PUTTING 'PEP' INTO THE HORNET
PUTTING ' PEP ' INTO THE HORNET
Some details of the tuning and supercharging service available at the Derby and London depots of M.A. McEvoy (London) Ltd.,
MANY a designer must be astonished at the power which can be extracted by suitable tuning from a power unit originally intended for touring, purposes, and in no case is this more marked than in that of the Wolsley Hornet. The Hornet Special of course embodies many of these itnprovements, but the research undertaken by Messrs. McEvoy & Co. of Leaper Street, I erby, has resulted in the production of a number
-) f components which still further increase the performance. Starting with the engine, one can obtain a special induction system with two downdraft carburetters and streamlined eyhaust manifold, stronger valve springs, and an overlap camshaft. Special pistons can be had with clever arrangements for oilsaving and duralirnin connecting rods which are quite free from whip. It has also been found possible to bore out the block to bring the capacity up to 1,500 c.c. A special crankshaft and a ribbed stunp complete the engine improvements, and any or all of these alterations may be
carried out independently. A four-speed Moss gear-box may be fitted to the early models, while cross-bracing the chassis effects a tremendous improvement in road-holding. All these features were embodied in the McEvoy Hornet which ran in the 500 Miles race, which put up a lap at 91.3, and which was running well until a camshaft coupling broke. The engine develops 55 h.p. at 5,000 r.p.m., and pulls a 3.9
top gear. The same car obtained a Premier award in the Sporting One Lay Trial and the Goodyear Bowl and the Shell Cup in similar events.
Parts similar to those used in the Hornet can also be obtained for Magnas, M.G. Midgets and other popular sports cars. The lay-out of supercharging installations forms an important part of the work carried out at Leaper Street, and this department is in charge of Mr. Laurence Pomeroy, junior. He is a staunch believer in supercharging for touring purposes, claiming that in this way one can get the brisk icee1eration characteristic of a large
engine in a light chassis without the bulk and heavy horsepower rating of the bigcapacity unit. By using a large supercharger in conjunction with the McEvoyPomeroy pressure throttle, the blower pressure reaches 5 lbs. at 1,000 r.p.m., but is never allowed to go beyond 7-10 lbs., so that the engine is not unduly stressed, and standard pistons and other components can be retained.
Zoller superchargers are manufactured throughout at the Leaper Street factory, and are run in for six hours, and are several times taken down and examined during the course of this period. The blower has a single rotor built up from steel discs and alloy segments. The shaft is mounted eccentrically to the axis of the casing, and the two blades which pass through the rotor are maintained in their correct positions relative to the casing by bosses on the end covers, which carry floating rings. Hard steel feet on the blades bear on the rings, and the latter which are the only part liable to wear can be replaced in 15 minutes, as we confirmed by timing the operation.
The pressure control is a piston-shaped valve which slides across the intake pipe from the carburetter. A small pipe communicates with the induction pipe, and when the pressure built up there is suffident to overcome the resistance of the spring-loaded piston-valve, the latter partly blocks the intake pipe and prevents a further rise in pressure.
The McEvoy oil-pump contributes much to the success of the installation. It delivers small quantities of oil accurately at low speeds, so that there is no danger of oiled plugs, and maintains an even increase up to 5,000 r.p.m., unlike many of the oil pumps on the market.
McEvoy supercharging has been successfully applied to M. G., Wolselev Hornet, Invicta, Alvis, Crossley, and O.M. cars, to mention a few. Messrs. McEvoy have published a series of Notes on High Density Induction which are very informative, and anyone interested in this subject should secure a set.