[hurryinhoosier62]

Originally Posted by Jay S

Seems like some grand statements from someone that doesn’t have there facts all that correct. That’s likely one of those people that thinks all that era’s 72cc heads are actually 72 cc, which they weren’t. Plus I can’t recall a 400 with 13s ever having 10.75 SCR from the factory. The 9.75 SCR 400 13 head example was probably even high, some 400s with 13s have low 9s compression, the 13 head ran 75-78 cc, cylinder scallops on one side add 1 or 2 cc’s, pistons down about .020”, and on top of that, probably were sporting a .045” to .050” thick head gasket from the factory. Even with a small 067 cam, they would run on 91 octane pretty easy.

Lots of that eras stock grocery getter 400 engines have been rebuilt once are running around with 8VR pistons now too, compression is in the 8s. Not many engines even are configured much like how they were from the factory. Different pistons, surfaces trued up, valve jobs, different cams.

I am trying to remember the last pump gas performance engine I have put together that had less than 9.5 compression. Actual compression, not what the factory soft ball rating. I think I would have to go back into the 1990s.

Excellent points, Jay. What many forget is the advertised compression ratios for Pontiac engines and the actual compression ratios were usually .5-.75 a point apart. The 10 to 1 engines were in reality 9.25-9.5 to 1.