Math wizards?
Thread Starter
Safety Car
Joined: Sep 2005
Posts: 4,213
Likes: 643
From: Orange County Ca
Math wizards?
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?
Instructor
Joined: Dec 2004
Posts: 236
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From: Clearwater Florida
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.
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.
Last edited by 67-73-76; Oct 15, 2006 at 09:13 AM.
Drifting
Joined: Aug 2004
Posts: 1,883
Likes: 124
From: Lubbock TX
This is the easiest to use and most accurate I've found:
http://www.rosspistons.com/calculator.php
HTH
http://www.rosspistons.com/calculator.php
HTH
Burning Brakes
Joined: Jul 2005
Posts: 787
Likes: 0
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.
Thread Starter
Safety Car
Joined: Sep 2005
Posts: 4,213
Likes: 643
From: Orange County Ca
Originally Posted by evil herbavore
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.
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.
Le Mans Master
Joined: Sep 2001
Posts: 7,739
Likes: 630
From: Unreconstructed, South Carolina
Originally Posted by glenkov48
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?
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.
Last edited by jackson; Oct 15, 2006 at 05:57 PM.
