From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Road test of a supercharged Alvis Speed '20' and a detailed analysis of a new Servais exhaust silencer.
| Identifier | ExFiles\Box 152\3\ scan0214 | |
| Date | 12th May 1936 | |
| The Motor 650 May 12, 1936. Contd. AN ALVIS SPEED "20" AT 22,000 MILES ham-fisted of drivers. The harder he pushes, the quicker it changes. I have heard expert gear-changers questioning whether this kind of transmission is really appreciated by those who know how to double-declutch. The answer is most decidedly in the affirmative. To take just one example, it reduces driving fatigue, and the driver retains the pleasure of controlling engine speed to a nicety to avoid shock upon re-engaging the clutch. Last, but not least, the constant-mesh gears are still quiet after 22,000 miles. The brakes, like the steering and suspension, are designed for safety, and I would say that this is one of the few systems with which a driver can really throw his weight on the pedal (when travelling at speed) without risk of a swerve or skid. The pedal pressure is a little higher than one normally expects nowadays, but the car is, of course, two years old in point of general design. There is a long right-hand brake lever, just within the scuttle, which is not merely a parking brake. From the owner-driver's point of view an excellent feature of this chassis is the one-shot lubrication system, through which oil can be delivered to all the vital parts merely by depressing a lever. The layout of the instruments is effectively devised for easy reading, and includes a large Smith speedometer and rev. counter. On the right there is an adjuster for the Andre-Telecontrol shock absorbers. A Zoller supercharger was fitted to this engine by Messrs. M. {Mr Moon / Mr Moore} A. {Mr Adams} McEvoy just a year ago, and has proved a successful experiment. It is mounted close to the scuttle, under the bonnet, and is driven by chain at two-thirds engine speed. The boost is controlled by a patented McEvoy - Pomeroy plunger valve, within a top limit of 10 lb. per sq. in. Modified for Supercharging Various modifications were made when the supercharger was fitted; these included lowering the compression, fitting Aerolite pistons with thick crowns, installing a more powerful water pump, and fitting new exhaust valves. The car has now covered 12,000 miles in supercharged form, and has given very little trouble. The supercharger itself remains quiet, flexibility on top gear is excellent, and easy starting is a feature. Sparking plugs present rather a problem, as a design which resists oiling-up in traffic is apt to become too hot under fast main-road conditions. I have, however, obtained good all-round results, from K.L.G. type 843 sparking plugs. A word of special praise must also be given to the Dunlop "90" Fort tyres which were fitted about two months ago. They show very little wear, after covering 10,000 miles, and have given adequate adhesion under all varieties of difficult road condition. Quite recently the car has been fitted with the Servais silencer; a novel design which damps exhaust pulsations effectively and cuts out noise without detriment to performance. A more thorough account of its properties will shortly appear in The Motor. To this list of appreciations must be added the following: the Tungstone accumulators, which spin the engine with remarkable energy on cold mornings, the Vacuum oil which has consistently been used in the engine, and the standard Lucas lighting system, which embodies two very effective P.100 headlamps, and a Notek auxiliary lamp which has proved a highly practical fitting. This car also represents an experiment with open coachwork, made after I had owned a long series of saloons. To fold the hood properly takes a few minutes, and side-screens are something of a nuisance. Against these drawbacks the open car offers an increase in the pleasure of motoring which amounts to at least 50 per cent. Just why it should be so is difficult to explain; open-car enthusiasts will understand and the saloon motorist is not likely to be converted by any words of mine. I will just say to those who find modern traffic irksome and crowded by-passes a nightmare—try the open car, or at any rate a drop-head model. After that, you will not need anyone to point out the difference which a roof can make. M.P. --- Prolonged Test of a Novel Silencer Exhaust Quietened Effectively Without Detriment to Performance (Left) Sectioned view of the Servais silencer, showing layers of expanded metal and steel wool. A NEW type of silencer has been developed which is proving very successful in a variety of applications ranging from cars to large oil engines. Known as the Servais, the silencer is of the "straight through" type and is arranged internally in such a way that exhaust pulsations are damped very effectively. The shape of the pressure waves is modified, without creating resistance to the flow, so that when the gases emerge their noise-producing qualities have been eliminated. The design is such that the silencer can deal effectively with pulsations of two frequencies: the "crack" produced by the sudden discharge of gas when the exhaust valve is snapped open, and the roar which follows as the bulk of the gas escapes ahead of the rising piston. In order to give this silencer a thorough trial we had it fitted to one of our staff cars: a supercharged Alvis Speed Twenty which develops sufficient power to make silencing quite a pretty problem. Various performance tests were carried out, such as acceleration trials with watch and Tapley meter, timed speed trials at Brooklands track, hill-climbing tests and the measurement of fuel consumption. These showed conclusively that extremely little power is lost when this silencing system is employed. The exhaust note at full throttle has been quietened effectively and at part throttle can scarcely be heard. These statements apply not only to the sound heard by those in the car, but also to the effect noticed by an observer on the road. The principal features of the Servais silencer, which have been patented, are clearly shown in a drawing reproduced. Within a cylindrical container, welded from steel sheet, there is a helical coil (of expanded metal) with steel wool packed between the convolutions. The innermost coil of expanded metal forms the passage through which the gases pass. The whole of this packing is of an elastic character which not only absorbs energy from the gas pulsations but is also (in virtue of its flexibility) unable to produce resonant noises in the exhaust piping or the outer cylinder. Another advantage of the flexible and elastic properties of the interior is found in the fact that carbon particles cannot become packed into the interstices. Any that may find their way into the expanded metal soon becomes shaken out again and blown through the tail pipe. It has also been found that the expanded metal is free from local overheating troubles. The design of this silencer can easily be modified to suit the characteristics of different engines and considerable research has been done in this direction with the aid of audiometer equipment. It is being tested by various car manufacturers and is available to private owners at prices ranging from 27s. 6d. The makers of the silencer are Servais Silencers, Ltd., 8, Old Pye Street, London, S.W. D41 June. 16. 36. | ||