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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: