compression

[seniorchristo]

I have calculated the compression ratio on my 1680cc twincam using Dave Bean's formula.
CR = SV + CV divided by CV
where: SV = swept volume
CV = clearance volume
and where: CV = HV + PV + GV + DV + LV
GV above is the volume of the head gasket

When using a .060" gasket, the compression ratio is 10.4 to 1
When using a .030" gasket, the compression ratio is 11.45 to 1

I didn't think head gasket thickness had such a large affect on CR but I recalculated and got the same results.

[englishmaninwales]

Are those the measurements for the CH gaskets in use i.e compressed thickness and using it’s actual fire ring diameter, not the bore dimension?

[seniorchristo]

I used those gasket thickness dimensions only as a rough guide to see how they affect the compression ratio. I actually plan to use a cometic CFM gasket which will compress to about .059".
I am surprised how much a .030" difference in thickness affects the CR. That is, if my math is correct.

[2cams70]

It does make quite a bit of difference the thickness of the head gasket.. What is LV in the formula?
I assume:
CV = compressed volume
HV = head chamber volume
PV = piston valve pocket volume
GV = head gasket volume
DV = dead volume (volume to head face at TDC excluding gasket)
LV???

[seniorchristo]

LV is headland value. That little bit of space below the piston top down to the top ring
On my engine it figures to .3 cc.

[2cams70]

Thanks - that makes sense