's for kohler engines. The only issue I see is theres such a narrow window where the cylinder is sealed to check. Not like on a 4 choke where you can test (basically) the entire area where the ring rides. And in 2 stroke it's much more important for the ring to seal everywhere imo.

That's a good read. I wonder if displacment plays a function as well for ring selection. I honestly haven't seen many sub 100cc engines with 2 rings, although my field of view is very narrow, compared to the over all 2 stroke engine class. (Mainly saws, ope, and our 1/5 engines)
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Really on a 2 stroke all the compression happens AFTER the last port opening right ? kinda has too.
My old Yamaha RZ350 had dual rings , it was a 2 cyl. 350 , Wiseco talked alot about reliability over time for 2 rings , not so good for Race motors , you want minimum friction in a race motor.
 
But remember its not just compression the ring needs to seal against. The down stroke forces the fuel air mix up through the transfers and into the combustion chamber. Thus if the ring leaks on compression it will leak during the transfer and intake phase too. If the ring leaks too much, now your getting a mix, of burned/burning fuel air mix, (from combustion chamber side) mixed with your fresh fuel air mix (from the crankcase) so it's much more imperative that the ring in a 2 stroke function properly. (If that's not very clear I'll try to doodle up something later)
 
Seandonato73 said:
...But, I typically don't think that's necessary, unless your planning on at least raising your port roofs a bit to help compensate for lowering them.
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remember that lowering the head will just lower the exhaust and transfer port durations. it is not valid for intake port

since the piston skirt is controlling the intake port, its duration will actually increase when you are lowering the head. if you want to, you can compensate that with some JB weld on the roof (not removing and further increase the duration of intake)
Rep732 said:
And thats what doug said he sets them at .020"... i was just curious for compression sake to try and get a little more out of it. If it won't yield any performance increase I guess there's no point to it
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what octane value ESP recommends for the .020" squish?
i think the lower the squish (and higher the compression rate), the higher the octane should be to avoid detonation....
 
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Typically esp is cutting the squish band to get the extra room for the stroker crank. Weve had guys measure squish down as low as 0.014" running on high test pump gas. 0.020" has never in my experience been an issue with 93octane gas.

You are correct on the intake port, however theres no down side to raising the port to factory numbers, assuming the piston skirt still travels high enough to compensate for it. I would think if you touched anything with a grinder the exhaust would be the easiest and most beneficial, transfers can be a bit tough for the beginner to keep the roof angle where it needs to be. But I digress, i was merely stating if you lower the cylinder too much youll throw your port numbers out the window. As most know bumping compression on a 2 stroke has gains up to a point, where the engine has to work harder to overcome the compression just to run, ie pumping losses at low to mid (not always) rpm ranges, this will decrease the efficiency of the engine instead of raising it. Imo it's better to have a lower static compression, and work on getting the port configuration, and pipe combination to get your dynamic compression higher, resulting in a higher amount of air/fuel charge trapped in the cylinder.
 
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