Compression ratio and displacement

Wilson2608

So CR is really just the relation of the combustion chamber ( and its variables such as head gasket) to the displacement of a cylinder(0.617) the larger the combustion chamber the smaller the CR?


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phunk

Exactly, displacement is all happening within the bore and stroke of the pistons, and has nothing to do with anything else.

Combustion chamber size applied to that displacement is what determines the compression ratio. There are a number of ways to change that chamber size without effecting displacement. You can change the shape of the piston dome, you can change the shape of the cylinder head chamber, you can raise or lower the wrist pin, you can change the headgasket thickness, and you can change the valve head shape. You could even change connecting rod length. All of these will change the total displacement of the chamber, but not effect the displacement of the engine.

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Wilson2608

I see! Okay I really appreciate all the fast and helpful answers. You've solved a age old question ( for me at least) hahah thanks guys!


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Jordo!

Yep -- an old tuner "band aid" trick, but it doesn't work very well...

For one thing, you have the wrong head gasket, so that can lead to leaks. For another, reducing compression without changing the pistons doesn't give you stouter parts or even necessarily reduce the likelihood of detonation (i.e., propensity for hot spots along the piston crown remain unchanged).

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DOOMMONKEY777

Use the Volume formula duhh,

V=[(π×d2)/4]×gasket

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SouthArk370Z

CR = ( pi / 4 * bore^2 * stroke + combustion chamber volume + gasket ) / ( combustion chamber volume + gasket )
works as long as you have flat-top pistons and, at TDC, the top of the piston is flush with the top of the block.

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Read T

If I ever want to lower compression can I just unscrew the spark plugs a little bit?

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Wilson2608

I don't think that will work , those plugs need to be tight because the pressure in the cylinders will probably be to much for them to handle.


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Rusty

Think you missed the sarcasm in his reply. LOL

On another note. I've seen a couple of valve jobs done that had a couple of valve seats cut too deep. So now the valves are deeper into the head, which lowers compression too.

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