Compression ratios are calculated with piston displacement volume and clearance volume. Piston displacement (sometimes called swept volume) is the volume displaced by the piston’s travel from bottom dead center to top dead center. Clearance volume is the combination of cylinder head volume (CHV), piston effective dome volume (EDV), deck clearance volume (DCV) and cylinder head gasket volume (HGV); line 1 through 4 calculate these volumes and then are used in step 5 to determine clearance volume (CV).

Line 1:

This calculation changes the cylinder head volume (CHV) from cubic centimeters or cc’s (the common unit of measurement) to cubic inches (cu. In.), the unit of measurement needed to figure the compression ratio. Multiplying the number of cc’s times the “metric conversion factor”, 0.061, gives the CHV in cubic inches.

Enter the cylinder head combustion chamber volume amount in cc’s in the box on line 1 and multiply.

Line 2:

The second step changes the piston “effective dome volume” (EDV) from cubic centimeters to cubic inches,

just as in the previous step. The EDV can be a positive or negative number, depending on whether the piston has a dome, a dish (or cup), or valve relief’s. The EDV of domed pistons is equal to the dome volume minus the valve relief volume. EDV figures for our performance pistons are available in the numerical list of performance pistons in the 604 piston catalog. Enter the EDV (in cc’s) for your piston into the box on line 2 and multiply.

The volume of dished pistons may be calculated by the following: 0.7854 times (diameter of dish) squared, times depth of dish = (-EDV) cu. In.

Note: If the above formula is used, enter the value as a negative number and at the end of line 2. This formula calculates EDV in cubic inches. It does not have to be multiplied by the conversion factor 0.061.

Line 3:

The third step calculates deck clearance volume (DCV). Deck clearance is the distance from the top of the piston (at top dead center) to the block deck surface. This measurement can be taken with a depth micrometer. Once the deck clearance (DC) is determined, it can be “plugged into” the formula in the box marked DC. Remember to square the bore diameter (BD times BD).

Note: Piston below deck is a positive deck clearance. Piston protrusion (above deck) is a negative deck clearance.

Line 4:

Step four calculates cylinder head gasket volume (HGV), where GID = gasket inside diameter, CGT = compressed gasket thickness.

Note: Remember to square the GID (GID times GID). If you know the cc’s of the head gasket cylinder opening, use the following conversion: cc’s times 0.061 = HGV cu. In. (and enter the value at the end of line 4).

Line 5:

Now we are ready to calculate clearance volume (CV). Enter the values from steps 1 through 4 on line 5. At this time all values should be in cubic inches; if your effective dome volume is a negative figure, enter it as such (-EDV). Remember when subtracting a negative number, the two negatives become a positive, (example: 4.64 -(-.37) = 5.01).

Line 6:

This line determines piston displacement (PD) for one cylinder in cubic inches. To determine total engine displacement, multiply by the number of cylinders. Enter the bore diameter squared and the crankshaft stroke.

Line 7:

Now we have the two values needed to determine the compression ration, piston displacement (PD) and clearance volume (CV). Enter the values on line 7 and complete the calculation.

The above steps and work sheet do not factor in the head land volume. Head land volume is the piston to bore clearance, between the top compression ring and the top of the piston. In most cases, the head land volume is minimal and would lower the compression ratio by one tenth (0.1) or less.