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Under Pressure: How to Measure Compression Ratios

Since race pistons in domestic V8 engines move up and down in excess of 100 times per second, replacing them is a normal part of the racer’s routine.

NHRA Top Fuel and Funny car teams replace them after every race pass and every second qualifying pass. Pro Stock teams replace them after every 40 passes approximately, and weekend warriors replace them every 12 to 18 months, sooner if their engines are nitrous-assisted. At piston replacement time questions of spec changes usually arise — especially the topic of compression ratios. 

When considering the correct compression ratio, it's important to remember that an engine’s compression ratio is calculated by taking the total swept volume (with the piston at bottom dead center) and dividing it by the total compressed volume (with the piston at top dead center). For example, if the total swept volume of a 632cu in big-block Chevrolet is 1380.34cc (cubic centimeters) and the total compressed volume is 86.69cc the compression ratio would be stated as15.92:1.

To find the engine’s total swept area in cubic inches, the following formula can be applied: 0.7854 x bore diameter x bore diameter x stroke length x the number of cylinders. To convert cubic inches to cubic centimeters multiply by 16.39. Using a burette is the best method of measuring the compressed volume (chamber volume plus piston volume).

Compression ratios are often influenced by rule book regulations. Also, engine efficiency is a deciding element in their composition. Higher compression ratios are not always the ones best calculated to succeed. When you over-compress the cylinder, you induce what's called a ‘pumping loss’ — it takes horsepower to squeeze the cylinder’s contents.

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