Cylinder Head Combustion FAQ

What’s the point of all of the different combustion chamber volume on RHS™ heads and which size is correct for my engine?

The combustion chamber volume of your cylinder heads is one of the most important variables controlling the compression ratio of your engine.

There are a number of engine variables that determine its compression ratio, among them are bore, stroke, combustion chamber volume, head gasket bore and compressed thickness, deck height, and piston top characteristics (i.e. dome, valve relief, or dish volume). It is important to keep in mind that once you have purchased your rotating assembly and cylinder heads, your compression ratio is virtually set in stone; short of machine work to your heads and/or pistons, nothing at that point will make significant changes in compression -little things like switching to a thicker or thinner head gasket can make “10th of a point” differences in compression, but that’s about it.

For an example of how chamber volume affects your compression ratio, take an engine with a 4.0” bore, 3.75” stroke, and 5.5cc valve reliefs in its pistons. Running a 72cc cylinder head with this engine will produce a 9.8:1 compression ratio. With a 50cc head, the same engine will have a 12.7:1 compression ratio.

As mentioned previously, with a cast iron-headed motor on the street, somewhere in the range of 9.5:1 is the practical limit for compression; with aluminum, 10.5:1 or so is a pretty good maximum figure. Of course, race engines are typically built with very high compression levels in order to build maximum power (14.5:1 and higher is not at all uncommon), but these are specialized motors designed to be run at high rpm, with stratospheric fuel octane levels, and a lot more camshaft than can be run on the street.

So when choosing a combustion chamber volume, it is important to have a very good idea of what the other specifications of your engine components are (or will be), so that you can be sure that the compression ratio that you end up with is appropriate for your application and intended use of your vehicle. Remember that an aluminum head is more tolerant of high compression, and all other factors aside, a higher compression motor will make more power than a low compression motor, provided that its unique compression-related requirements (fuel octane, camshaft, etc.) are met.