Official Montana 500 Website

Compression Ratios

Here is some information that might be of interest to people. Thanks to Larry Young of Tulsa, Oklahoma for this. Larry cc'd various heads. These are the results he got. The total volume of the cylinder is 44.2 cubic inches. The total volume of the part of the piston that sticks up above the bore is 2.86 cubic inches. The head gasket holds .92 cubic inches. The total combustion chamber is the head volume minus the volume of the part of the piston that sticks above the bore, plus the volume of the head gasket. Using the stock head as an example: 17.9 minus 2.86 plus .92 = 15.96 cubic inches for the total volume of the combustion chamber. Cylinder volume plus combustion chamber volume divided by combustion chamber volume equals the compression ratio. (44.2 + 15.96)/15.96 = 3.77:1

The first number will be the head volume, the second will be the calculated compression ratio.

Stock head 17.9 = 3.77
Stock (milled 1/8") 15.6 = 4.24
Waukesha-Ricardo 16.6 = 4.02
Z-head 13.7 = 4.76
Z-head (milled .050") 12.8 = 5.07
Reeder 12.2 = 5.31

Now the same figures for an engine bored .060" over.

Stock head 3.86
Stock (milled 1/8") 4.34
Waukesha-Ricardo 4.11
Z-head 4.88
Z-head (milled .050") 5.2
Reeder 5.44

The Montana 500 is Sanctioned by The Montana Cross Country T Association.