Below are notes on my free spreadsheet of 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. The download link is at the bottom. This is a good way to get used to using spreadsheets. It’s easy, you just enter the needed data into the light blue cells and the hidden formulas will use that info to display to you the results.
Hot Engine Port Durations Calculator
Everyone is measuring the cold port durations of their engine which is about 1 degree higher on the exhaust port than the hot duration and 1.5 higher on the transfers. What happens is that the con rod expands with heat more than the cylinder does and pushes the piston upward so that the hot “deck” measurement is less. Read more at www.dragonfly75.com/moto/findduration.html
Carb Sizing 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.
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.
Maximum Delivery Ratio RPM
From the ideas expressed in the research paper “Some Development Aspects of Two-Stroke Cycle Motorcycle Engines” I have a calculator that uses the square root of the intake volume divided by the compressed crankcase volume to show the RPM at which the highest delivery ratio is obtained. This assumes that a CCR of 1.5 (which most engines have) gives a peak delivery ratio RPM of 10,000.
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.
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 www.dragonfly75.com/moto/various.xlsx and make use of it if you have Excel on your computer. To look at my for-sale calculators please go to www.dragonfly75.com/moto/calculators.html
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