thanks. That’s also the minimum. You’ll find if you actually cc the dish it’s more than that. It’s a cast dish, not machined.

But let’s take that number anyway. what was the thinking on dishing the piston starting in 68 when the 67 was flat and had the same 10.5 rated comp ratio and the same cubes?

there is no argument they dished the piston for a reason. The 67 C head was 80 +/- and 10.5

any explanation?

 

Yes, the factory cast pistons would/could CC more. Many people did use factory style forged pistons, so, the spec was needed for those pistons. Oldsmobile supplied to NHRA specs shown below. You should note the 75 CC's minimum for all 400's except the 67 W-30 OAI which uses 71.9 CC's.





. Many people did use factory style forged pistons, so, the spec was needed for those pistons. Oldsmobile supplied to NHRA specs shown below. You should note the 75 CC's minimum for all 400's except the 67 W-30 OAI which uses 71.9 CC's.

 

Again, why did they dish the 400 piston in 68/69? i thought we were talking about as they came from the factory..none of that info is relevant

 

The increased stroke in 68 changes the compression ratio calculations (increases CR), hence the dish.

 

The swept volume is the same for early and late 400's. longer strokes dont increase comp ratio if the cyl swept volume is the same

 

woops. not sure what I was thinking earlier. I have nothing else other than some guesses. Seems like at least by 1970 they were most comfortable having actual production compression ratios at <10:1.