Question about CR and heads

[str8stang036]

I have been looking at low compression pistons for a 351w project I want to start. When I look at the CR for the pistons its also dependent on the size of the CC of the head. For example: 9.06CR for a 63CC head.

My question is does the CR go up or down dependig on the size of the chamber? What else determines the CR going up or down?

 

[Grn92LX]

Smaller combustion chambers increase compression. Also, the block has an effect on compression depending how much "in the hole" the piston is. 0 decking makes the piston flat w/ the block deck. Head gasket thickness has an effect also.

Mike

 

[Joes95GT]

If you are at a really high elevation, your compression ratio will be changed slighty changed as well.

Joe

 

[str8stang036]

So I am guessing that headgaskets lower the CR, since it pushes the head off of the block slightly. Making a bigger combustion chamber right? And the more "in the hole" the piston is the lower the CR?

 

[Joes95GT]

The thickness of the headgasket would determine the compression ratio. Some companies measure CR's with an .037 and some use like a .042 headgasket. So the thicker the headgasket, the higher the number, and the higher the number, the lower the compression ratio. Your also right about deckheight and piston clearance. If the piston comes .005 from the top of the deck, still in the cylinder, it will have a lower compression ratio than if it were .005 out of the hole. Basically, on an N/A motor, the closer the piston comes to the top of the combustion chamber, the higher the compression ratio.

Joe

 

[str8stang036]

Thanks guys for the reply. Its help me understand CR a little better.