Jim---The compression ratio generally given for an engine is the
geometric ratio, determined by the cylinder volume. This would be taken
when the piston is at the bottom, (BDC) and again with the piston at the
top of the stroke. (TDC) The gap, or volume left between TDC and the
head combustion chamber is then factored in. As an example--If the total
volume of a cylinder from BDC up to and including the head were divided
into nine equal parts, the compression ratio would be 8.0: 1.
Confusion can come when trying to reconcile why, as you pointed out,
your compression gauge doesn't show 15psi (atmospheric pressure) X 7.75
or approx. 116 lb. I'll try to explain this, but don't know how
successful this will be. The gauge reading when everything is
according to factory specs will be in the 140 psi area. To get this
figure involves more than just the geometric compression ratio. It also
includes such things as volumetric efficiency, as how much air can the
piston pull in thru the valve porting, and hold it, while this air is
then squeezed into a pressure reading. The squeezing of this trapped
air heats it up and increases its rated pressure.
Reducing the space in the head thru milling not only raises the
geo/ratio, it also reduces the 'left over' volume. This raises the
gauge pressure thru the extra heat generated by squeezing the air more.
To add a little spice to all of this, one could have a geometric
compression ratio of 12.0: 1 and get a cylinder pressure reading of 140
lb., if the camshaft was radical enough to allow compression to 'bleed
out' during the valve overlap.
Compression and pressure are related, but different.
Clear as mud?
Dick Taylor
|