I was surfing the web when I happened across this chart on one of
KB Piston's pages. I thought it might help someone who was building a performance engine to figure out the best way to go about it. The article also gives a good definition of the difference between Compression Ratio and Compression Pressure.
FWIW, the comp ratio in Old Yeller is 9.8:1.
PUMP GAS (regular)
8.5:1-Non-quench 2 valve head road use standard sedan, without knock sensor.
8.5:1- Quench head engine for tow service, motor home and truck with torque cam.
9.0:1- Street engine with proper .040" quench, 200° @ .050" lift cam, iron head, sea level operation.
9.5:1- Same as 9:1 except aluminum head used. Light vehicle and no towing.
10:1- Used and built as the 9.5:1 engine with more than 220° @ .050" lift cam.
10:1- 4" and smaller bore, high RPM cam, cold plugs, good fuel distribution, full power limited to 30 seconds W.O.T.
RACE GAS
13:1- is the highest compression ratio suggested with unrestricted gasoline engines.
13:1- Plus o'kay with restricted intake engines at high RPM.
ALCOHOL
15.5:1- is the highest compression ratio suggested for unrestricted alcohol fuel engines. Probably will lose power at 15.5:1 in most applications.