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 Confusion on Wallace CR Calculator (gasket bore)
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Tribal Scout

9 Posts

Posted - 22 Jun 2017 :  10:59:51 AM  Show Profile  Reply with Quote
Hi there. Trying to calculate the static CR on my 455 bored .030 over. I have used the calculator a few times, but left the field for "Gasket Bore Diameter" blank. I assumed since it has the typical gasket thickness figured in .041, that the calculator by default assumed the gasket bore diameter is the same as the actual bore diameter. NOPE. Can't get it to make sense! When I figured it in as the actual cylinder bore diameter it dropped my CR from 9:4 to 8:7. Seemed a lot, so made the bore diameter on just the gasket go from 4.2 to 5..did not make much difference. Then to test the other direction, I made the gasket bore smaller than the actual cylinder bore. As in the gasket would be overlaping into the combustion chamber..reving CC volume. NOPE! still way under 9:4 with Added volume from the gasket in the bore.

I don't want to mess this one up..as it is a big difference between and plays heavy on cam choices. What am I missing here and what is the way to figure this out. I would assume the lower CR was right, but everywhere I have read here and elsewhere says 100cc heads on a .030 overbore on a 455 is over 9:0. Im getting 8:82 for this with .190 deck height and 101cc heads with gasket bore at the exact same as the cylinder bore. And 8:79 with a little oversize gasket bore as I assume they would be in real life..4.3" Any help would be greatly appreciated.

Sitting Duck

1531 Posts

Posted - 22 Jun 2017 :  11:15:54 AM  Show Profile  Reply with Quote
Here's a thread of mine from another site,from years ago. It starts out as a DCR vs. SCR thread,but I think this is kind of what you are looking for as the online calculators can be deceiving. Lots of good info on this thread (None from me !) It turns out even some of the gurus get it wrong! See post #28
Jim Hand even kicks in.


71' GTO Original 400 M20 3.23 posi
13.95@102.1 MPH on street tires

Edited by - 67drake on 22 Jun 2017 11:43:05 AM
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Steve C.

1866 Posts

Posted - 22 Jun 2017 :  11:44:58 AM  Show Profile  Reply with Quote
A fwiw, in the restoration section here you will find another calculator.


Keep in mind the compressed thickness of a particular head gasket in use. The performance FelPro 1016's are .039 crush and the 8518's are .045. The popular Butler gasket is about .045.

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Tribal Scout

9 Posts

Posted - 23 Jun 2017 :  8:04:19 PM  Show Profile  Reply with Quote
Thanks guys. The link was full of great info from lots of people. The calc. that SteveC posted is great! And because it automatically figured gasket bore, I was able to compare the two and realize the Wallace is not accurate unless you input that correctly rather than leave it blank. So I now know my static CR on my 455 will be about 9.15:1 with the info I have if I zero deck my block. Out of town, but will accurately CC my heads and pistons when I get home and make final decisions from there. But great to know what range I am truly in and where I plan to go from here.
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