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Discussion Starter #1
I am trying to calculate the compression ratio for a stock 1970 351C 4v and the numbers don't agree with my shop manual which says it should be 11:1. I am using the following dimensions for my calculation.

bore 4.0"
stroke 3.5"
head 62.8cc
gasket 0.03" (a guess)
piston to deck clearance 0.035
valve relief vol 2cc (a guess)

With these numbers I calculate 10.2:1. Am I missing something or did these engines have lower compression than advertised?

Perry
 

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You're first problem is that you are guessing at some volumes, therefor your calculations are way off. Head gaskets usually have a crush measurement of about .045" this will equal about 9CC IIRC You need to know exactly the volume of the valve notches and deck hieght.

Last comment, that's a factory advertisement anyway. As built can be comepletely different from the "close" machining tolerences.

If I had to wager a guess, I'd say you're about 10:1 in reality.
 

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If I use 0.045 for the head gasket the compression gets even lower. The deck height is a nominal values I found in several sources. Even if I had flat pistons with no reliefs I would be at 10.1:1. The reliefs will just make it lower. I am going to measure the actual volumes when I rebuild so this is mostly academic. What I am trying to figure out is do I need dished pistons to get to about 10:1.

Thanks,
Perry
 

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Yes you will.
I have domed with releifs custom made for a total cylinder change of 0cc with 67cc heads and was well over 12.5:1

A flat top replacement with your other numbers puts it at approx 10.5:1

using your .034 for gasket and .035 for deck with a 4.03 overbore since you are rebuilding and a 3.5 stroke.

A flat piston with 4 cc removed from it would get you to 10:1 or just a dish with a 4cc releif. These numbers are all calculated roughly using the deck and gasket info you provided.
 

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With all the specs. you first gave, I've come up with 10.08:1 CR.

Ouch, using a .045 would really put you in the dumper. This is why I tell people you must measure and calculate compression ratio. It appears that the factory "advertised" ratio is just that, an advertisement. Close to a full point off if those # are correct.
 
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