I'm looking for a rule of thumb, or mathematical formula, or just personal experience on this.



For an N/A engine, can you correlate the potential HP of a motor to its compression test results (using a standard piston engine tester)?



More specifically, for a stock port 12A, it seems that about 150-155 FWHP is the limit. From what I've read, the best compression numbers you'll see are about 120 PSI with a standard tester.



So, if you have a motor that tests at 90PSI, is there a rule of thumb that this engine would have x% of the potential of an engine with 120PSI readings? I know that there are lots of variables that affect how much HP you actually make, and I know that it's hard to do this compression test exactl the same each time (b/c of cranking rpm , etc). I'm trying to find out how much compression affects the ability of an engine to make full power - assuming the comp. test is accurate, and the motor is properly tuned.



Thanks,

Neil

[quote name='sevenracer' date='Jun 17 2005, 09:27 AM']I'm looking for a rule of thumb, or mathematical formula, or just personal experience on this.



For an N/A engine, can you correlate the potential HP of a motor to its compression test results (using a standard piston engine tester)?



More specifically, for a stock port 12A, it seems that about 150-155 FWHP is the limit. From what I've read, the best compression numbers you'll see are about 120 PSI with a standard tester.



So, if you have a motor that tests at 90PSI, is there a rule of thumb that this engine would have x% of the potential of an engine with 120PSI readings? I know that there are lots of variables that affect how much HP you actually make, and I know that it's hard to do this compression test exactl the same each time (b/c of cranking rpm , etc). I'm trying to find out how much compression affects the ability of an engine to make full power - assuming the comp. test is accurate, and the motor is properly tuned.



Thanks,

Neil





Fortunately there is little connection between these two factors. The difference between a nice tight fresh engine an an old dog at cruise may be obvious. The 2,000 to 2,200 ranges gives the compression and power strokes plenty of time to leak down. But at partial throttle at these RPM there is little power being produced.



The RPM where maximium power is produced may be from 8,000 to well over 10,000. and in this case the leakdown time is minimal and has little affect on power.



Slight differences in timing, induction system, muffler style and so on would make as big a difference.



In any case there is a difference and givin the chance to build up a fresh engine, you would want all seal pieces at the minimum clearance spec.



Lynn E. Hanover










[/quote]

I guess that explains why my low comp. motor has a hard time starting hot, idles poorly, and hesitates sometimes cruising, but still runs great when I try to tear the **** out of it.