It is often desirable to lower compression ratio by elevating the cylinder base with a shim plate. A formula to calculate the required thickness of the shim is given below:
h/s = (CR1 - CR2)/((CR1 - 1)(CR2 - 1))
h = thickness of shim
s = stroke of engine
CR1 = known compression ratio before shimming
CR2 = compression ratio after shimming
Note: the formula is dimensionless - this means thickness of shim and stroke can be in inches, mm, or furlongs....just be consistent! Also consider that one gasket at cylinder base is the norm and using a shim will require two gaskets....the compressed thickness of the second gasket should be considered part of the overall shim thickness.
The formula can be worked backwards to calculate the compression ratio, CR2, that will result from a given shim thickness, h.
The formula works for the CR change that results from skimming the head, if a negative h, is used to represent the amount of head skim.
Questions or comments? PM me, texasSlick
h/s = (CR1 - CR2)/((CR1 - 1)(CR2 - 1))
h = thickness of shim
s = stroke of engine
CR1 = known compression ratio before shimming
CR2 = compression ratio after shimming
Note: the formula is dimensionless - this means thickness of shim and stroke can be in inches, mm, or furlongs....just be consistent! Also consider that one gasket at cylinder base is the norm and using a shim will require two gaskets....the compressed thickness of the second gasket should be considered part of the overall shim thickness.
The formula can be worked backwards to calculate the compression ratio, CR2, that will result from a given shim thickness, h.
The formula works for the CR change that results from skimming the head, if a negative h, is used to represent the amount of head skim.
Questions or comments? PM me, texasSlick