blunsford

A local engine builder told me that on my 454 one point increase in compression equals about 60 HP. Does that sound right? I would have guessed less.

Also, how many HP can you produce before you have to go to forged pistons instead of stock pistons?

GDaina

You can't assign an arbitrary number like that...variables have to be considered. Size of cam, head flow vs. stock...higher compression will yield more h.p. with other mods than increasing C.R. on a stock motor.

If you are going to add a cam, good flowing heads, bore, then why not just go with forged pistons?

What do you have in heads, cam, pistons, etc?

Matt Gruber

more like 4%, or 20 hp

Ganey

I suppose this chart needs some explanation. It shows the increase in power in % of a C/R increase. EXAMPLE: If the original CR is 9:1 on right of chart & it is increased (A line) to 12:1 (B line) on left of chart, then the increase in power is about 4.5%. If the power is increased from 9.5 to 10, the increase is about .08%.

Matt Gruber

torque is more noticeable. a big cam could be quite soggy at 1500-2000 with 8.75, but be acceptable with 10:1
60tq seems a stretch, however 40 quite possible

Chuck Harmon

I think Ganey's graph may be a bit conservative. I calibrated DD2000 to match the actual dyno of my L88. Using the actual compression, seat to seat timing and lift specs for the cam, valve size, head flow, cfm, bore & stroke; I used different head and exhaust choices in their menu to obtain a hp/torque curve that most closely matched the actual. When I change only the compression, the hp change was almost exactly 20hp for each 1 point change between 10:1, 11:1 and 12:1. This works out to be about 4% for each point.

There are so many factors that can affect horsepower, I think any chart that excludes other factors is only a good "from the hip" guide. I am sure that Ganeys' chart is probably right on for some engines. But others may find more or less.

Chuck

Chuck

MotorHead

Would it not have more to do with the cam and piston together, I mean you could have a cam with big duration and the motor would fall on it's face at 8:1 and just start working properly at 10:1 :D

Matt Gruber

an interesting note,
if it is perfectly tuned and already at maximum CR,(for the fuel being run)
power will DROP with extra CR, due to pinging.
Timing can be retarded, but HP will still be DOWN.

Chuck Harmon

That is my exactly my point! :cheers:

Chuck

LemansBlue68

All things being equal, thermal efficiency increases 3% with each point in compression in the vicinity of 8-12:1. This works out to about a 3% increase in torque. Going higher than 12:1 the law of diminishing returns starts to kick in. That's why diesels don't go much higher than 20:1 (they also don't have issues with knock). As CR is increased, the tendency to knock increases as well so higher octane fuels have to be used to take advantage. Otherwise, spark timing must be retarded which decreases the benefits somewhat because the engine can't run at the most efficient spark advance.