351c cj compression...

[keith1562]

I'm doing a full rebuild of a 351c cj. These engines have notoriously low compression. Mine is a 72 and the pistons are dished, it also has open chamber heads. I intend to replace the pistons with flat tops. What kind of compression ratio should I expect with these pistons?
Any input, positive or negative will be appreciated.
Thanks,
Keith

[Steven Harris]

I would be purely guessing.

Someone else will surely know.

[JTS71 Mach1]

There are a whole lot of things you would need to know, to acurrately answer this question. Just going by how much the compression fell in 72 & 73 when they started using the dished pistons (swimming pools) a good guess would be about 9.5 to 1 to possibly 10 to 1. You could do the math. Cylinder volume at bottom dead center, to top dead center. Then measure the depth of the piston in the block, add that to the volume of the combustion chamber. Then figure the Volume of the combustion chamber with the piston at top dead center. Lets just say you had 10 inches of volume at BDC, and at TDC. you had 1 inch of volume, that would be a 10 to 1 ratio. You can do the math and come up with a fairly accurate measurement. But I'll bet I'm pretty close. JTS 71 Mach1

[keith1562]

Thanks jts71 that's the kind of ballpark figure I was hoping for. I've got a 1969 Cougar with a 351w, with 9.2:1 compression. It runs great on pump gas so that was the range I was looking for.
Thanks again,
Keith

[JTS71 Mach1]

Glad to help. JTS 71 Mach1