Blown motors

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In which H. L. Biggs recalls some Marshall-supercharged cars with which he was associated, and which should be of considerable interest now that low tax and high efficiency are all-important.

S. C. H. DAVIS’S well-remembered saying “the component one knows the least about is that most likely to give trouble” had a great deal to do with my seeking employment with Marshall, Drew & Co. the designers and manufacturers of Marshall superchargers. [The Marshall is the most widely used of commercial Roots-type superchargers.— Ed.] I had, in addition, memories of John Marshall’s performance in C.U.A.C. events in earlier days, riding Norton motor-cycles, to crystallise my opinion that here was a firm run by someone who definitely knew the motor sportsman’s requirements.

Marshall superchargers were already well known in the racing world in connection with M.G. Magnettes and Altas, and at the time when I joined the firm they were commencing the production of blower installations suitable for the average sports-car. The actual work consisted of assembly of the blower from parts already machined in the works, and fitting, entailing the building up of mountings, drive, controls, induction system, oil feeds, etc.

One set had already been delivered for Summerfield’s “P” type M.G. Midget, and the first actual set I was busy with was a similar type for E. J. Cope’s “P” M.G. The blower, a Type 65, was mounted on a quarter-inch plate platform between the dumb irons. It was driven at engine speed through a short shaft employing special couplings, in which the drive was transmitted through hardened steel balls running in grooves cut by drilling holes longitudinally on the common circumference, internal and external, of the driven and driving halves of the spigoted members, thus allowing for easy removal. A balanced induction pipe with buffer ends was made to bolt on to the existing twin-carburetter pipe casting, with a flange welded at the top to take the blow-off valve. A short pipe opposite this flange took the hose coupling to the long 1½ inch diameter pipe from the blower outlet. The accelerator operation was taken by a cross bar on the back board to the near side, to control the throttle through a long quarter-inch diameter rod and ball joints. A special, large-choke-diameter S.U. replaced the two smaller carburetters used unblown, and the mixture control was operated by Bowden wire.

These early type Marshall blowers used straight-cut gears, and provided the oil was led to the gears, they acted as a pump, passing the oil through and ejecting it from a suitable pipe where it could be returned to the sump. In this particular car a Tecalemit inertia pump, as used for chassis lubrication, supplied oil to the gears. A suitable cowl was beaten up to cover the blower and clean up the frontal appearance of the car. This set gave a supercharger pressure of 9 lb. per square inch at peak r.p.m. It was returned later as a stone, drawn through the carburetter, had jammed between the rotors, tearing off a flake of duralumin, the rotors being constructed of that metal, with steel spindles. A new blower was fitted, the Tecalemit pump removed, and oil taken to the gears from the camshaft-feed, via a filter.

Each new installation embodied improvements, and the next job I remember was one for that well-known competition driver, Norton Bracey. The general details were similar to those of Cope’s M.G. set, but a drive-coupling of smaller diameter allowed us to fit the blower closer to the radiator; a Vokes’s air-cleaner was also fitted to prevent a recurrence of the trouble experienced with Cope’s car. Leading some of the blower-gear feed-oil back into the induction-plate provided upper cylinder lubrication, but this was discarded later. A neater cowl was made up, with an octagonal gauzed hole in front bearing the M.G. badge, and various parts were chromium plated giving the car a very pleasing appearance. It rendered a good account of itself in competition work in many spheres.

The most interesting “P” type M.G. we supercharged was that belonging to H. G. Symmons, described in a contemporary as “the most perfect of its type.” It had obviously had every care lavished upon it, being fitted with a Type 75 E.N.V. pre-selector box, Scintilla Vertex magneto, pump cooling, a special rear axle assembly, while the ports were beautifully polished and mated. “Q” type valves and all the necessary adjuncts to M.G. speed were incorporated; unblown it had done 71 miles in the M.C.C. High Speed Trial, the fastest lap at 85 m.p.h. The Marshall set was built up and installed using a considerable number of refinements, blower-gear oil-feed being taken from the camshaft pipe-line through an Enots adjustable drip feed to the gears and back to the sump via an adapter fitted under the dipstick boss. A camshaft cover breather pipe led to the Vokes filter on the carburetter air intake. I took this car up to Andre’s, where it was fitted with triple-bladed Hartfords on the front axle and Telecontrols at the rear. Symmons had a really beautiful blower cowl, in light alloy, carrying the M.G. octagonal motif throughout the design, made up by an aircraft panel beater, with a special short bumper bar to protect the blower. The car was run in many events with success, and was timed over the mile, stripped, at 101.5 m.p.h. It was sold in September, 1935, to the late Derek Leon and, having had a different cylinder head fitted, covered 77 miles in the M.C.C. High Speed Trial, in the course of which drive, the blower end plate shifted, causing the rotors to touch the casing. At this time I had left Marshalls and was in my own business. I rebuilt the blower parts into a new casing, stiffened the blower mounting and stripped the engine, grinding and polishing the ports of the standard head, reboring the block, fitting special Aerolite pistons, etc. The crank was reground, all bearings remetalled and the rods rebalanced. After running it in, I prepared the car for the 1936 M.C.C. High Speed Trial, and it lapped at nearly 90 in practice, with the blower giving 10 lb. per square inch at 6,500 r.p.m. I rode with Leon in the event, and we covered 80.98 miles, the car never being really extended. Leon used an ethyl-benzole mixture, Champion L.A. 14 plugs and Speedwell oil, and the water and oil temperatures never exceeded 80°C., mean r.p.m. 5,500, blower pressure 8 lb. per square inch. The standing lap was covered at 71 m.p.h. and the total distance covered was, I believe, the greatest ever accomplished in one of these events by an 850 c.c. M.G. After many vicissitudes this car was sold to Yvonne Morel, who substituted a crash box for the pre-selector.

To return to my time at Marshalls, one trip I had was to collect a blown M.G. “N” Magnette from Clapham, where it had been left with seized blower gears; it had been run at Lewes Speed Trials, and had a perfectly open exhaust. I freed the blower gears and had an interesting run back endeavouring to avoid the police, the exhaust note being colossal! We removed the small “65” blower and fitted the larger Type 110. I heard later that the car was putting up an exceptional performance, doing 105, fully equipped. In a never-ending endeavour to bring the price of blower sets within the reach of all, many experiments were made with belt-driven sets. An M.G. “K.N.” Magnetic saloon, blown with a Tyre 65, driven by two whittle belts from a crankshaft double pulley at 1.2 engine speed, was the next job in hand. The blower was mounted on a plate, bolted behind the induction pipe, and it gave 4 lb. pressure and a fuel consumption of 28 m.p.g. Another experimental belt-driven set was fitted to an open 9.5 h.p. four-cylinder Triumph “Gloria,” on which the blower was mounted on a plate on extended cylinder nuts about midway along the block and an extension shaft was carried forward on an out-rigger bearing to bring the drive pulley in line with the crankshaft drive.

An early M.G. Magna was the first car in which the cast nose-piece carrying the extension shaft was used, this being evolved from various experiments in an endeavour to arrive at a universal fitting. Other small cars fitted with such sets included “J” type M.G. Midgets and Austin Sevens, etc.

An interesting job at the works was the late P. F. Jucker’s T.T. Replica Frazer-Nash with Gough-designed o.h.c. engine. It had, originally, a chain-driven Type 110 supercharger, but this drive was removed and the blower mounted flat against the timing case and driven by an extension of the magneto driving shaft, which was squared and spigoted into in corresponding hole in the rotor shaft. Jucker’s private car, an open white Isotta, always brought a spot of colour to the somewhat drab surroundings of Clarendon Road whenever he visited Marshall’s works.

Successes of the Marshall blower in trials will be well remembered by all, none more than those gained by the M.G. “Cream Cracker” and “Musketeer” teams. The “Musketeer” Magnettes were among the most interesting of genuine trials ears built up from more or less standard parts. They had L type chassis, N-type engines bored to 60 mm. and using P.B. pistons, and P-type bodies and rear axles, giving them crab track; the differential was, of course, locked. They used a small Marshall belt-driven unit giving 5-7 lb. per square inch boost at maximum r.p.m. and would do 100 m.p.h. fully equipped and climb 1 in 2½ on a second gear of 11 to 1. The later “T”-type “Musketeers,” after a season unblown, during which Macdermid experimented with supercharging, used a standard Marshall set blowing at 6 lb., and having ridden with Macdermid, I can vouch for the potency of the combination. Another fierce blown job was Mac’s personal P.B. M.G. Midget, boosted at 15 lb. by a Type 260 Centric, and using a 1⅝ inch S.U. carburetter. I had the pleasure of driving this little car, which was also good for the 100, the blower wailing like that of any Mercedes.

I have, since I left Marshall’s, rebuilt blowers for Haesendonck, Goodenough and Green, and would undoubtedly have done more work of this nature had not war intervened.

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