SUPERCHARGING FOR ALL!
THE success of the J.2 M.G. Midget has been an accomplished fact since its introduction, but the merits of the J.3, a supercharged version of the same car, are less widely known, excepting for its fine performance at Montlhery, where several records, including 1 to 24 hour at 70.61 m.p.h. were put up at the close of 1932. The car we took on test was the actual record-breaker, and we found the combination of speed and tractability almost incredible for a car fitted with an engine of only 750 c.c.
The chassis layout differs little from that of the J.2 described in the September 1932 issue of MOTOR SPORT. The single overhead camshaft engine has inlet and exhaust ports on opposite sides, 14 mm. sparking plugs, and a
two-bearing crankshaft. A 6a Powerplus supercharger is driven direct from the front end of the crankshaft, and the S.15 . carburettor is supplied from the 11 gallon rear tank, which has a reserve supply, by means of an Autopulse pump. The J.3 crankshaft is heavier than that of the 3.2 and has a shorter throw, so as to bring the capacity down to 746 c.c.
The clutch is strengthened, and transmits the drive through a four speed gear-box, with a constant mesh third gear, to the spiral bevel back axle. With the increased power given by the supercharged engine a final ratio of 4.89 has been found suitable, allowing high road speeds without fuss. The front axle is dropped and the chassis passes under the rear
one, with a consequent .lowering of the centre of gravity. The springs are flat underslung semi-elliptics sliding in trunnions at their rear ends. Cable-operated brakes are used, and they and the other chassis points are lubricated from nipples grouped under the bonnet.
The car, we were informed, had a maximum speed of about 93 m.p.h., an amazing speed when one considers that two years ago the Mile Record for 750 c.c. cars stood at 94 m.p.h. Changing over from a car of three times the engine capacity and much larger chassis dimensions, we were a little doubtful as to whether this speed would prove agreeable on a small car. Tyre pressures were checked, the shock
absorbers tightened up hard in front and slightly less so behind and we set off from Abingdon. The pneumatic upholstery damped out any harshness from the tightened suspension, and it was evident that the trial was going to be a success. The steering is unusually light, and so is liable to be overwound by the heavyhanded, but once this feature had been recognised, the car was driven with complete confidence. So much so that it was taken down a main-road slope at 90 m.p.h. and proved perfectly controllable, while on corners the short chassis enabled one to get round with the minimum of effort. The repairs on Brooklands Track prevented a complete circuit being made, but starting from the Fork side of the
damaged bridge, the car reached 70 m.p.h. by the time it was passing the Vickers Sheds. A strong head wind on the Railway Straight kept the speed down to 80 m.p.h., but once under the shelter of the 13yfleet banking it rose steadily to 88 and 90 would undoubtedly have been attained if it had not been necessary to stop before again reaching the bridge. The acceleration chart reveals an excellent performance which might have been bettered at the top end if the strong wind had not made it difficult to do 80 against it. At the maximum engine revs. of 5,500 the speed in third gear is 70 m.p.h. and there was no sign of period throughout the range. 45 m.p.h. seemed about the safe maximum in second The brakes were as
efficient as the high maximum speed demanded and from 40 m.p.h. the stopping distance was 55-57 feet. They were smooth and progressive in effect. The J.3 is a very successful example of supercharging properly applied. On its normal fuel-50% Ethyl and 50% Benzol —it runs smoothly down to 500 r.p.m. on top gear, though for a fast get-away a change-down straight into second is a help. The K.L.G. 718 plugs, which are of course of the 14 mm. type, are as happy in traffic as on the open road. The bear ings of the blower are lubricated from the sump and the blades get their supply from upper-cylinder oil used in normal quantities in the petrol. Because of this the plugs do not become swamped with oil after an all-night stand in the garage, and the car started instantaneously on all occasions. A possible criticism of the blower-installation is that being fitted on the front end of the crankshaft, a starting
handle cannot be fitted, but with a light car like the M.G. a push start should not be difficult. The blower and the engine were both mechanically silent, but the exhaust note
becomes prominent at 3,000 r.p.m., which often seems to happen on a four-cylin.dier engine. Keeping above or below these revs, the exhaust note in traffic is inoffensive, while in the open country the trumpet note effect fits in well with the exhilirating effect of the car’s acceleration.
With a touring car, as distinct from the semi-racing machine, one must have good suspension and comfortable upholstery. Even with the shock-absorbers fully tightened, as they were throughout the test, road shocks were hardly noticed, and the seat was adjustable both for distance from the pedals and for rake. The controls all came easily to hand, and the racingtype brake lever was of great use in traffic.
The gear-c4ange from top to third was a joy, and with the 70 m.p.h. maximum there was no difficulty in getting quickly past any vehicles one normally encounters. All the gears were quiet, and third is a constant-mesh ratio. The gate of the remote-control gear-box is not very definite, as it is possible to catch the lever in the slot leading to reverse gear when changing down to second in a hurry, and one is also liable to put the lever into reverse instead of first gear in a traffic block and to rush rapidly backwards. The enthusiast would overcome this by fitting a gate off one of the larger cars, which is interchangeable with the J. type. The horn button is fitted to the dash where it can be operated with one finger, and the dipping switch with it. The
The accelerat;on chart of the J.3 Midget.
latter simply cuts out one headlight when required. The lamps allow 60 m.p.h. to be maintained with safety. The J.3 is a car which fits in with one’s every mood, and is just as happy hum
wing along at 40 m.p.h. as rushing round corners at full revs, with the inside wheels six inches from the gutter. The good power-weight ratio makes frequent gearchanging unnecessary, and one settles
down easily to speeds of 65 to 70 m.p.h., the revs. at the latter speed being just under 4,000. Its maximum with the screen up is about 80 m.p.h. Heavy rain on the second day of the test prevented the maximum road speed being determined, but it gave a good opportunity of trying the hood. When not in use, this folds down in a locker behind the seat. The sticks are of normal type, pivoted inside the body, and the front part of the fabric clips on to the
screen. The latter is rigid and sufficiently high to give a good field of view, but the suction wind-screen wiper did not work when the blower was exerting pressure. The hood locker is quite spacious, and could hold a pair of useful suitcases, especially if their dimensions were chosen to fill the space available. A removable flap gives access to the back-axle and the transversely placed shock-absorbers. Other manufacturers
Other manufacturers please copy !
Wet roads in no way affect the car’s steadiness and even on tramlines, where the small tyres might have been expected to give trouble, there was no tendency to misbehave. The brakes were also applied fiercely in emergencies without causing any deviation from the desired path. Considered irrespective of price, this supercharged 750 c.c. car gives real comfort for two people combined with a performance as high as any normal driver could want. Driven with a little consideration, there no reason why it should not keep its tune over long periods without attention, while the tax, insurance, and expenses should be
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