Veteran to classic: J-series MGs

Performance uprating

In the last years of the David Brown era, Aston Martin’s Engineering Director Dudley Gershon fought against financial pressure to abandon development of the Aston Martin V8 engine. His conviction that the lessons of racing need to be amalgamated with production
specifications grew largely from the pre-war experience of MG in developing its competition cars. Here, he details the thoroughness of the programme which resulted in the supercharged J4 MG.

Modern cars are often deceptive. Very similar body shapes conceal engines of vastly different output, with chassis detail modified to suit, and the availability of spoilers, wheels and so on from kit shops often upgrades the appearance of the low-performance version until it resembles the fast version. This is a growing trend, and several popular cars now have a range in which maximum speeds might vary within the realm of 90-125mph for any given configuration.

Years ago, taking the 1930’s as an example, the options in a given model were generally restricted to two or more engine sizes giving only a small difference in performance, usually by increasing cylinder bore. Jaguar was probably on its own in offering a range extreme of 13/4, 21/2, and 31/2-litre engines with almost the same body, but even then top speeds varied by only about 20mph.

There was one notable exception to this restraint. The MG company, regarding sports performance and racing as a design progression towards the improvement of production models, took extraordinary steps with its products.

It transformed the popular M type, in a series of steps which involved drastic frame re-design and engine/transmission uprating, into the C type which was raced successfully almost worldwide during 1931-33. Later C types were fitted with two-seater TT-type bodies, modified to include doors; these were used for the J2 production model, introduced in August 1932, which had a twin-carb 36 bhp engine. The final version of the C delivered about 70 bhp in supercharged form, whereas the original M barely reached 20 bhp.

The J2 was immensely popular on introduction, and was also made available in supercharged form as the J3, which had about 60 bhp on tap and was suitable for minor events but not for serious racing. Following the introduction of the J2, the racing version of the C type developed into the J4, which was available from 1933, with a larger supercharger, bigger brakes, and longer wheel base. The engine delivered 72-90 bhp according to blower size and fuel permitted.

In 1934 the J4 type was developed into the Q, which was given a three-bearing crank (matching the touring version P type which had then replaced the J2). This engine produced 95-124 bhp, with exceptions around 140 bhp for brief record attempts using larger superchargers, special fuels and sometimes heads.

Common to all these engines in racing form was a reduction from 850cc to 750cc obtained by down-stroking a much sturdier and counterbalanced crank. The J3 was also a 750cc unit, but it had a simple forged crank, rather like that of the J2 though with the short stroke and larger bearings.

At this time, in around 1935, I owned a J2 which I decided to convert into a J4, assuming this would be easy. Not knowing any better (but thinking I did!), I ordered a No 7 Powerplus blower, a Laystall crank, an outside exhaust system with regulation Brooklands silencer and an ENV gearbox.

I intended to get the work done in about three weeks, but as soon as I started I hit trouble. Realism, in the form of a stream of new factual information, stopped me in my tracks.

Every single part of the car, from cash to wheels, was either of different design or material or both (or was an added extra), and thus needed replacement or modification. This applied to supercharger and reduction box (added), carb, valves, springs, pistons, rods, crank, clutch, gearbox (another type), propshaft, axle nosepiece, driveshafts and wheels. The steering, brakes, body structure, suspension, wiring system, exhaust system, and a host of other details were also different.

The immensity of the task and the fact that there were no short cuts, showed me that it was no use trying to be clever; my only course was to change the car for a genuine J4. This I was lucky enough to locate and buy.

The J4 looked very similar to the J2, except for the outside exhaust, the blower, and the absence of doors. But the vast difference in specification was one obvious reason why a new J4 (only 17 were built — my ex-Dennis Mansell example was No 4) cost £675, while the J2 was only £225.

I used this car for local trials and, when I could afford it, for such events as the Junior Handicap (Nuffield Trophy) at Donington, where I crashed it in practice and broke a crank in the race due to my lack of restraint and scant attention to the rev counter.

If anyone wonders why I bothered to change the car to get the proper end product, rather than just risk boosting the J2 by supercharging a strengthened engine and forgetting the rest, my mind was made up after taking a ride in a J3, which was simply a supercharged version of a J2.

The J2’s top speed was around 75 mph, although the original Autocar test got 80 mph on a car later admitted to have been a bit special. The J3 would do 90 mph (and possibly more), but it was lethal at above 80 mph — all over the road with everything flexing. On the other hand, the J4 was rock steady and properly under control right up to its maximum of more than 100 mph. I do not believe any other manufacturer has developed a racing version of a touring model (and vice versa) in such a thoroughly detailed way before or since. Whether by design or metal spec. almost every component was changed in the process.