Best compression ratio
Thread Starter
Drifting
Joined: May 1999
Posts: 1,610
Likes: 3
From: Goulburn, Australia NSW
Best compression ratio
I'm looking at some 383 stroker kits over here and can select from a choice of 3 different compression ratios......10.9, 10.3 & 9.7. I have a set of AFR aluminium heads that should be able to handle a little more compression than stock so what would be the best ratio to go with? This is an occasionally driven street car that would only see the odd bit of track work....any suggestions?
Race Director
Joined: Apr 1999
Posts: 11,520
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From: CORVETTE 77 385 C.I. TEXAS
Re: Best compression ratio (Brutus)
Best C/R depends on a no. of things including cam. It is better not to overdo it as you have little to gain & a lot to lose.
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%.
Probably you want 9.7.
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%.
I'm looking at some 383 stroker kits over here and can select from a choice of 3 different compression ratios......10.9, 10.3 & 9.7. I have a set of AFR aluminium heads that should be able to handle a little more compression than stock so what would be the best ratio to go with? This is an occasionally driven street car that would only see the odd bit of track work....any suggestions?
Team Owner
Joined: Feb 1999
Posts: 34,454
Likes: 665
From: Republic of Texas
Re: Best compression ratio (Brutus)
Power wise the higher the compression the better. Since you are sunning aluminum heads I'd go for either the 10:3 or the 10:9. If you car choosing a cam with a fair about of bleed off the 10:9 should not be a problem and will also give you the most power.
:cheers:
:cheers:
Thread Starter
Drifting
Joined: May 1999
Posts: 1,610
Likes: 3
From: Goulburn, Australia NSW
Re: Best compression ratio (Ganey)
I had already purchased a Comp Cams XE262 with 218/224 lift @50 thou, but that was to go with a rebuild that would have only taken the bore out to 355.
Would I be correct in thinking that a 383 could handle a bit more cam?
Btw, the AFR heads I have are 68cc combustion chambers so given that most combinations I've seen advertised base their c/r on 64 cc heads I was wondering where that would leave me with the c/r from these kits?
Would I be correct in thinking that a 383 could handle a bit more cam?
Btw, the AFR heads I have are 68cc combustion chambers so given that most combinations I've seen advertised base their c/r on 64 cc heads I was wondering where that would leave me with the c/r from these kits?
Race Director
Joined: Dec 1999
Posts: 19,610
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From: Forked River NJ
Re: Best compression ratio (Brutus)
You should definitly go with more of a cam in a 383 motor. That is a mild cam in a 350. It would be real mild in a 383. I would go for the higher comp ratio. I am running about 10 to 1 with AFR heads and a lot of duration in my cam. I should have gone higher. I can run 87 octane with no problem.
Team Owner
Joined: Feb 1999
Posts: 34,454
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From: Republic of Texas
Re: Best compression ratio (Brutus)
Would I be correct in thinking that a 383 could handle a bit more cam?
:cheers:
Team Owner
Joined: Apr 1999
Posts: 21,953
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From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Re: Best compression ratio (Brutus)
http://www.smokemup.com Get into the auto math and figure out your real compression. tighter quench can get allow more compression without detonation. I now have 11.2 c/r and it runs fine on 91 octane with a big cam.
For a nice H-flat cam I would recommend the Crane powermax 278. i had it in a 355 with 1.6 roller rockers and it was fun. the 383 would tame it down. I also had the Comp Cams XE274 in a 355. the crane had a broader torque courve.
:seeya
For a nice H-flat cam I would recommend the Crane powermax 278. i had it in a 355 with 1.6 roller rockers and it was fun. the 383 would tame it down. I also had the Comp Cams XE274 in a 355. the crane had a broader torque courve.
:seeya
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Race Director
Joined: Mar 2001
Posts: 11,322
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From: Living in the Hartland
Re: Best compression ratio (Brutus)
Just to inject a question that I am curious about and might help you:
What other benefits are their to higher comp since power gain is not much with higher comp? Seems the car would gain some increased revability and help low end torque with higher comp.
What other benefits are their to higher comp since power gain is not much with higher comp? Seems the car would gain some increased revability and help low end torque with higher comp.
Pro
Joined: Jun 2001
Posts: 672
Likes: 0
From: Quebec
Re: Best compression ratio (Fevre)
The new generation of cam like the Extreme energy XE from comp cam don't need big duration to create high flow. The more air/fuel mixture you get in and the more h.p. you'll produce.
Now, with little duration, you need less static compression to create dynamic compression. The pression built up more when the intake valve close earlier.
The old performance cam used a lot of duration to create flow. That's why they could put 11:1 static compression in a street engine without detonation.
The dynamic compression is the key to determine the optimal static compression. There's a nice caculator somewhere in the archive that give dynamic compression in relation with camshaft duration and static compression. The number that's ideal is close to 8:1 dyanamic. You must not be over 8.2 to 1.(at least with cast heads).
Hope this help.
Stephan
[Modified by American Boy, 12:45 PM 6/27/2003]
Now, with little duration, you need less static compression to create dynamic compression. The pression built up more when the intake valve close earlier.
The old performance cam used a lot of duration to create flow. That's why they could put 11:1 static compression in a street engine without detonation.
The dynamic compression is the key to determine the optimal static compression. There's a nice caculator somewhere in the archive that give dynamic compression in relation with camshaft duration and static compression. The number that's ideal is close to 8:1 dyanamic. You must not be over 8.2 to 1.(at least with cast heads).
Hope this help.
Stephan
[Modified by American Boy, 12:45 PM 6/27/2003]
Team Owner
Joined: Apr 1999
Posts: 21,953
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From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Re: Best compression ratio (American Boy)
American Boy - my dynamic compression is @ 8.46 Right on the edge of high octane race type motors. I have thermal coated pistons which are supposed to give you an addition .5 - 1.0 more static compression before detonation.
Fever - more compression is more TQ and HP across the whole rpm range of the motor. It's on a declining ratio. Where a 1 point change from 9 to 10 or 9.5 to 10.5 is the @4% change. It's the little things that all add up.
Fever - more compression is more TQ and HP across the whole rpm range of the motor. It's on a declining ratio. Where a 1 point change from 9 to 10 or 9.5 to 10.5 is the @4% change. It's the little things that all add up.
