Video: MAHLE Explains How To Calculate Compression Ratio

MAHLE Motorsports makes some really helpful videos for engine tech. They are extremely helpful for starting, or dialing in your projects. This video tells us how to calculate engine compression the easy way (don’t worry, it’s not done using common core math).

Compression ratio is a value that represents the ratio of the volume of the engine combustion chamber from its largest capacity to its smallest capacitor. The actual ratio is calculated by determining the total volume of the engine, divided by the engine clearance volume. This specification is fundamental to know, especially for engine builders.

The total volume includes the swept volume plus the clearance volume. The swept volume is another way of saying cubic inch displacement, although to get the compression ratio, you only consider one cylinder. Clearance volume is determined by different variables: combustion chamber volume, piston dome or dish volume, the gasket thickness and diameter, and where the piston stops.

combustion-2Swept volume is pretty simple. Using a small block Ford, the cylinder was measured out as 41.66 cu. in. Converting to cubic centimeters to make it consistent, it comes out to be 682.7cc.

Clearance volume is a little more complicated. Combustion chamber, gasket, and piston volumes are available from the manufacture’s specs. To calculate the deck clearance volume, you’ll need to know the bore diameter and deck clearance.

Putting it all together, you’ll use all of these variables to get the compression ratio. Once you have gathered everything simply plug it into the downloadable combustion-3calculator from MAHLE (wasn’t that nice of them to make that?) here.

Calculating compression ratio can be extremely confusing, and most explanations are hard to follow. MAHLE provides the most comprehensive explanation we’ve heard in a while, so hopefully you’ve learned something. Plus, since their calculator does the really hard work for you!

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