A Cylinder Head Primer:

The fact that a two stroke engine has no valves does not mean that the design of the cylinder head is not important. In fact, because a two stroke fires on every stroke, the compression ratio is crucial if overheating is to be avoided.

The photo to the left shows a stock GP200 head. The oval shaped recess is the combustion chamber and the circular area surrounding it is the squish band. The purpose of the squish band is to force the fuel mixture which is furthest from the spark plug into the combustion chamber, where it can be burned more efficiently.

In the photo on the right the combustion chamber has been filled with a measured amount of oil to determine its volume. Compression ratio is simply the relationship between the amount of space in the combustion chamber and the amount of space in the cylinder (not including the head volume) when the piston is at the bottom of its stroke. There is some deliberation about "corrected" compression ratios, but this only comes into play when a major change in exhaust port timing is made.
This photo shows two GP200 heads which have been modified for use with the TS1 Humiliator kit. The head on the left retains the stock combustion chamber, which has a volume of about 18cc. Note that this volume has been reduced slightly after the squish band was reshaped. The combustion chamber in the head on the right has been enlarged to 22cc. A TS1 225 kit has an actual capacity of 223cc, giving final compression ratios of approximately 12.4:1 for the head on the left and 10:1 for the head on the right.

Some people make the mistake of measuring head volume including the squish band area by lying the head on a plate of glass and filling the head with oil through the spark plug hole. Because all Lambretta pistons have a domed crown, this will give a false reading because at top dead center the dome protrudes past the top of the cylinder and into the squish band area of the head. A while back we saw a brand new head that we had reprofiled for 10:1 compression for sale on eBay, advertised as having 7:1 compression. The seller had probably measured the volume including the squish band area. One last note about measuring head volume is that the most accurate way to do it is with the head installed on the engine and the piston at top dead center. Because we machine many heads without the advantage of having the customer's complete engine, we are very careful about measuring volume as accurately as possible. We have found that because the surface of the oil sits level in the combustion chamber when filled as in the picture above, the amount of oil that would be displaced by the dome of the piston and into the squish band area at top dead center in the 'real world' test of bolting the head up to the engine is an accurate approximation.

Kurt Lingier, president of the Lambretta Club of Belgium, was kind enough to offer this drawing which illustrates this theory of measurement. The black line is the imaginary fill level of the oil when measuring a separate head. The blue space at point B equals approximately the yellow space at point A. Thus measuring the combustion chamber with the oil method on a separate head should give a nearly identical reading as the same head fixed to the engine with piston at TDC.

The rule of thumb we follow for compression ratios is that for 91 octane pump fuel in North America, 10:1 is the maximum safe compression ratio. For 94-95 octane pump fuels found overseas 11.5:1 to 12:1 is fine, and for racing fuel of 100 octane or more the upper limit is in the 14:1 range depending on many engine variables such as whether the engine is air or liquid cooled, exhaust system design, RPM range, etc.

A few more things to consider regarding squish bands and psi readings:

1. Squish tolerance is very important and is generally agreed to be best set at .75-1.5mm, depending on a few factors such as the rpm range of the engine, bore and stroke dimensions, etc. If the squish is too small, the piston can actually hit the head at high rpm due to the rod and piston stretching as they reach tdc. Squish clearance can be checked by putting a strand of solder down the spark plug hole, turning the engine over to smash the solder at TDC, and then measuring the thickness of the smashed solder. A large bore and stroke engine with heavy moving parts and a long connecting rod is the most likely to have this problem if it is set up with too small of a squish band and revs much higher than stock. Most scooter engines in the 150cc-225cc range do well with a 1mm squish setting.

It follows that the only proper way to make large changes in compression is to actually alter the combustion chamber dimensions. Of course small changes in compression can be had by adding or removing gaskets, but only by staying within the proper squish tolerance.

Reading an engine's psi at kickover speed does not take into account the charge that is lost out the exhaust port at this low speed. An engine with a larger (in particular taller) port will give a lower reading than an otherwise identical engine with a shorter port (i.e. less port timing). A well designed expansion chamber system actually stuffs some of this lost charge back into the engine right before the port closes, so it follows that this engine may need to run slightly lower compression than a similar engine with a stock exhaust system, or run higher octane fuel. PSI readings are not useless, but they don't always correlate between engines.
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