Pistons, HP and Compression
Thread Starter
Racer
Joined: Sep 2003
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From: Indianapolis IN
Pistons, HP and Compression
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?
Also, how many HP can you produce before you have to go to forged pistons instead of stock pistons?
Race Director
Joined: Feb 1999
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From: In Dreams There Is Truth Ohio
Re: Pistons, HP and Compression (blunsford)
A local engine builder told me that on my 454 one point increase in compression equals about 60 HP. Does that sound right?
Also, how many HP can you produce before you have to go to forged pistons instead of stock pistons?
What do you have in heads, cam, pistons, etc?
Race Director
Joined: Apr 1999
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From: CORVETTE 77 385 C.I. TEXAS
Re: Pistons, HP and Compression (blunsford)
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%.
Race Director
Joined: Oct 2000
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From: central FL, near the beach.
Re: Pistons, HP and Compression (blunsford)
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
60tq seems a stretch, however 40 quite possible
Melting Slicks
Joined: Apr 2000
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From: San Jose California
Re: Pistons, HP and Compression (blunsford)
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
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
Race Director
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Re: Pistons, HP and Compression (Chuck Harmon)
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
Race Director
Joined: Oct 2000
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From: central FL, near the beach.
Re: Pistons, HP and Compression (blunsford)
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.
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.
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Melting Slicks
Joined: Apr 2000
Posts: 3,286
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From: San Jose California
Re: Pistons, HP and Compression (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
Chuck
Melting Slicks
Joined: May 2001
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From: May help you? You can sure as hell try!
Re: Pistons, HP and Compression (blunsford)
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.