# Compression Ratio

Written by Jerry Ratzlaff on . Posted in Fluid Dynamics

The compression ratio, abbreviated as CR, a dimensionless number, in an engine is the ratio of the maximum volume to the minimum volume in a cylinder.  The equation and calculator below can help find the compression ratio.

## compression ratio formulas

 $$\large{ CR = 1 + \frac{\pi}{4} \; \frac { BORE^2 \; STROKE }{CCV \;+\; HGV \;+\; PVD } }$$ $$\large{ CR = \frac{ V_d \;+\; V_c }{ V_c } }$$

### Where:

 Units English Metric $$\large{ CR }$$ = compression ratio $$\large{dimensionless}$$ $$\large{ BORE }$$ = bore $$\large{in^2}$$ $$\large{mm^2}$$ $$\large{ V_d }$$ = clearance volume $$\large{in^3}$$ $$\large{mm^3}$$ $$\large{ CCV }$$ = combustion chamber volume $$\large{in^3}$$ $$\large{mm^3}$$ $$\large{ V_d }$$ = displacement volume $$\large{in^3}$$ $$\large{mm^3}$$ $$\large{ HGV }$$ = head gasket volume $$\large{in^3}$$ $$\large{mm^3}$$ $$\large{ \pi }$$ = Pi $$\large{3.141 592 653 ...}$$ $$\large{ PDV }$$ = piston deck volume $$\large{in^3}$$ $$\large{mm^3}$$ $$\large{ STROKE }$$ = stroke length $$\large{in}$$ $$\large{mm}$$