Mr. Cyril G. Pullin, the famous racing motor cyclist, and Douglas exponent, sends us his views of the present situation in respect of silencing at Brooklands as follows :
If all competing machines are fitted with a silencer made to conform to the terms of the new regulation, they are apparently allowed to use the Track. quite apart from their relative degree of silence, which,. of course, elitninates the human element in deciding whether a special silencer of patented design is sufficiently quiet or not, and as the regulation is worded, it appears to put all competitors on an equal footing.
The regulation silencer can be made in a large variety of forms, and motor cyclists have apparently gone baldheaded for one particular type of silencer, which in my opinion is the most undesirable.
This form of silencer is certainly not very effective from a silence point of view, when fitted to engines of very small swept volume, as ,for instance on a 175 c.c. having an exhaust pipe of 1.5″diameter, the outlet pipe in this container in this instance would have to be the same size as the inlet thereto, the container itself being little more than an exhaust pipe of slightly increased diameter, having only to be 1,050 c.c.
The relative silence of a 175 c.c. single cylinder machine and a 1,000 c.c. two-cylinder machine is distinctly marked, and big machines suffer considerably more from the silencing effect than the smaller one.
A point one must bear in mind when dealing with the question of silencing racing machines is, when the gasses are being exhausted through straight pipes of suitable length and diameter, they are discharged into what we may practically term a negative pressure, and the inertia of the gasses in the pipes set up an extractor action which, with suitable valve timing, can be taken advantage of, but with the regulation silencer which I first mentioned, the gasses are being discharged into a container which is working at a positive pressure and calls for an entirely different valve timing (and its dependents).
The Brooklands competitors feel very keenly the unfair disadvantage we are working at in competition with European and American competitors, but we are certainly on the right road to produce quiet fast motor vehicles so much in demand by the press and public in general.
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