Compression Ratio

Compression Ratio is a rating of the amount of "squeeze" the engine puts on the air-fuel mixture before it is ignited.  Mathematically, it is the ratio of the total cylinder volume at bottom dead center (BDC) to the volume at top dead center (TDC).

    Compression Ratio = Cylinder Swept Volume + Combustion Chamber Volume
                                                      Combustion Chamber Volume

Compression Ratio Example

Say you have a single cylinder engine with 1000 cc of air when the piston is at the bottom of the stroke (900 cc in the cylinder plus 100 in the combustion chamber). Then when the piston is at the top of the stroke, the "cylinder" has no volume, and the only volume is from the chamber's 100 cc.  That would give you a compression ratio of 10:1.

The higher the compression ratio the more energy can be extracted from the air-fuel mixture. Therefore a higher compression ratio is desirable. But a higher quality (octane) of gasoline is required for high compression ratio engine. If you use a lower quality gasoline the engine may "knock".

Dynamic Compression Ratio

In the real world the intake valve is usually still open when the piston is at Bottom Dead Center. Therefore the "squeeze" does not start to happen until the intake valve closes.  In order to take this into account "Dynamic Compression Ratio" was developed. It is always going to be lower than "Static" Compression Ratios.

    Dynamic Compression Ratio = Cylinder Swept Volume (when intake valve closes) + Combustion Chamber Volume
                                                                                  Combustion Chamber Volume

Programs that take Compression Ratio into account:

We have produced 2 programs that are specific to Compression Ratio. Compression Ratio Calculator v2.3 and Compression Ratio Calculator Plus v2.3.

Our Engine Analyzer Pro v3.9 program can also do simplified Compression Ratio calculations along with Our Engine Analyzer v3.2 and Engine Analyzer Plus v3.2.

And our Engine Log Book will keep track of the Compression Ratio of each engine build. Plus it will directly connect with our Compression Ratio Calculator v2.3 and also has simplified Compression Ratio calculations built in.

Plus we have 2 programs that work with out Datamite systems that also have very simplified Compression Ratio calculation options: Dyno Datamite v3.7 and Port Flow Analyzer v3.5