how would a person figure the compression ratio on a banshee?

 

The determine the Uncorrected Compression ratio:

You need to get the trapped volume. This is the volume that is trapped at TDC.

Trapped volume= FPV + HGV + DHV - PCDV

Where:
FPV is the flat plate volume of the combustion pocket.

HGV is the volume of the head gaskets thickness

DHV is the deck heigth volume the positive or negative hiegth converted to volume.

PCDV is the piston crown dome volume. The volume the piston crown displaces and the periphery of the piston itself to the top ring.

Once you have establised the trapped volume then take:

(Cylinder volume + trapped volume)/ trapped volume = UCCR:1

The corrected compression ratio really only takes into account the volume above the piston at the point that the exhaust port is fully closed to the volume above the piston at TDC. The UCCR method is by far the better way of determining a suitable compression ratio in a motor, especially when you start to really lean on it to get the power out of it. One of the dynamics that a two stroke engine has, the trapping efficiency, will actually allow you engine over a narrow "powerband" to trap more volume than the cylinder actually is. Your 250 cc can trap more than 250 cc's, in highly tuned engines as high as 275+ cc's.

If you would like help in determining the FPV, HGV, DHV, and PCDV email me. That's why there are all sorts of programs available to make this easier. You still have to do the work of getting the numbers, but then you let the program do the rest of the work.