Wes Duenkel April 17, 2018

The compression ratio isn’t just a number: it’s one of the greatest determining factors in engine building. Compression ratio determines the type of fuel, how much boost, and has a significant influence on an engine’s power and torque potential.

Calculating compression ratio is pretty straightforward, but involves a little math. So, grab your calculator and let’s crunch some numbers.

Compression Ratio is the total volume of the engine divided by the clearance volume of the engine. What does that mean?

Imagine if you unscrewed one of the spark plugs and filled the cylinder with water. Measure the water it took to fill the cylinder with the piston at bottom dead center, and then divide that by the amount of water needed to fill the cylinder with the piston at top dead center. The ratio of the two different volumes is the compression ratio.

Piston design has a large impact on compression ratio. Most piston manufacturers list the volume of their pistons.

Whether the piston is above deck or in the hole at top dead center also affects compression ratio. Measure this with a dial indicator and magnetic base.

Combustion chamber design is also a large factor in compression ratio.

The total volume includes the swept volume (bore x stroke) and the clearance volume (piston dish/dome, deck, head gasket, and chamber volume).

Here’s the equation

Compression Ratio = (Swept Volume + Clearance Volume) / Clearance Volume

Example Variables

Cylinder Bore = 4.030-inch

Cylinder Stroke = 3.000-inch

Piston Dome (or Dish) = 7cc dome

Piston-to-Deck Clearance = 0.010-inch

Head Gasket Bore = 4.100-inch

Head Gasket Compressed Thickness = 0.039-inch

Combustion Chamber Volume = 60cc

Calculating Swept Volume

First, let’s figure the swept volume of one cylinder:

Swept Volume = (Cylinder Diameter / 2) Squared x 3.14 x Stroke

Example: For an engine with 4.030-inch bore and a 3.00-inch stroke, the result is 38.2 cubic inches (one cylinder of a 306 cubic inch engine).

Important: we need to keep our units consistent, so it’s easiest to convert this volume into cubic centimeters (cc). The conversion is:

1 cubic inch = 16.387 cubic centimeter (cc)

Example: 38.2 cubic inches = 626.8 cc

Swept Volume = 626.8cc

Clearance Volume

Next, we calculate the clearance volume. This includes the piston dish/dome, deck, head gasket, and combustion chamber volume.

Equation:

Clearance Volume = Piston Volume + Deck Volume + Gasket Volume + Combustion Chamber Volume

Most piston manufacturers list the volume of the dish or dome of their pistons. (If you don’t know, you can use a cc’ing kit to measure it.) Remember: dish volume increases clearance volume, and dome volume reduces clearance volume.

Example: Our piston has a 7cc dome.

The block deck height, piston compression height, rod length, and stroke all affect how much a piston is “down” or “out of the hole” when at top dead center. This affects the clearance volume of the engine, and must be calculated.

Equation:

Volume = (Cylinder Diameter / 2) Squared x height

Example: Our piston is 0.010-inch below the deck at top dead center. This equates to 0.13 cubic inches, or 2.1 cc.

Gasket thickness also affects clearance volume. (The calculation for the gasket thickness is the same as the deck volume).

Example: The head gasket is 4.100-inch diameter, and 0.039-inch thick. This yields 0.51 cubic inches, or 8.4 cc.

Obviously, combustion chamber volume is also a major component of clearance volume. Check your heads’ specifications for the chamber volume specification. However, if you’ve installed new valves or modified the combustion chamber, you’ll need to measure it with a cc’ing kit.

Example: The cylinder head manufacturer lists 60cc of combustion chamber volume.

Thus, the total clearance volume is the sum of the piston volume (-7cc), deck volume (2.1cc), gasket volume (8.4cc), and combustion chamber volume (60cc).

Clearance Volume = 63.5cc

Plugging in our figures into the first equation:

Compression Ratio = (Swept Volume + Clearance Volume) / Clearance Volume

Compression Ratio = (626.8cc + 63.5cc) / 63.5cc

Compression Ratio = 10.9:1

Run your numbers twice or three times to make sure you’ve not making any errors—but don’t worry. There are also plenty of online calculators that will help you calculate compression ratio. Even if you use an online compression ratio calculator, it’s important to understand the underlying math.