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Squish
Oct 16, 2021 23:29:49 GMT -5
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Post by paulg on Oct 16, 2021 23:29:49 GMT -5
I’ve got a 2 stroke Polaris style atvs engine that I was wanting to put a 70cc bbk on. I have no idea what to set the squish band. Without the base gasket I measured 1.15mm. With the base gasket I got 1.65. Anyone please care to chime in?
Thanks!
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Post by aeroxbud on Oct 17, 2021 5:14:33 GMT -5
For a sports cylinder, somewhere between 0.8-1mm is usually ok. Some higher performance kits say 0.6-0.8. But that's going to give high compression. I would also check to see what the piston to port opening looks like at bottom of the piston travel. My Polini went much better with the ports flush, and a slightly higher squish.
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Post by 90GTVert on Oct 17, 2021 7:29:00 GMT -5
As aeroxbud eluded to, it's usually best to select the base gasket based on where it puts the cylinder. Ideally, you'd like to see the port floors level with the edges of the piston crown at the bottom of the stroke. Squish can then be accepted if it's not unreasonable, or the head or cylinder may be able to be shaved to reduce squish. With flat surfaces it's easiest. If there are grooves for o-rings, you can probably get away with a little change, but much cutting can cause problems with sealing.
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Post by pete130 on Dec 22, 2021 1:37:15 GMT -5
I’ve got a 2 stroke Polaris style atvs engine that I was wanting to put a 70cc bbk on. I have no idea what to set the squish band. Without the base gasket I measured 1.15mm. With the base gasket I got 1.65. Anyone please care to chime in? Thanks! Thats a lot for a 70cc but I have seem 2mm on some high powered 2t but street use .8 is good race motors usually run tighter my 94cc runs .55 but I have at .65 got to rember less squash more comprehension, and at high revs it can work against you, I have a.motor that won't rev any more because of high compression, I would try look up factory manual
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Squish
Dec 23, 2021 15:22:47 GMT -5
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Post by fly on Dec 23, 2021 15:22:47 GMT -5
Should start with manufacturer spec, but they are pretty conservative. I think Athena specs .8mm for the Evo kit now, but I run between .6-.7mm with zero problems.
Decreasing case volume, as in HPC cranks, is good, tends to make more power but in a narrower band, as I understand it. Same goes with the opposite direction, as in trenching - you don't want to overdo it. Ideally you're looking for primary and secondary compression at ~1:1. But somebody correct me if I'm wrong.
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