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Primary compression
Modern short stroke water cooled engines that employ large transfer port areas do not require a high primary compression ratio, in fact a high primary compression ratio substantially increases the pumping losses and decreases the power available. However, this does not apply to the long stroke of the Villiers unit that uses small transfer ports, even though pumping losses do increase there is a net increase in power. For road racing a minimum geometric primary compression ratio of 1.4:1 is required, 1.5:1 would be better but it is difficult to achieve without extreme measures. The higher ratio is used to squirt the charge through the narrow port. The speed of the charge entering the cylinder is further enhanced by the narrowing of the port, its inlet area should be at least 150% larger than its outlet. A good primary compression will improve tractability and performance whether for road or off-road riding, don't allow this pressure to be diluted through the use of a hollow gudgeon [piston] pin.
Modern engines should be considered as having transfers fulfilling a "store and forward" function for the fuel with the real control being exercised by the exhaust. The modern exhaust shape required easy access to a supply of fuel mixture, exactly what the large transfer system supplies. What a modern system cannot do is sustain a long deep draw of fuel from the crankcase through inadequate transfer passages.
In the long stroke small transfer engines, there is significant advantage to be gained from the truly explosive entry of the compressed gases from the crankcase into the cylinder, sweeping the remaining exhaust gases out into the exhaust port in a manner described by Schnurle many years ago. As the piston descends and before the transfer ports are uncovered, a hollow gudgeon pin will bleed off some of the primary pressure that you have fought to create. The hollow pins should be plugged with an alloy slug or slugs which should have no more than a 1 thou clearance fit and be coated with Loctite or similar. Interference fits will swell the gudgeon pin causing fitment problems. The lack of interference fit is required to allow the Locktite to key and bond, a zero or interference fit will not allow the Locktite to work properly. At racing speeds, the difference in power can be measured on the dynamometer.
The geometric primary compression ratio (GPCR) is useful in determining whether crankcase stuffers are necessary. Be under no illusion: good primary compression is important. If your GPCR is less than 1.2 then action is definitely required.
