Math for Motorheads
Craig S. Walker
Power:
If we want know Horsepower, we need to know torque and the rpm that torque occurs at. Hence:
Torque X RPM / 5252 = HP
Likewise if we have a dyno chart that shows HP and we know the engines rpm at that point we can calculate as follows for torque:
HP X 5252 / RPM = Torque
You will often hear of brake mean effective pressure when discussing horsepower. To determine Mean effective pressure (MEP) perform the following:
MEP = (HP X 792,000) / (displacement X rpm)
Also:
MEP = (torque X 150.8) / displacement
Bore and stoke:
Cylinder volume = pi / 4 X bore2 X stroke
Displacement = pi / 4 X bore2 X stroke X number of cylinders
Bore = Sqrt of ( displacement / [ pi / 4 X stroke X number of cylinders])
Stroke = displacement / (pi / 4 X bore2 X number of cylinders
Compression Ratio:
Compression ratio = Cylinder volume + chamber volume / chamber volume
Chamber volume = cylinder volume / compression ratio - 1.0
Displacement ratio = cylinder volume / chamber volume
If you would like to calculate how much to mill your heads in order to achieve a new desired compression ratio:
Amount to mill = ([New disp. ratio - Old disp. Ratio] / [New disp. ratio X old disp. ratio]) X Stroke
Piston speed:
Piston Speed (ft. per minute) = (stroke in inches X RPM) / 6
RPM = (piston speed in ft. per minute X 6) / stroke in inches
Brake specific fuel consumption:
BSFC = fuel pounds per hour / brake horsepower
Quarter mile ET and MPH (approximate….seems better for cars than bikes)
ET = Cube root of [ (weight/HP) X 5.825 ]
MPH = Cube root of [ (HP/ weight) X234 ]
HP = weight / (ET / 5.825)3
Weight = (ET/5.825)3 X HP
Also:
HP = (MPH / 234)3 X weight
Weight = (234 / MPH)3 X HP
This is a chart that I had no idea what to do with; therefore, I'm putting here.