Measuring combustion chamber volume

Revision as of 18:23, 9 June 2013 by Coopchennick (talk | contribs)

One very important aspect of a cylinder head is the compression ratio (CR) that it will result in. Of course, the compression ratio can be adjusted by adding or removing gaskets but this will also affect the squish clearance, which may not be desirable. When optimizing a cylinder head, gaskets should be selected to produce the desired squish clearance, the combustion chamber volume should be measured and the CR calculated, and then the head can be modified or replaced if the CR is too high or too low.

The basic procedure shown here to measure combustion chamber's volume is to bring the piston to TDC and fill the chamber with a measured quantity of oil until the chamber is full.

An alternative way to describe a combustion chamber is by measuring the pumping psi using a compression gauge.

What you need

  • Engine off the moped
  • Two-stroke oil
  • Small (~10 cc) syringe. Get it at a pharmacy.
  • Tape

Step One

Set the squish clearance. Small changes in the squish clearance by adding/removing gaskets has a considerable effect on the CR.

Step Two

Bring the piston to TDC.


Step Three

Put the cylinder head on if it isn't already and remove the spark plug.

Head on no spark plug.jpg

Step Four

Fill the syringe with oil.


Step Five

Fill the combustion chamber with the oil until it reaches the top of the spark plug hole. The reason for filling to the top of the spark plug hole, rather than the bottom, is simply that it is difficult to tell when the oil level has exactly reached the bottom. This will be adjusted or later.