Compression Ratio Conflict
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Junior User
Joined: Feb 2002
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From: Winnipeg, MB.
Compression Ratio Conflict
I have a 1972 2V 351 Cleveland. I have seen numerous times that the CR is about 8.5. I bought a book that deals with rebuilding 351C, M and 400's and it says 9.5. Is this a misprint or what?
Junior User
Joined: Dec 2003
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My Motors repair manual lists it as 8.6. I ran a stock 1971 351c in my truck for several years. I did replace the cam with a Crane RV cam but everything else was original factory. It ran fine on regular gas. I rebuilt the engine a few years ago but I still have the original pistons out in the shed. I think they are flattops but I think they sat below the deck.
If you are referring to Tom Monroe's book there is at least one joker in the deck. He lists chamber volume as 76 cc's. All six of the 351c, 400 and 351M heads I have were around 80 cc's.
I am running the KB 148 pistons with a 13cc dish at zero deck and I have a measured 8.3 CR. I have to think a stock flattop sitting below the deck would yield similar compression. No way to know for sure without measuring your engine, but I tend to believe the lower CR.
If you are referring to Tom Monroe's book there is at least one joker in the deck. He lists chamber volume as 76 cc's. All six of the 351c, 400 and 351M heads I have were around 80 cc's.
I am running the KB 148 pistons with a 13cc dish at zero deck and I have a measured 8.3 CR. I have to think a stock flattop sitting below the deck would yield similar compression. No way to know for sure without measuring your engine, but I tend to believe the lower CR.
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Joined: Dec 2003
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Just for grins I ran Mr. Monroe's numbers. Swept volume=720.7cc. + .035" deck clearance=8.2cc. + two valve reliefs=4cc.+ .040" head gasket=8.4cc. + chamber volume=76.2cc. = Total=817.5cc. 817.5cc - 720.7cc = 96.8cc. Compression ratio=817.5 / 96.8 = 8.45.
It looks like the 9.5 is a typo. Wouldn't be the first book with an error in it!
It looks like the 9.5 is a typo. Wouldn't be the first book with an error in it!
