Static Compression Ratio
 

Ok, I need help to find where my calculation is wrong. I'm trying to create a spreadsheet for calculating static compression ratio. When I plug in the numbers from my old GTO to test it out, it is coming up light. Here's what I have.

Chamber Volume = 72 cc's
Gasket Volume = 9.28 cc's (4.3" bore x .039" crushed thickness) Fel Pro 1019
.020" in the hole = 4.37 cc's (4.12" bore x .020" below deck)
Valve Relief Volume = 6.7 cc's (TRW Forged Slugs)
Cylinder Volume = 819.25 cc's (4.12" bore x 3.75" stroke)

Formula = (72+9.28+4.37+6.7+819.25)/(72+9.28+4.37+6.7)
=911.6/92.35
=9.87:1
Factory rating was 10.75:1. What is wrong with my math?

Nothing. I get the same answer.

Stan

The factory advertised compression ratios listed are generally optimistic by 1/2 to one full point. This has become a pretty well known fact in the Pontiac hobby.

Compression?
 

So then with a compression ratio of 10:75, what would the cranking pressure be? How about 9:5 to 1, Just asking as my 127 headed 421 had a cranking pressure of 220 psi. How does long duration versus short duration affect this? Please edumacate me.:confused: Also i am at 4700 feet elevation dose that make a difference?

The higher the CR the higher the Cranking PSI
The lower the CR the lower the Cranking PSI
The earlier the IVC the higher the Cranking PSI
The later the IVC the lower the Cranking PSI
Yes 4700 feet elevation will have less Cranking PSI than the same engine will at sea level.

Stan

PS What are the specs for your engine which cranked 220 psi @ 4700 feet?

That makes sense, glad my math was correct. I always wondered how a '68 428 GP could have the same CR as a '68 400 GTO with the same 16 heads. Obviously, it can't/doesn't.

The 428 engines all used a piston with a 12cc dish so they could use the same heads as a 400 and maintain close to the same compression ratio.

Similar deal with 421 and 389 engines. The 421 pistons had very large deep valve reliefs as compared to the shallow small reliefs on 389 pistons. Definitely a big difference in valve relief volume between the 421 and 389 pistons, very likely a 6cc difference between the two.

http://i137.photobucket.com/albums/q...h/DSC03699.jpg

http://i137.photobucket.com/albums/q...h/DSC03698.jpg

Your formula is also not taking into account the average 1.5 CCs in compression ring back land clearance for both rings if your top ring is not hapless.

Thanks, B-Man. I never knew the 421/428 had larger valve reliefs. I've never actually seen one. I only worked on 400's and 455's.

Cr vs CP
 

Stock bore and stroke 421 127 heads #8 cam original SD dual point distributer in a 4000 pound 61 Bonneville 220 muncie 390 rear. race gas. Heads were from Micky Thompsons sale at his shop in Calif. never cc'd them so have no idea if they were milled down but they are ported pretty far. The original dual quad manifold ports line up perfectly. Flat top piston, HO block arma steel crank. Do have a NOS 990 crank but I'm saving it.:old: Tough on tires.