SUPERCHARGING FOR SPORTS CARS AND TOURERS AN IMPROVED MARSHALL ROOTS-TYPE BLOWER AND A NEW CENTRIFUGAL FAN.
advantages the supercharged power-unit have been appreciated for the last ten years, but it is only quite lately that the superchargers themselves have been made sufficiently efficient and reliable to justify their being fitted to the ordinary fast car intended for road-use. The Roots-type blower, which consists of two figure-of-eight shaped-vanes geared together and running at close proximity in a casing, offers considerable advantages, for the vanes are in perfect balance, and since they do not touch, require no oil. Unless carefully made, however, the “
Roots” is inefficient at low speeds, and may require to run at several times engine speed to produce the necessary pressure while the gears themselves often cause a scream which is out of place on a quiet-running car. These and other points have been successfully dealt with on the latest Marshall superchargers.
The rotors are made of steel and are integral with their shafts, and the lobes are hollowed out for lightness. The outside parts of the vanes are almost flat and have two grooves cut in them, and the eddies caused by these and the small clearance between the vanes and the outside of the casing only .003 of an inch, produces an almost perfect seal. The rotor shafts are carried in ball and roller bearings in the finned casing, which is made of electron or a light aluminium alloy, and a vernier dog-coupling ensures the rotors being set at exactly 90 degrees to one another. The gears are ,wide and have straight teeth, profile-ground, and running with a clearance of only .0006 inches. No noise can be heard when running and when suitably connected they act as a gear-pump, and lubricate themselves from the engine, automatically drawing oil from the sump and forcing it back again. The gears are carried in an external casing sealed from the pumping chamber by phosphor-bronze rings, which are pressed against shoulders on the shafts by light helical springs. The rotors run dry, and there is no possibility of the plugs being fouled by an oily mixture. Marshall superchargers are made in five sizes, and are suitable for cars ranging from 750 c.c. to three litres. They may be driven off the front end of the crankshaft, and since the drive-shaft runs right through the casing, a starting handle dog can be fitted on the outer end. Alternately they may be driven by enclosed chain from any projecting shaft, such as the one which carries the fan-pulley. For some of the intermediate capacities the blower has to be geared-up slightly,
but as the safe maximum is 18,000 r.p.m., no trouble is likely to be experienced from this source. Prices have been considerably reduced and now range from 230.
For normal sports cars the boost is usually 6 lbs., while on semi-racing jobs, 12 lbs. are used. The compression ratio of the engine has of course to be reduced if one wishes to continue on normal fuel, but in spite of this a substantial gain of horse-power is found over the whole range, while the superchargers produce a useful pressure from about 1,000 r.p.m., tremendously improving performance at this end of the scale. On certain cars the transmission is not sufficiently strong to withstand the increased power available with the Rootstype blower, and for them a centrifugal fan has been designed, which materially improves distribution at low speeds without producing an embarrassing in
crease of power. A vertical shaft driven by bevels of the crank-shaft or other suitable point carries a finned disc within a casing, and sucks mixture from a down-draft carburettor, and forces it into the induction pipe. This fan blower is easily fitted, and requires no lubrication, as the shaft is carried in ball-bearings packed with grease. The cost is £9 10s. Od.
Marshall superchargers are manfactured by Marshall, Drew & Co., Ltd., of 140, Clarendon Road, North Kensington, W. 10, who have successfully fitted them to M.G., Frazer Nash and Aston Martin cars, and facilities are also available at this address for any type of high-efficiency tuning.
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