How to Find the Efficiency Ratio of a Cylinder

Determining your cylinder's efficiency ratio requires a variety of measurements.

Volumetric efficiency is a common measuring principle for determining the capacity of cylinders used in combustion engines. According to the Car Craft website, the compression ratio formula is the most accurate method for determining the efficiency of a cylinder, and can involve a variety of factors, including cylinder, clearance, piston, gasket and chamber volumes. For best results, pre-measure the volume and dimensions of your cylinder and enter the numbers into the compression ratio formula to determine the volumetric efficiency.

Instructions

1. Calculate the swept volume of your cylinder by multiplying the bore volume times itself, and then multiplying the sum by the volume of the stroke. Multiply this final number by .7853982 to receive the swept volume of your cylinder in cubic inches. For example: if the bore and stroke are 4 and 3.48 inches, respectively, 4 x 4 x 3.48 x .7853982 equals a swept volume of 43.730 cubic inches.

2. Determine the clearance volume by measuring the distance between the top of the piston to the engine block deck surface. Multiply the bore volume times itself again and multiply that by the distance between the top piston and the engine block deck. Multiply this last sum by .1996 to determine the clearance volume. For example, if the bore is 4 inches and the distance is 1 inch, or 2.54 centimeters, your formula is 4 x 4 x 1 x .1996 for a clearance volume of 1.5968 cubic inches.

3. Add the volumes of your head gasket, piston dome and chamber together. The Car Craft website recommends consulting the manufacturer’s catalog for your individual parts to determine the volumes of these cylinder components, since they generally require internal measurements. Combine the volumes of your cylinder swept, clearance, head gasket, piston dome and chamber together to a single sum. For example, if your vehicle is a Chevy 350 with a swept of 43.730, a clearance of .188, a piston dome of .275, a head gasket volume of .478 and a chamber volume of 4.638, your sum would be 49.309.

4. Divide this number by the sum of your clearance, piston dome, head gasket and chamber volumes to receive your final compression ratio. Using the above measurements, 49.309 would be divided by 5.579 to receive a rounded sum at 8.84, or a compression ratio of 8.84-to-1.

Tips & Warnings

• Some engines are constructed to be flush against the engine block, in which case your deck volume would be zero while calculating your clearance volume. If your engine is flush against the block, your deck volume will be zero.

• Always allow time for your engine to cool before taking measurements.

References

• Car Craft: How To Calculate Compression Ratio
• Muller: Compression Ratio
• The Old One: Piston And Piston Ring Clearance Specifications
• Great Plains Aircraft: Compression Ratio Formula