Notes on my free spreadsheet of mini 2 stroke calculators

Some of these calculators have never been available till now although engineering types have always been able to figure this stuff out.

Maximum Average Piston Velocity Calculator

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. I rearranged the formula also to make it show the maximum RPM needed to achieve an maximum average piston speed of 25m/sec.

Cranking Engine PSI Calculator

I came up with my own formula to estimate the cranking psi calculator going only by the compression ratio and riding elevation. Of course the more the rings are worn the less the pressure is, so the formula assumes the rings are within spec. 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.

Compression Ratio Calculator

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. Luckily I found a good formula to use to help calculate the volume the piston dome occupies.

Trapped Engine Volume Calculator

This one was tricky due to piston dome area but I figured out how to do it. I think we should all be going by this calculation instead of the old way which basically gives the engine volume from BDC on up but without the head volume. That’s what the 4 stroke engine volume calculators do. Crazy, like 2 strokes aren’t important enough to have their own formulas. Go figure.

Mikuni Main Jet Crossover Calculator

The jet crossover formula is my own concoction that tries to duplicate the best information I’ve come across on the subject (including contacting places that sell the jets). Too many reports on the internet though are totally off and some even say the opposite of what’s true.

Carb Air Velocity Calculator

This calculator is cool because it shows what your carbs air velocity is and then shows what the extremes of trail riding and racing would be best at, and what size carb you’d need to achieve that. The calculator mimics what the motorcycle manufacturers have done over the years so don’t bark at me if you don’t like it.

Pipe Cone Angle Calculator

This one is handy to use if you are gathering information on your pipe but don’t have the formula for figuring out the cone angles. Of course the steeper the angle the stronger the return wave but also the shorter the return wave, and both the diffuser wave and the baffle wave need to have a certain length depending on what your goals are concerning the powerband. To really figure this all out right you need a good expansion chamber calculator based on a return wave simulator.

You can download this spreadsheet of calculators from the link at the bottom of the first paragraph at www.dragonfly75.com/moto/calculators.html and make use of it if you have Excel on your computer. I give credit to James Dean, the creator of the JD Jetting spreadsheet for Keihin carbs, which I used to select the best needle for my ’89 Honda CR250, for getting me into making spreadsheet calculators. (The JD spreadsheet is no longer available in case you’re wondering.)