Compression and HP
 

For a N/A engine, is there any way to correlate max potential HP to engine compression?

I have two 12A engines, one tests at about 90PSI (~40PSI pulses with valve open on tester) with a standard compression gauge, and the other tests at about 115-120PSI (~60PSI pulses, valve open).

I'm wondering how much less power to expect between the two, all other factors being equal.

I'm assuming that the relationship is non linear (the weaker engine will make more than 75% of the HP of the strong one), but I don't have any direct experience to back that up.

Any info on this?

Thanks,
Neil

Compression on the tester involves many factors, including port timing and cranking speed. You could probably notice a drop in tested compression just by changing to thicker oil or having a weaker battery/starter, simply because of the resulting slower cranking speed.

peejay is right, cranking rpm will influence a compression test

I understand that cranking speed affects the test numbers.

I'm trying to figure out if you can compare the HP potential of two motors based on compression tests. This is based on the assumption that the motors were tested exactly the same - same rpm ,etc.

I race a first gen in the IT7 class. All motor internals are stock. The fastest guys probably have about 150hp at the flywheel. I'm assuming that if you tested their motors, you would get really good compression numbers - definitely over 100PSI, probably close to 120.

I need to know what I can expect to get out of a 90 PSI motor assuming it's tuned properly. My guess is that it's more than 75% of that 150hp, but I 'm looking for anybody who has direct experience with this, or who knows the engine theory that correlates compression to power output.

Thanks,
Neil

I imagine if you were to try to find a correlation, you would need to use the same starting system for each motor, running the exact same oil, and as many other factors controlled, because I think that the typical difference in compression between two properly broken in race engines would be washed out by the other influences.

Edit: ^^ and identical porting, pref. all stock ^^

I'd say the best way to tell which one is going to be a stronger motor, if they're rebuilds anyway, is what they look like inside. What brands of what parts, new or used in spec, are in them, what it's previous mileage had done to it prior, etc.