The Cylinder Head part.3 (compresion)

Heat is the enemy of two-stroke engines and stretching the compression ratio to
give a 10% power increase will possibly result in a 3% power rise at the most; the rest
will be lost in heat energy and pumping losses. However, at lower engine speeds the
cylinder will not be completely filled with fuel/air mixture and the power may jump by
5-6% because there is not such a heat loss.
This is, in fact, the real benefit of raising the
compression ratio, not to increase maximum power but to pick up mid-range power
and possibly widen the power band.
Because so much confusion exists in the motorcycle industry relating to
compression ratio we need to define exactly what we are talking about when we use the
term. Ever since the first internal combustion engines, regardless of whether the
engines were two-stroke, four-stroke, diesel, petrol, etc., compression ratio was taken
to mean the ratio of the volume of the cylinder with the piston at BDC to the volume of
the cylinder with the piston at TDC (FIGURE 2.6).

 This relationship is expressed in the
formula:

CCV, combustion chamber volume, is made up of the volume of the combustion
chamber, plus any space existing between the piston crown and the top of the cylinder,
plus the head gasket. This volume can be worked out geometrically but it is much
simpler to bring the piston up to TDC. Seal around the edge of the piston with a thin
layer of grease. Fit the head and head gasket and measure the volume with water or
paraffin, using a burette graduated in 0.lcc.
As an example of how these formulas work we will consider the long stroke
Bultaco Pursang 125. This engine has a bore of 51.5mm, a stroke of 60mm and,
according to the manufacturers, a compression ratio of 14:1.

Therefore the engine has a compression ratio just a touch lower than specification.

As this engine will be running at the speedway using 110 octane fuel (Avgas 100/130)

the compression ratio will be increased to 15:1. The standard motor is designed to run

on 95 octane fuel.

The formula to find the required combustion chamber volume is:-

Therefore the combustion chamber volume must be reduced by 9.8 - 8.93 = 0.87cc

To find how much the head must be skimmed to reduce the volume by 0.87cc we

use the cylinder displacement formula transposed to read:-

The above compression ratio is now referred to as the uncorrected compression

ratio. The Japanese have introduced a new way of measuring the compression ratio,

called in various circles effective, corrected, actual or trapped compression ratio. This

can be very confusing because an 8:1 corrected compression ratio is about equivalent to

a 15:1 compression ratio calculated by the old method.