The engine I just built has Keith Black flat tops, with the 120 CC heads, and it's at 9.2:1 comp ratio. My info shows 76 CC heads will have 13:1 with these pistons. There are also pistons available for 76 CC heads that get 10:1.
The pistons I refer to are the hypereutectic KB type.
The standard heads for a '76 are the 120 CC type according to the info I have. They can be numbers that end with 493, or the late '76 type ending in 552.
Hope this helps:thumbsup:
425 heads should give it about 9.75:1 since it gives 74-75 a 10:1 rating. The flow may not be what you want because of port sizing, but the valves may be close. 68-73 76cc heads give about 11.5 to 12:1 on the 74-75 engine, so that should be somewhere between 11 to 11.5:1 on the 76 IF the block and/or heads have not been shaved. The 76 with stock 8.25:1 pistons is the only year model I would consider that swap on- however, premium or racing gas is all that will work in it and the compression is so high, you might damage the motor. Personally, with a 76, I would rather buy new pistons for the desired compression or shave the heads and deck the block to try to bump the sock pistons up to 8.5:1. This way I could run cheap gas at part throttle with no knock and have heads that are engineered for the cubic inches.
(I think the 8.25:1 of the 76 would make it a good turbo or small blower project engine.)