The 300SL Mercédès-Benz, which created such a sensation in sports-car racing two years ago, until the manufacturers withdrew it from competitions because “they had learnt all they needed to know,” was brought to Silverstone on October 14th for the delectation of a few favoured motoring journalists. Rudolf Uhlenhaut, head of the Mercédès-Benz experimental and racing department, drove each journalist for two or three fast laps (of the full Silverstone circuit. This was a wonderful experience, for Uhlenhaut is well known to be nearly as fast as Fangio when it comes to poking a Mercédès round the circuits, and while he may have kept just a little in hand during these demonstration runs, the 300SL was driven very fast indeed, using all the available space on the corners. Uhlenhaut used only top and third gears, bringing the car out of the corners by skilful opening of the throttle, the speed rising to something like 112 m.p.h. along the straights.
Later came the opportunity of trying the 300SL for oneself and while fast “lappery” masks rather than clarifies impressions of a strange car, especially one of this power and speed, we were impressed by the manner in which 6,000 r.p.m. came up in the indirect gears, equal to nearly 100 m.p.h. in third, the comfort and support of the cloth-upholstered bucket driving seat, the high-geared, taut, somewhat heavy steering, the very fine road-holding, swing-axle rear i.s. notwithstanding, and particularly by the great power of the light-to apply, completely fade-free brakes, with their turbo-finned drums. Naturally, the extreme power and urge of the 300SL impressed. It also created favourable comment for the manner in which it stood up to this continual high-speed without falter, the exhaust showing neither blue haze nor black smoke, the engine starting promptly, and no trouble of any sort remotely thought of, except for a big nail which punctured one of the Dunlops, although this car, with the first production fuel-injection engine, was used as a road-hack and probably did nearly 200 miles of fast work at Silverstone. The accelerator has a short travel and when it is depressed the result is interesting! The car snaked somewhat when taking an adverse camber and it is possible, that a de Dion back-end would hold it down better, although it is certainly an outstandingly stable and safe car.
It is a truly delectable motor-car and not surprisingly costs £4,392 15s: 10d. in this country, in spite of the fact that Mercédès have studied cost-reduction and hence use bolt-on wheels, drum brakes and a sheet steel body, etc. Although the price is high, there is, they appreciate, a limit, even for sales to the U.S. A.
The instrument panel is in the modern, even ornate style, with quite modest speedometer and rev.-counter dials before the driver who, in this l.h.d. example, had a hand-brake lever on the left of his seat and a central remote gear-lever, nicely placed and controlling a gearbox with admirable synchromesh. Entry and egress to the two-seater cockpit with its luggage shelf behind the seats entails cocking the legs to negotiate the deep chassis sill, as the spring loaded lift-up “Le Mans” doors are used, but head-room as you enter or emerge is unrestricted, as the doors form part of the roof. This Construction is essential if a properly-stiff chassis frame is to be employed without a heavy weight-penalty; the frame is, indeed, formed of 25 mm. by 1 mm. “bicycle” tubes. The sliding windows seem rather small but visibility is excellent and head-room ample, whilst the luggage boot is capacious.
Mercédès -Benz are building 25 of these 300SL cars a month at present, selling mainly to America, and will soon increase production to 50 a month. They aim to build 500 in all, so that the car qualifies to race in the Gran Turismo class.
The engine is a development of the 300 and 300S six-cylinder overhead camshaft 85 by 88 mm. 2,996 c.c. unit. With direct fuel injection, which, from our remarks above, it must be obvious that Stuttgart has got absolutely “taped,” the power output is 212 b.h.p. at. the clutch, using a compression-ratio of 8.55 to 1. We write “at the clutch” advisedly, for American enthusiasts get 240 b.h.p. by removing cooling fan, exhaust mufflers and such like impediments. The b.m.e.p. is 150.5 lb./sq. in.
The engine peaks at 5,800 r.p.m., but is safe up to 6,000 r.p.m. and will tolerate 6,400 r.p.m. in top gear if the low-ratio back axle is fitted.
Uhlenhaut explained that Mercédès-Benz have adopted fuel-injection because it provides the same strength mixture in each cylinder, so that a higher compression-ratio can be employed than is possible with normal carburation, when the limiting factor is that of the leanest cylinder. The car we tried was using pump National Benzole and S.A.E. 20 Mobiloil. It shows not the slightest tendency to “pink.” Besides this advantage of fuel injection, this system provides excellent low-speed torque, from 400 r.p.m, upwards, and makes the engine easier to operate under cosmopolitan conditions, inasmuch as jets do not require changing at high altitudes, as the fuel pump is provided with bellows for automatic altitude adjustment and also has automatic temperature-and cold-start adjustments.
Dirt is the enemy of the fuel-pump, as it is of a diesel-injection pump, but a double filter obviates most of the trouble. The only shortcomings of fuel-injection have been difficult hot-starting, cured by using an electric auxiliary pump to prime the system with cool fuel, and a tendency to “hunt,” which however is confined to idling speeds and will be cured. A long induction pipe, feeding air to the inlet valves, damps out induction pulsations.
The cost of fuel-injection is approximately £60 greater than that of normal carburation, but later on Mercédès-Benz will probably adopt it for their lower-priced cars.
The 300SL will be available in open as well as coupé form next spring, but not as a four-seater, as Stuttgart consider such addition of weight quite out of keeping with their conception of a Gran Turismo motor car.
Three different back axle ratios are available, 3.68 to 1 for normal touring, giving a maximum speed of 150 m.p.h., as used on the car we drove, 3.42 to 1. which provides a maximum of 153 m.p.h. and 3.25 to 1. which gives an all-out speed of 100 m.p.h. — quite a sports car! Using the “town” gear-ratios 40 m.p.h. is possible in first gear. 67 m.p.h. in second gear and 95 m.p.h. in third gear of the four-speed gearbox, but the higher axle ratio gives 75 m.p.h. in second gear and 107 m.p.h. in third. The weight of the coupé, ready to drive and full of fuel, is given as 25.5 cwt., suggesting a dry weight of about 23 cwt. Were a light-alloy body used some 80 kilo. could be saved, but the price would increase.
Fuel consumption is quoted by Uhlenhaut as 44 m.p.g. at 30 m.p.h., 38 m.p.g. at 40 m.p.h., 35 m.p.g. at 60 m.p.h. constant speeds, the average range being between 14.8 and 24 m.p.g. for fast driving, including Alpine work. Uhlenhaut said that motoring “as fast as you can travel on British roads.” including town-driving, the 300SL returns 18 to 20 m.p.g., a very conservative consumption for a 3-litre car of this kind, proving that low weight and a good aerodynamic form pay dividends here.
For sports-car racing Mercédès-Benz have the new straight-eight 3-litre fuel-injection 300 SLR (picture on page 622) which Motor Sport described last month.
Uhlenhaut said that next year this car will run in the important sports-car races and they hope to vanquish Ferrari, and Lancia if the latter marque runs. Open and closed versions of the 300SLR will be produced, but it is expected that the drivers will prefer the open cars for races like the Mille Miglia, because of the problem of keeping clean the windscreen of a coupé.
Disc brakes are being investigated dt Stuttgart, but will not necessarily be used. Already tests of a German-supplied Chrysler disc brake have been completed and Ullenhaut now places high hopes in the Dunlop disc brake.
He welcomes the increasing competition in Grand Prix racing and looks forward to 1955, when the sports/racing 300SLR will make its debut and the Mercédès -Benz G.P. cars are expected to be faster than they were this year. — W. B.