Compression Ratio Calculator

Determining the Actual Compression Ratio for Your Engine
Article Date: March, 2022

Article and Photography by Mark Quasius

It's important for an engine builder to know what the compression ratio will be on an engine. Higher compression produces more power and is more efficient. But higher compression ratios also require premium fuels and introduce more stress on an engine so you'll need to determine how far you want to go. Really it's nothing more than comparing the volume of the cylinder at bottom-dead-center to its volume at top-dead-center. This can be calculated if you are good at long mathematical equations but I've done that for you and created a form below so that you can enter in your parameters to see what the compression ratio of your engine will be. This will help you determine what pistons and cylimnder heads you will want to select for your engine build project.

First of all you'll need some basic information about your engine. The bore and stroke is easy enough to find but you'll also need to know the volume of the cylinder head's combustion chamber and the volume of the piston's dish. Piston suppliers will have the piston dish volume available in their literature or on their webnsite so that's easy enough. Cylinder head volume may be a bit harder. If you can't find that information or have made some changes to the chamber by porting and polishing you may need to measure that with a cylinder head CC'ing kit, available at a number of performance equipment outlets. Lastly you'll need to know how far the piston rises up in the cylinder. Not every piston goews to the very top of the cylinder block so you will need to measure the offset with a dial indicator or digital caliper. you'll also need to know the thickness of the head gasket so keep your caliper handy. Once you have these six measurements just plug them into the form below to get your answer.

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