How to determine the compression ratio

Whether you're building a new engine and need a metric, or you're curious about how fuel efficient your car is, you should be able to calculate an engine's compression ratio. There are several equations required to calculate the compression ratio if you are doing it manually. They may seem complicated at first, but they are really just basic geometry.

An engine's compression ratio measures two things: the ratio of the amount of gas in a cylinder when the piston is at the top of its stroke (top dead center, or TDC), compared to the amount of gas when the piston is at its bottom. stroke (bottom dead center, or BDC). Simply put, compression ratio is the ratio of compressed gas to uncompressed gas, or how tightly the mixture of air and gas is placed in the combustion chamber before it is ignited by the spark plug. The denser this mixture fits, the better it burns and the more energy is converted into power for the engine.

There are two methods you can use to calculate the compression ratio of an engine. The first is the manual version, which requires you to do all the math as accurately as possible, and the second—and probably the most common—requires the pressure gauge to be inserted into an empty spark plug cartridge.

Method 1 of 2: Measure the compression ratio manually

This method requires very precise measurements, so it's important to have very accurate tools, a clean engine, and double or triple check your work. This method is ideal for those who are either building an engine and have the tools on hand, or those who already have the engine dismantled. To use this method, it will take a very long time to disassemble the engine. If you have an assembled motor, scroll down and use method 2 of 2.

Necessary materials

• Bore gauge
• Calculator
• Degreaser and clean rag (if necessary)
• Manufacturer's manual (or vehicle owner's manual)
• micrometer
• Notepad, pen and paper
• ruler or tape measure (must be very accurate to the millimeter)

Step 1: Clean the engine Thoroughly clean the engine cylinders and pistons with a degreaser and a clean rag.

Step 2: Find the hole size. A bore gauge with a scale is used to measure the diameter of a hole or, in this case, a cylinder. First determine the approximate diameter of the cylinder and calibrate with a bore gauge using a micrometer. Insert a pressure gauge into the cylinder and measure the bore diameter several times at different locations inside the cylinder and record the measurements. Add up your measurements and divide by how many you took (usually three or four is enough) to get the average diameter. Divide this measurement by 2 to get the average hole radius.

Step 3: Calculate the cylinder size. Using an accurate ruler or tape measure, measure the height of the cylinder. Measure from the very bottom to the very top, making sure the ruler is straight. This number calculates the stroke, or area, that the piston moves up or down the cylinder once. Use this formula to calculate the volume of a cylinder: V = π r² h

Step 4: Determine the volume of the combustion chamber. Find the combustion chamber volume in your vehicle owner's manual. Combustion chamber volume is measured in cubic centimeters (CC) and indicates how much substance is required to fill the opening of the combustion chamber. If you are building an engine, refer to the manufacturer's manual. Otherwise, refer to the vehicle owner's manual.

Step 5: Find the Compression Height of the Piston. Find the compression height of the piston in the manual. This measurement is the distance between the centerline of the pin hole and the top of the piston.

Step 6: Measure the piston volume. Again in the manual, find the volume of the dome or piston head, also measured in cubic centimeters. A piston with a positive CC value is always referred to as a “dome” above the compression height of the piston, while a “poppet” is a negative value to account for valve pockets. Typically a piston has both a dome and a poppet, and the final volume is the sum of both functions (dome minus poppet).

Step 7: Find the gap between the piston and deck. Calculate the amount of clearance between the piston and deck using the following calculation: (Bore [measurement from step 2] + Bore diameter × 0.7854 [constant that converts everything to cubic inches] × distance between piston and deck at top dead center [TDC] ).

Step 8: Determine Pad Volume. Measure the thickness and diameter of the cylinder head gasket to determine the gasket volume. Do this in much the same way as you did for the deck gap (step 7): (hole [measurement from step 8] + hole diameter × 0.7854 × gasket thickness).

Step 9: Calculate the compression ratio. Calculate the compression ratio by solving this equation:

If you get a number, say 8.75, your compression ratio will be 8.75:1.

• FunctionsA: If you don't want to figure out the numbers yourself, there are several online compression ratio calculators that will work it out for you; Click here.

Method 2 of 2: use a pressure gauge

This method is ideal for those who have an engine built and want to check the car's compression through the spark plugs. You will need the help of a friend.

Necessary materials

• pressure gauge
• Spark plug wrench
• Work gloves

Step 1: Warm up the engine. Run the engine until it warms up to normal temperature. You don't want to do this when the engine is cold because you won't get an accurate reading.

Step 2: Remove spark plugs. Turn off the ignition completely and disconnect one of the spark plugs from the cable connecting it to the distributor. Remove the spark plug.

• Functions If your spark plugs are dirty, you can use this as an opportunity to clean them.

Step 3: Insert pressure gauge. Insert the tip of the pressure gauge into the hole where the spark plug was attached. It is important that the nozzle is fully inserted into the chamber.

Step 4: Check the cylinder. While you hold the gauge, have a friend start the engine and accelerate the car for about five seconds so you can get the correct reading. Shut off the engine, remove the gauge tip and reinstall the spark plug with the correct torque as directed in the manual. Repeat these steps until you have tested each cylinder.

Step 5: Perform a pressure test. Each cylinder must have the same pressure and must match the number in the manual.

Step 6: Calculate PSI to Compression Ratio. Calculate the ratio of PSI to compression ratio. For example, if you have a gauge reading of about 15 and the compression ratio should be 10:1, then your PSI should be 150, or 15x10/1.