free 2 stroke calculators

Michael Forrest /

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 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.)


Re: free 2 stroke calculators

Michael Forrest /

Re: free 2 stroke calculators

Michael Forrest /

Crappy free programs

Some programs are even based on the formulas proposed by Gordon Jennings from way back in the early 70's. Man they were just beginning to investigate back then. Basically they knew next to nothing.

Jennings main shortcomings;

0. He assumes the same temperature and sound wave speed inside all pipes.

1. Because of his temperature error his calculations for the tuned length were off by 3.5" compared to a Maico 440 pipe, and 1.75" compared to a Maico 250 pipe. (both pipe specs along with full engine data, horsepower data, and EGT were given to me for analysis.)

2. He uses formulas to calculate everything but belly length, letting it take up the remaining distance. This is totally wrong because the belly length should be set to allow a slight overlap of the diffuser and baffle waves. That set length is optimally almost always below 4" (100mm).

3. He had no idea whatsoever about the return waves the exhaust pulse causes, their return time or their time duration. His ideology was not based on the reality of what was happening inside the pipe but just on scant trial and error data.

4. He calculates the header length as a multiple of its diameter. It's correct length is that which locates the secondary waves in correct relation to exhaust closing time at top RPM.

5. He calculates the belly diameter as a multiple of the header diameter although the header can be too narrow or too wide. Proper sizing depends on the exhaust pulse pressure.

6. He calculates the stinger diameter as a fraction of the header diameter whereas it should be calculated by the exhaust pulse pressure.

7. He calculates the stinger length as a multiple of its diameter whereas in reality the same back pressure can be had by a fat long stinger as by a skinny short stinger.

8. He recommends not going over a combined baffle angle of 30 degrees (15 from centerline) although many successful pipes use much larger angles. According to physics there is no limit to the angle. It's just that the shorter the baffle, the shorter its return wave. The return wave length should match the RPM, the timing between transfers closing and exhaust closing, and the desired RPM spread of the power band. And its length is dependent on the length of the exhaust pulse.

9. He was calculating for a single coned diffuser which gives weak results compared to a multi-coned diffuser of gradually increasing angles.

10. He calculated for a 2 to 1 ratio of angles between the baffle and diffuser which is too limiting. Any way when you go to a multi-coned diffuser there is no single angle of overwhelming importance to compare the baffle angle to. So in modern pipes there is no ratio formula.

Well he was a pioneer 46 years ago but anyone who uses his formulas now is way behind the times. We have progressed so much farther than his formulas and now have computers to simplify the complex process. (But thank you Gordon for helping along pipe research in the early days. RIP)

Re: free 2 stroke calculators

Hows the SEO going?

You seem to post the same thing every other month.

Re: free 2 stroke calculators

Michael Forrest /

I just made that spreadsheet so how could it be a repeat?

Re: free 2 stroke calculators

Dirty30 Dillon /

Why not update the other thread rather than create a new one?

Re: free 2 stroke calculators

You should look at writing a PDF book on this stuff, I would read it.

Re: free 2 stroke calculators

Michael Forrest /

Cool but most of what I know is at my two sections of my web site:

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