# CR help



## wildj82 (May 1, 2010)

Hi All, need some help calculating CR. 400 cu #16 68 HO heads. Flat tops with beveled edge according to previous owner. I know the heads are 72 cc and one post thought to sub 11cc for piston. Any help would be great. Motor is assembled in car. Just trying to pick new cam.


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## PontiacJim (Dec 29, 2012)

Wallace racing on line has a compression calculator. You need to know head cc's, engine bore size, how far down in the block the pistons are, cc's of the valve reliefs, thickness of your head gasket and its diameter. A few cc's can make a big difference.


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## wildj82 (May 1, 2010)

PontiacJim said:


> Wallace racing on line has a compression calculator. You need to know head cc's, engine bore size, how far down in the block the pistons are, cc's of the valve reliefs, thickness of your head gasket and its diameter. A few cc's can make a big difference.


Trying to determine the valve relief and bevel cc's anyone ever measure one


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## geeteeohguy (Feb 2, 2008)

The most common 'beveled edge' pistons seem to be the dreaded 8 valve relief rebuilder specials. If you have these pistons, they are very prone to detonation, due to the poor quench designed into them. They will ping and rattle even at 9:1 CR levels. To measure the valve relief in the piston, use a graduated syringe and water or light oil, like ATF. Water with surface tension remover added is more accurate. (water wetter or the stuff you buy for fountains to keep mosquitos from breeding).


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