Why calculate compression ratio when building/rebuilding an engine?

When building an engine with all new parts or with used parts, it is wise to measure the deck height and the volume of the cylinders and combustion chambers to determine compression ratio and volumetric balance.

An engine with too high a compression ratio will run hotter, ping, or require higher octane gasoline. A compression ratio of around 7:1 is ideal if you expect to use regular octane gasoline in a properly timed daily driver. If the measured ratio is higher, adjustments can be made by shimming under the cylinders.

Identifying and correcting volume disparities across cylinders result in a more efficient, smoother running, less stressed engine. If volumetric balance is off, corrections such as removing material from the smallest chambers can be made.

The compression ratio calculator spreadsheat below allows you to easily enter measurements and select a shim/CR combination to suit your needs.

If you are unfamiliar with the meaning of terms like "deck height", "volumetric balance", and "compression ratio" and would like to learn more, you should consider purchasing the Sermons of Bob Hoover CD. Any automotive engine rebuild manual worth owning should outline the significance of these terms. Mr. Hoover's CD also includes detailed information on attentive air cooled VW engine construction, as well as valuable lessons in maintenance and tuning.

Download Microsoft Excel 97 Spreadsheet containing compression ratio calculators  [17KB]

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