Quoting John Baechtel in Performance Automotive Engine Math "Depending on the condition of the piston rings and valves, a healthy engine has a cranking compression between 150 and 180 psi. A good performance engine can easily have a cranking compression more than 200 psi. Some are a little higher and a few are much lower."
The 790cc Triumph twin standard from the factory, volumetric efficiency = 92%, 52 horsepower, 9.2 to 1 static compression ratio, 8.6 to 1 dynamic compression ratio, calculated dynamic cranking pressure = 173 psi
The 790cc Triumph is now an 865 cc engine with the following, volumetric efficiency = 109%, 84 horsepower, 10.5 to 1 static compression ratio, 9.2 to 1 dynamic compression ratio, calculated dynamic cranking pressure = 191 psi. Measured is near this based on a bench test prior to fitting.
Volumetric efficiency calculated by PipeMax and dynamic and cranking pressures calculated using program on
www.rbracing-rsr.com. The figures are for reference. They show that power can be had in a similar size twin without big compression. Both engines are street motors that run on regular gas.
Determine your cranking pressure using the rbracing calculator and your inlet valve closing degrees at 0.050 info from your cam data card, the rod length from Carillo, and the bore and static compression ratio from your piston manufacturer. This is your calculated dynamic cranking pressure. Compare it to your measured. Are there big differences? If so, check your rod length, cc your head and cyl to get the actual static compression ratio, and check your cam timing. Adjust so measured is sorta close to calculated.
Standard engine building procedure for me is to do the step in the preceding paragraph on the bench before the motor goes into the frame. Simply hook up the compression gage and turn over the engine with the starter motor. An engine would not be run if it had 300 psi cranking compression. It would be taken apart and fixed.
