What should the compression on a fresh 425 with 10.25 Egge pistons be (roughly). How much variation per cylinder? I'm getting readings from about 130 - 150. But the battery was low and I didn't open the carb so will do again.

 

Enter your info. on your build into this calculator...

http://www.projectpontiac.com/ppsite...nt/view/16/30/

 

Thanks, tried it but getting NaN in the compression columns? The following are the build specs:

Bore - 4.155
Stroke - 3.975
Combustion Chamber - 80cc
Deck Height - .015
Head Gasket - .040
Piston Dish - .045
Connecting Rod length - 7.00
Cam Intake duration - 266
Lobe seperation - 110

Also, was looking for actual compression readings not ratios (my mistake). Thanks.

 

There were two types of dished pistons for the 425. The 10.5:1 CR and the 10.25:1 CR, using two different dish volumes of 4.5cc and 7cc. Using a 4.5cc dished piston, your as built 425 has roughly a 10.1:1 CR. I think you needed to convert the piston dish amount to a volume in cc's to get the calculator to work. I would guess your cranking cylinder pressure with that cam should be around 165psi. With a dynamic compression of around 8.2:1 you should be able to run pump premium with the engine tuned properly.

http://www.projectpontiac.com/ppsite...nt/view/16/30/

 

After getting the battery charged and running the compression tests again, I'm running 150 - 160 in each cylinder cold. Does ths sound about right on a rebuilt motor?

 

That sounds pretty good.