I am just wondering what the stock CR is for a stock 78 351 with 69cc heads- I am thinking somewhere between 8.0-8.5? Also what would be an estimated CR for that engine with 58cc heads and a .040 overbore?

 

Depends on which 351 piston you go with.They're not all the same for all years. Just the head chamber change itself will raise it a point and a half. Decide what piston you go with before head selection you need to pair both to avoid having too much or not enough compression. This also affects cam selection as well.

 

I am not changing the pistons in the engine, just the heads. I want to know the compression ratio so I know if I can use the 58cc heads on pump gas. The previous owner of the engine said that the pistons are stock replacement .040 over. Also I have a B303 cam and 1.7 rockers and would like to know if I can run them on the engine with the 58cc heads.

 

You're going from 8 to 1 to about 8.94 to 1, you should have no problem running pump gas. You might get by with 87 octane, but certainly no more than 89 to run it. That is IF they went with the same year piston that it came with (I came up with a 25 cc head volume for the 78 pistons). With rebuilds someone else did, you never know. If by chance it's got the 69-70 flat tops, the compression will be bumped to 11 to 1. The 78 pistons had a huge .105 dish in the top. The 69-76 truck piston had an even deeper .24" deep dish. If you pull the heads and the pistons are flat tops with 4 valve reliefs, you're going to have 11 to 1 with 58 cc heads. Better wait to make sure of what pistons are there before buying heads. If it does have the flat tops, you can always go with a 64 cc head.

 

Thanks alot for the info. I pulled the heads and the pistons are dished. I have a pair of C9TE heads from a 69 302 that I am going to install onto the 351.