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In December 1935, Oliver Thornycroft of Ricardo & Co, read a paper to the Institution of Automotive Engineers entitled The General Question of Supercharging . Among those who attended and contributed to the discussion were Messrs. Lawrence Pomeroy Senior and Junior; G.E.T. Eyston; M.A.McEvoy; Murray Jamieson; and . . . the reason for writing this . . . Cecil Kimber.
Thornycroft's paper takes as read, the benefits of supercharging for aircraft and racing cars. What he sets out to examine is the practicality of supercharging mass produced engines, running on pump fuel; i.e. petrol engines for small cars and diesel engines for commercial vehicles. He provides a lot of data taken from engine tests carried out by Ricardo for the Air Ministry, but comes to no hard conclusions other than the question of supercharging has to be considered as a fresh problem for every intended application .
No surprises so far.
Understandably, much of the experience offered by the contributors to the discussion is based on racing and high performance cars, not the pursuit of weight saving and economy which is Mr 'T's primary interest! One senses him becoming a little frustrated by this. In a reply to a contributor who extolls the virtues of the centrifugally blown Graham-Paige (straight line acceleration to 85 m.p.h ) he says the car for the million needs high torque at low speed and preferably, no torque at all beyond 60 m.p.h.
However, the audience is not to be diverted onto cars for the million, and we have W.A. Robothom of Derby Investigating the possibility of supercharging the Bentley, being impressed by the Graham-Paige but considering the 5 litre Mercedes not our ideal of a refined motor car .
All good clean fun, and interesting reading with the hindsight of 60 years of automotive development. The real curiosity for us though is contained in the contribution from Cecil Kimber, who spoke immediately after 2 gentlemen who for different reasons considered that mass supercharging would not catch on. His contribution is reproduced in full:
Mr Cecil Kimber: I do not take so gloomy a view as that held by the two previous speakers as to the future of supercharging motor-car engines. The company with which I am associated has had experience of blowing small engines, and from two engines, of 750 c.c. and 1,100 c.c. respectively, we have obtained over 150 b.h.p. per litre. With the 750 c.c. engine, unsupercharged, the best we could get, using high compression, special cams and so on, was about 48-50 h.p., whereas by fitting a blower we produced about 110 h.p. It is not difficult to calculate how much larger and heavier the engine would have to be in order to secure a similar increase of horse-power without supercharging.
During the last 18 months I have been running a car with a supercharged engine, and have covered about 30,000 miles on it. I do not need to use any special fuel, but can fill up with any that is available at any pump. The car will run perfectly happily at speed from 500 to 6,300 r.p.m.; the higher speed represents just over 100 h.p. The size of the engine is 1,250 c.c. This will enable the possibilities of supercharging to be appreciated. The blower on the car is of the Rootes type, and we consider that for a small engine for ordinary touring purposes, this type is the best so far. One reason is that we do not experience the difficulty of lubricating oil passing through into the cylinder and giving rise to trouble with the plugs. At the lower speeds, although it may be blowing at a lower pressure than the eccentric vane type, the Rootes blower gives the greater net again of engine power in our experience, because less power is needed to drive that type of blower.
During the course of the 30,000 miles I have covered in the car I have fitted an eccentric vane type blower, and I experienced nothing but trouble owing to oiling up and the collection of petrol in the induction system, and so on. That was possibly due to the fact that the blower was fitted on the car between the front dumb-irons, and it had rather a long induction pipe. Otherwise, the car has behaved remarkably well, and I should be very sorry to cut off the blower now.
A practical engineering difficulty about adding a blower to a small engine in order that it should give the performance of a big engine is that of securing silence. It is not a question of eliminating merely the noise from the blower , or pneumatic noise, or anything of that sort. I suppose the general stresses under which all these smaller parts are working, taking them together, call for a lot more quietening than is the case with bigger engines, which are working much more easily. I think the ideal blower for the motor car is possibly of the type that the Americans are now introducing, and is particularly applicable to their engines. They use a big engine which, considered from the British small engine point of view, is very inefficient, but at any rate it develops a lot of power low down. It has small gas passages, giving increased torque low down; then, when it tails off, the centrifugal blower, which until that stage has been doing nothing but mix the gasses nicely, really starts to come into its own, and instead of the power curve falling off, it continues to rise in a straight line. That suggests that possibly the ideal blower for a motor car will be the small centrifugal type ( which is light and comparatively cheap to make), provided that the blower runs at a constant speed, irrespective of the engine speed, so that it gives a boost low down and does not give an increased boost higher up. Incidentally, the blowing pressure on the car I am driving is about 8 or 9 lb. Maximum.
It has not been my experience that the use of a blower increases oil consumption; the car I have been driving is still doing about 2,000 m.p.g., which is quite reasonable. Another advantage of the blower is that throughout the 18 months or so during which I have been driving the car, the engine has never needed decarbonising; the head was taken off once, but there was absolutely no carbon at all.
I do not think that if we could get blowers down to a very low price, we should see more of them on touring cars. No manufacturer yet has had the temerity to put one on the market, possibly because of the innate conversation of the public, who think of blowers only in connection with racing cars.
It is my own experience, therefore, that an enormous increase of h.p. can be obtained by fitting a blower. The normal h.p. of the engine of my car, unsupercharged, is 56; with the blower the h.p. is from 95 to 100; so by adding about 30 or 40 lb. weight, I secure all that increase of performance. On the other hand, not only is an unsupercharged engine giving a corresponding horse-power much heavier than the supercharged engine, but the weight of the chassis, springs, and so on, must increase in proportion, so that altogether the car becomes much heavier. That is a point which it is worth considering in favour of using the blower for the ordinary touring car.
Installation of engine and Zoller supercharger in M.G. Special car as mentioned in the text. A 1250cc engine in a Triple M car is an interesting conundrum. The photo appears to be of a 4-cylinder engine, but is not an early MPJG or XPAG. The Corsica was a 6-cylinder M.G., so what was Cecil Kimber describing?
Kimber in fact came close to prophesying what was to happen, because 40 years later the centrifugal blower ( although not mechanically driven, but powered by an exhaust driven turbine) started to become widely available on production cars, including M.G. saloons. Turbocharging for diesel engines was practised well before 1935, but its application to petrol engines had been experimental only. Because of the higher exhaust temperatures of petrol engines, it needed the post war development of high temperature alloys for gas turbines to allow the development of the reliable very low price blower of Kimber's prediction.
The car referred to as Kimber's personal transport is presumably his Corsica drophead K Magnette. When overhead cams gave way to pushrods, the Magnette was replaced by a VA saloon. I cannot help wondering if, faced with such a change of pace, the idea of a centrifugally blown VA ever got as far as the back of an envelope. . .
They are still greatly in favour with Triple M and T-Type owners, both on and off the track, and as the years have progressed new models have appeared, including Volumex and Sprintex, even the Eaton.
They are not so effective on the A and B series engines found in MGA, Z-type, MGB and Midget.
I suspect a short stroke engine is not as amenable to supercharging as the older long stroke OHC and XPAG engines of the 1930s, 40s and 50s. Some would say motor racing is no place for romance and sentiment, but there is not much to beat the sound of a Roots blower at maximum revs echoing down the pits straight at Silverstone!
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