Combustiom chamber vs compression ratio

[galaxy]

Let's say I want 10.0:1 compression. Now let's say my engine builder says "this engine has 9.5:1 compression with a 65cc head" Is better in any way or more efficient to add heads with a 58cc chamber to raise compression or to have the engine built to have a true 10.0:1 with the 65cc head?

I guess what I'm really asking is what are the performance differences/charasteristics (other than the apparent effect on compression) between different size heads with all other things being equal?

[DAVID GAGNARD]

Galaxy;

You do not need to change the heads,you could simply cut (shave/mill) the heads down to the desired cc's.... Most closed chamber heads go like this, for every .006 you shave off the head you lose 1cc in the combustion chamber....You would have to calculate how much you would need to get from 9.5 to 1 to 10 to 1....You will also need to check for valve clearance....I would not cut more than .020 but it depends on the heads you are using... .020 would take the 65cc heads to approx.. 62cc and raise your compression some.....

Performance on a 58 cc head vs. a 65cc head assuming all other things being equal should only amount to the increased compression and corresponding increase of hp by the increased compression....

Another way to raise compression cheaply is decking or cutting the top of the block. First install the crank and a couple of pistons and measure the piston height at top dead center,I've seen some pistons as low as .075 from the factory,then you can determine how much you could deck the block down to without the pistons popping out of the block....

David

[mstngjoe]

I will assume you are talking about a SBF engine. If so, those 65cc heads just will not work if you want 10.0:1 cr.

Here's an example:

Bore............4.030
Stroke.........3.00
Cylinders.....8
Head CC......65.0
Piston vol.....-6.0cc (valve reliefs)
Gasket thickness + deck height--------.034

This is assuming you run the piston .005 OUT OF THE BORE & a .039 compressed head gasket.

With this setup you would have 9.02:1 static compression ratio.

My advice would be to find some 58cc heads and run a zero deck height with the .039 gasket. That gives you an excellent quench of .039. You should be able to run 10.5:1 with aluminum heads no problem.

Here is a link to some engine calculators:

http://www.kb-silvolite.com/temp/calc.php?action=disp

Good luck and have fun.

[CStraub]

It has been well explained here to mill the heads. To answer the power difference. At 10 to 1 compression, for every point of compression gained, you increase power by 7% of the CID of the engine. IE 306 at 11 to 1 =21HP gain. Now a 11 to 1 aluminum headed engine is equal to a 10 to 1 iron headed engine due to the thermal ineffeciny of aluminum.

CStraub