Chamber size??

AdamR said

after 71 they dropped compression by moving the piston pin up. this dropped the piston way down in the bore about .100"

The earlier 318s that are rated in the 9:1 are are lower then that. probably in the *.5-8.8:1 range.

The real bad news is your 318 is likely in the 7.8:1 range.

The only way to know for sure what your compression is ,is to measure everything. Head CC's, Deck hieght and gasket thickness.

Click to expand...

Beside compressed thickness of the gasket you need to include the bore size of the gasket. Fel-Pro for instance is 4.150. That adds significant volume that will reduce the compression ratio even more.

If you are using pistons with valve reliefs that needs to be taken into account to.

Also, the advertised compression ratio for a piston typically is assuming some set of dimensions for all the other parts. You won't have that compression unless the parts are at the nominal numbers the piston manufacuturer used. In most all cases you will have a significantly lower compression ratio than wht the psiton is advertised for if the heads & block have not been decked.

Here is a nice little on line compression ratio calculator;
http://www.csgnetwork.com/compcalc.html