If you take a stock '69 L-36, 427/390 with 10.25 cr stock 96cc chamber heads and change to #241 '71 105cc heads, can the compression ratio be calculated with only this info?

 

There are many factors involved to calculate CR...right down to the head gasket.
Hopefulluy Jackson will see this thread and let you know exactly what is required

The man knows his stuff

 

If the heads in your post have not been shaved, I have different numbers for the combustion chamber volume than you do.

According to the books "Chevrolet by the Numbers" by Alan Colvin,

1969 427/390 HP motors were fitted with head #3931063. The combustion chamber volume is 100.967 cc. They were used on late 68 and 69 Corvettes and other applications.

The 241 head is listed as head #3999241 for 1972 454/270 HP Corvettes and other applications. It has the tapered spark plug seat and oval ports. It was used into the 1974 model year and the cumbustion chamber volume is 113.060 cc.

In order to calculate the static compression ratio you have to know;

(1) the swept volume (bore and stroke)
(2) the head gasket thickness volume
(3) the deck clearance volume
(4) the piston dome/dish/valve relief volume
(5) the combustion chamber volume

The formula for static compression ratio is:

Flat Top Pistons with no Valve Reliefs
(1+2+3+5) divided by (2+3+5)

Domed Pistons
(1+2+3-4+5) divided by (2+3-4+5)

Dished Pistons and Pistons with Valve Reliefs
(1+2+3+4+5) divided by (2+3+4+5)

I think I've got this correct but someone should check my math.

 

This is the easiest to use and most accurate I've found:

http://www.rosspistons.com/calculator.php

HTH

 

Since you probably aren't trying to compete in some professional racing class where the last 1.33 Hp matters, as long as you know for sure the chamber volume of those particular P/N's it would be close enough to divide the old heads' volume by the new heads' volume and multiply that result by the old compression ratio.

 

Thanks everyone. Yea, not trying to get down to the last decimal point, just a general idea. Seems like different books list different specs for different heads. So with pop up pistons and unknown head gasket thickness, or even knowing if the heads have been milled, it appears that a previous owner installed these heads in order to run low octane gas. Engine runs fine. Was just thinking of the lazy way out. I think I'll go have a beer and don't think about it again.
PS The way I found out that the heads were changed, was that it was running with the old style plugs, which were correct for the 69 engine, and they actually fit in and the engine ran, but obviously not very well.

 

All things being equal on 427BBC ... except a SINGLE change from 96cc chamber to 105cc ... if , IF it is 10.25:1 now w/ 96cc ... it should end up right at 9.45:1 w/ 105cc.

BTW ... I do not KNOW the cc of either head ... I'm not disputing either ... I just don't know ... so, I used your 96 & 105 figures.

Also ... for grins & giggles ... using the 101 vs 113: if 10.25:1 now / 101 ... should be right at 9.21:1 w/ 113. Again, I don't KNOW cc of either head.