How to compute compression ratio
How to compute compression ratio
If you're tired of just guessing at what your compression ratio is, why not figure out exactly what you have? It's not very hard to do, and if you're assembling a performance engine, you should already have all the numbers you need to do it.
Compression ratio is the volume of the cylinder at BDC (Bottom Dead Center) divided by the volume of the cylinder at TDC (Top Dead Center). The measurements you will need to do this are bore, stroke, combustion chamber volume, head gasket bore size and thickness, deck clearance (distance from the top of the block to the top of the piston) and dome/dish volume of your pistons (where applicable), as well as the volume of the valve reliefs in the pistons.
The first step is to figure out the volume of the cylinder itself. The formula for this is � the bore x � the bore x 3.1415927 x stroke. Using a 360 as an example, the bore size is 4.000, the stroke is 3.580. The displacement for the cylinder (piston travel only) would
be (2 x 2 x 3.1415927 x 3.580). This comes out to 44.99 cubic inches. Remember, this is just the volume of the piston travel, not the entire cylinder.
Next, you need to figure out the volume of the combustion chamber in cubic inches. First you will need to get a burette and find out the volume in cubic centimeters (cc's). Once this volume is known, it can be converted to cubic inches by dividing it by 16.387. So, for
example, with a chamber volume of 58 cc's, dividing by 16.387 comes out to 3.54 cubic inches. Write this figure down - you'll need it later.
Head gasket volume can be calculated using the same formula used for the cylinder displacement, using the gasket bore size, but instead of stroke, the gasket thickness is used. In our example 360, the gasket displacement would be 2 x 2 x 3.1415927 x .040, assuming a .040" compressed thickness. This comes out to .50 cubic inches. Deck clearance is figured the same way - .065" deck clearance is .81 cubic inches. Again, write down these values as they will be needed for final calculations.
This leaves only two variables - valve reliefs and dish/dome volume. Your piston manufacturer should be able to tell you these values.
If your pistons have a dome on them, it will subtract from the overall chamber volume - dishes will add to it, as will the valve reliefs. Let's assume our 360 has flat top pistons and 5 cc valve reliefs. This means that there will be no value for a dish or dome, so all
that needs to be taken into account is the valve reliefs. After this measurement gets converted to cubic inches (.305), you are ready to calculate your static compression ratio.
To calculate the total volume of the cylinder at BDC, all the values will be added together. Here are the volumes for the example 360:
piston travel - 44.99 cubic inches
combustion chamber - 3.54 cubic inches
head gasket - .50 cubic inches
deck clearance - .81 cubic inches
valve reliefs - .305 cubic inches
44.99 + 3.54 + .50 + .81 + .305 = 50.145 cubic inches at BDC
Next, volume of the cylinder at TDC is calculated, which includes everything above except for piston travel, so 44.99 is subtracted from 50.145 to come up with 5.155 cubic inches at TDC.
Divide 50.145 by 5.155 and that is the actual static compression ratio of 9.73:1
Now there should be no more need for you to have to guess at what your compression ratio is - just because those pistons you got are advertised as 10:1, it doesn't necessarily mean that's how it's gonna be.
