How To Calculate Engine Compression Ratio And Displacement

The difference is that the dome volume.
Doing the maths.

In the old days, calculating the CR meant getting out the slipstick (really while ago)
Or working through a group of formulas on a hand-held calculator. Today, finding online calculators that quickly spit out the results is merely Google click away.
But because the old saying goes,
A computer is merely nearly as good because of the quality of data it gets.

Measurements needed to determine the CR:

• Cylinder bore diameter
• Crankshaft stroke length
• Head gasket bore diameter
• Head gasket compressed thickness
• Combustion chamber volume
• Piston dome volume
• Piston deck clearance volume

There are a few high-tech calculators online that invite even more,
Like rod length and distance from the primary compression ring to the highest of the piston. The latter will help provide volume above the highest ring,
But this measurement doesn’t usually affect the ultimate calculation significantly and
It is employed only in very critical applications.

Most gaskets, such as this JE Pro Seal unit, provide the gasket volume and compressed thickness values to help compute the CR.

Online calculators generally offer a choice of entering all measurements in either inch
Or metric, apart from the combustion chamber and piston dome volumes,
Which are always entered as cubic centimetres or cc.

Many of today’s aftermarket suppliers provide their respective measurements for off-the-shelf parts,
Which is quite half the battle in quickly determining your engine’s CR within reasonable accuracy.

“Too many of us get hung abreast of tenths of some extent in CR but fail to know the
Consequences of fluid dynamics thanks to appropriate cam selection and phasing,
For instance,” says Stevenson. “If everything else is well-matched, a difference in 0.1 of a ratio is negligible for love or money shy of maximum effort professional racing.”