"The Brooklands Silencer" by J. E. G. Harwood, B.A. (Oxon.)


The Brooklands Silencer (known in 1927 as The Brooklands "Official Receiver")

(Reprinted with appreciation from 'Speed and How to Obtain It', by J. E. G. Harwood, B.A. (Oxon.) 2nd Edition, 1927)

Chapter XV - Tuning

"…The rules concerning the "Official Receiver" are as follows . . 1. An ordinary touring car or motorcycle must be fitted with an ordinary touring silencer if and when used on the track for private and touring purposes, as distinct from competitions. 2. Except as above mentioned, all cars and motor cycles when running on the track shall be fitted with the type of silencer hereinafter described.

3. Construction

(a) A pipe from the exhaust valve or valves shall be led into a receiver, which shall be situated as close as possible to the engine.

(b) This pipe shall penetrate into the receiver to a distance of 2 inches and no more.

(c) The capacity of the receiver mentioned in paragraph (a) shall not be less than six times the volume swept by the pistons of one cylinder of the engine, and such receiver if cylindrical shall not be of greater length than four times its maximum diameter, and be of irregular shape of equivalent proportions. The top, bottom and ends of the receiver, in side elevation, shall be straight lines and shall be parallel to one another respectively. The ends shall not be at an angle of less than 45 degrees to the top and bottom lines of the receiver. In addition the extensions of the bottom and inlet pipes into the receiver must be parallel to one another and to the top and bottom of the receiver.

(d) An exit pipe shall lead from this cylinder as far as the back axle. This exit pipe shall protrude into the receiver specified in paragraph (a) to a depth of two inches, and no part of this exit pipe shall be of greater cross-sectional area than the minimum area of the exhaust port of any one cylinder.

(e) The pipes leading into and out of the receiver shall not be opposite each other in the receiver, but shall be out of line to the extent of one and a half inches measured at points on the circumference, and not between pipe centres, so that if the pipes were continued there would be a space between the pipes at a place where they overlapped sufficient to allow of the passage of a one and a half inch gauge.

(f) No device may be employed in the receiver which would tend to produce a straight through flow of the exhaust gases between the inlet and outlet pipes.

(g) The exhaust gases must not pass direct from the exit pipe to the atmosphere, but must be finally emitted from what is commonly known as a "fish tail" on the end of the exit pipe. The orifice of such "fish tail" shall be approximately rectangular in shape, and of the following dimensions -

For engines with a capacity up to and including two litres… small dimension, not more than one quarter inch… large dimension, not more than six inches.

For engines with a capacity over two litres . . . small dimension, not more than one half inch . . . large dimension, not less than twelve inches.

The length of the fish tail when fitted shall be measured from the end of the exit pipe to a point situated at the centre of the orifice, and the length of the fish tail shall be equal to the larger diameter of the orifice. Thus, if the orifice of the fish tail is 9 inches by 1/4 inch the distance from the end of the exit pipe (where the tail commences) to the centre of the orifice shall be 9 inches. The surface of the fish tail shall be flattened as far as possible, and shall taper from the end of the exit pipe to the orifice. (h) The after half of the sides of the fish tail, that is to say, the half of the fish tail nearest the orifice, may be perforated with holes not greater than 3-32nds in diameter. The number of holes is not limited.

4. In spite of the fact that a competitor may have complied with the above regulations the Brooklands Automobile Racing Club, through its official or duly appointed representative, shall have the right at any time exclude any vehicle from the track or grounds within its jurisdiction if, in the opinion of such official or representative, such vehicle has made or might make undue noise.

5. If while any vehicle is using the track during the progress of a race, record breaking attempt or test, or for any purpose whatever, its silencer should become detached, deranged or broken in such a manner as to allow of the emission of the exhaust gases through any other place than the orifice of the fish tail, such vehicle shall if competing in ally race, record breaking attempt or test, be liable from that moment to be disqualified from taking any further part in any such event, and shall in any case throttle down and leave the track forthwith.

6. The Brooklands Automobile Racing Club, by its official or duly appointed representative, reserves the right to examine the interior of any silencing apparatus fitted on competing vehicles, and may disqualify any competitor whose apparatus in the opinion of such official or representative does not comply with these regulations, and may withhold, withdraw or deal with, as it thinks fit, any prize, certificate or other award to which such competitor would otherwise have become entitled as a result of the use of the offending vehicle".