Compression Ratio limits for Big Blocks
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Race Director
Joined: Jul 2004
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From: Scottsdale Arizona
Compression Ratio limits for Big Blocks
Are the rules for maximum compression ratio similar to those for small blocks? I've been offered a pretty nice 496 with a static CR of 10.25: 1 with iron heads. I don't have the cam specs but at first glance, I would say this is too high for the crappy premium gas that you'd find in CA. Is this correct?
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Race Director
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From: Scottsdale Arizona
I understand that but my question is more general. All other things being equal, I would call 10.25:1 high for an SBC. Is the same true for a BBC?
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Not a problem with a decent street cam.
JIM
JIM
Pro
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From: Northern Florida
I now have a bored 468 cid motor, reworked to 10-1 cr , Edelbrock ALum. Heads with hydr. roller cam at 242 duration and .553" lift ---- with timing way advanced. I can run 91 octane without pinging but do get some run-on after being switched off ; 93 octane cures all my problems .
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Tech Contributor
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From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
I used to have a 454 cid motor , 11-1 cr. , and i had to add a can of octane boost in with the 93 octane gas so it wouldnt ping. The flat tappet cam was 220 duration at .050" and a lift of .480" .... stock iron heads -- the timing was way advanced however.
I now have a bored 468 cid motor, reworked to 10-1 cr , Edelbrock ALum. Heads with hydr. roller cam at 242 duration and .553" lift ---- with timing way advanced. I can run 91 octane without pinging but do get some run-on after being switched off ; 93 octane cures all my problems .
I now have a bored 468 cid motor, reworked to 10-1 cr , Edelbrock ALum. Heads with hydr. roller cam at 242 duration and .553" lift ---- with timing way advanced. I can run 91 octane without pinging but do get some run-on after being switched off ; 93 octane cures all my problems .
Pro
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From: Northern Florida
I have no 'vaccuum' advance ; its a Mallory electronic eye Distributor with module , and I had an experienced Tuner re-curve the Distributor on his bench to set it up for maximum low end torque. Made a night and day difference.
Tech Contributor
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From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
If your tuner thinks a vacuum advance hurts low end torque, you need to find another tuner.
Melting Slicks
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From: Mo
Pro
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From: Northern Florida
The Tuner changed the advance characteristics of the electronic Distributor i have. He doesnt think a vaccuum operated advance hurts low end torque , nor did i indicate such.
Pro
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From: Northern Florida
Le Mans Master
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From: Newark DE
My STOCK spec '65 L76 (327/365hp) is 11.25:1. Runs just great on 93 pump gas. once or twice I couldn't get 93 so used 91 and it was fine on that also with no pinging or detonation.
I see no reason why a BBC would react differently than a SBC
Race Director
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From: Millington Illinois
St. Jude Donor '08-'09
Less issues with a 10.25 to 1 BB vs a 10.25 to 1 SB. Cubic inches gives you more torque. Cubic inches also tames a cam. The more CI the lower the operating RPM of a given duration cam. No problem.
Tech Contributor
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From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
If your tuner was "up to speed" he'd have already told you that run-on is going to be a problem with that setup you have.
Add a vacuum advance to your engine and be amazed at how well it idles and the improvement in gas mileage you'll get. OBTW, your exhaust valves will thank you for it too.
Race Director
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From: Millington Illinois
St. Jude Donor '08-'09
Le Mans Master
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From: Austin TX
I'm going to say that 10.25 with iron heads is too much compression for 91, and maybe 93 octane (generally speaking). Cam choice will determine how it works. 10.25 would be a good compression ratio with aluminum heads.
Thread Starter
Race Director
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From: Scottsdale Arizona
10.25 to 1 Iron head street engine.
http://forums.corvetteforum.com/c3-p...?highlight=777
http://forums.corvetteforum.com/c3-p...?highlight=777
with iron heads too.I think one of my U-joints broke as a result of me looking at the dyno sheet.