Compression ratio 1974 BB 454
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Joined: Jan 2007
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From: Sandnes Norway
Compression ratio 1974 BB 454
I have a 1974 BB and my mechanic at the garage says that my measured compression ratio is between 130 and 138, and that is a little low he says. My consoleplate says 8,25:1 comp ratio and that shoult be about 117-118 i think. Am I wrong in some way here?
Race Director
Joined: Jun 2000
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From: FL
Cranking compression ratio has very little to do with static compression ratio. It has a lot to do with when the intake valve closes. If you have a small cam that closes the intake quickly, you can easily build cranking pressure with a lower static compression. In the same vein, a larger cam will bleed off cranking pressure due to the later intake closing event.
Race Director
Joined: Jun 2000
Posts: 10,046
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From: FL
Here is a good article:
http://www.chevyhiperformance.com/te...e_compression/
Pro
Joined: Apr 2001
Posts: 605
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From: las vegas
What is more important is the evenness of the cranking pressure. Anything within ten percent is considered good. Yours is almost half of that at 130/138. A good valve job might take care of some of the overall compression, and new rings and cam would take care of the rest. Unless you are going to be pushing this engine or just want to rebuld it I would leave it alone. You did not mention how many miles are on this engine, as that might tell you if it is time for a rebulid. Vegas
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From: Sandnes Norway
Thanks
It looks like it is too low then. Regarding miles on the engine: Car was bought on E-bay and seller stated that the 36000 odometer miles are original , but that's what all collector car sellers say so I'm not sure if it's true or not. Most likely it has rolled at least one turn.
What I'm going to use this car for is only light Sunday cruising so the performance is not so important but it sure is funnier the stronger it is. Maybe I should consider rebuilding it.
Regards
It looks like it is too low then. Regarding miles on the engine: Car was bought on E-bay and seller stated that the 36000 odometer miles are original , but that's what all collector car sellers say so I'm not sure if it's true or not. Most likely it has rolled at least one turn.
What I'm going to use this car for is only light Sunday cruising so the performance is not so important but it sure is funnier the stronger it is. Maybe I should consider rebuilding it.
Regards
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Joined: Jan 2007
Posts: 96
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From: Sandnes Norway
Hi again
Lot's of stuff to read and learn about CR. Found a good reference for the basic's here
http://e30m3performance.com/myths/mo...comp_ratio.htm
Regards Clue
Lot's of stuff to read and learn about CR. Found a good reference for the basic's here
http://e30m3performance.com/myths/mo...comp_ratio.htm
Regards Clue
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Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
I wouldn't worry about the numbers too much if the engine runs ok...Just tune and drive the car and have fun. You can rebuild down the line. You start second guessing all the stuff, the car will become a money pit... and quick.
"Don't fix it, if it ain't broke!" does have some real merit. Also, any 454 that is running decent should have plenty of power for a Sunday afternoon drive.
