This one was tricky due to piston dome area but I figured out how to do it. Luckily I found a good formula to use to help calculate the volume the piston dome occupies. This calculator is easy enough in concept but getting the "above TDC mm" is a bit impossible because domed pistons throw simple formulas off since that especially has to be taken into account when figuring out the combustion volume when the piston is at TDC so I made a "piston dome cc" calculator at J19 to help. The formula tries to mimic a cranking psi graph I found on the net which looked right from going by my own experience with engines I've owned. Of course the more the rings are worn the less the pressure is, so the formula assumes the rings are within spec. I came up with my own formula to estimate the cranking psi calculator going only by the compression ratio and riding elevation. I rearranged the formula also to make it show the maximum RPM needed to achieve an maximum average piston speed of 25m/sec. ![]() The average (mean) piston velocity is what most calculators go by instead of the maximum velocity which is fine because they maintain a set relativity to each other no matter what. Maximum Average Piston Velocity Calculator Some of these calculators have never been available till now although engineering types have always been able to figure this stuff out. Below are notes on my free spreadsheet of mini 2 stroke calculators.
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