Obtaining Performance Figures for the Shorrock-supercharged Austin Healey Sprite
Last month we had an opportunity of taking performance figures for the Shorrock-supercharged Austin Healey Sprite, for which the Donald Healey Motor Company, of The Cape, Warwick, are the sole distributors. A C/75/B Shorrock vane-type supercharger blowing at a maximum of 7 lb./sq. in. is driven by a double vee-belt at just below engine speed, and draws through a 1½-in.S.U. carburetter. This puts the power output up to 64 b.h.p. at 5,500 r.p.m.
The car we tried was an extremely nippy little vehicle, equipped with wire wheels having knock-on hubs, Girling disc brakes at the front, one of those eye-catching wooden-rimmed steering wheels and a hard-top, but alas no heater -- the sliding side windows could not, by any stretch of imagination, be said to be draught-proof. Incidentally, one has to be prepared, should one leave the car out all night in a frost, to find the windows iced up, which, as the car has internal door handles, means that the proud owner is unable to gain access to his Sprite until the thaw sets in.
With its brisk acceleration and quick steering (21/3 turns lock-to-lock) there can be few faster cars through traffic than this blown Sprite, especially as the 5.20 X 13 Avon "High Speed" tyres make it an exceedingly sure-footed car on slippery surfaces. The suspension is very hard, which accentuates the many rattles and creaks from the body, and some vicious kicks are imparted through the diminutive steering wheel. The engine goes up remarkably willingly to 6,500 or even 7,000 r.p.m., and the car is clearly under-geared and would be much better with an overdrive or higher axle ratio. As it is, 3,000 r.p.m. in bottom gear equals 11 m.p.h., in second gear 18 m.p.h., in third gear 32 m.p.h. and in top gear 47 m.p.h. These are indicated speeds, the speedometer having about the usual degree of inaccuracy, i.e., at 50 m.p.h. it indicates 53 m.p.h. and at 60 m.p.h. it reads 65 m.p.h.
Correcting the speedometer, at 6,000 r.p.m. (and this or very slightly more sounded quite enough, in our estimate) the maximum speeds are approximately 22, 35 and 59 m.p.h. in the indirect gears, and in top gear 84 m.p.h. Under favourable conditions it is possible to obtain 6,500 r.p.m. in top gear, given a sufficient length of clear road. On the test car there were symptoms of what sounded like impending big-end failure, although oil pressure was normally above 10 lb./sq. in. The normal water temperature was 160 deg. F., rising to about 175 deg. F. in traffic, but refusing to go above 190 deg. F. during performance. testing.
Having corrected the speedometer we took our usual performance figures, which are quoted in the data panel. The discrepancy between the figures in parentheses and the figures for best times is explained by severe clutch slip, which spoilt some of the runs. Endeavouring to feed the clutch in failed to cure this, but when it was banged home it was possible to obtain normal acceleration. As will be seen, this supercharged Sprite is a brisk little car, but its consumption of fuel is out of all proportion to the performance available. The tank is supposed to hold six gallons but more likely takes only five gallons, the range being a mere 137 miles. Having replenished the empty reservoir we recorded less than 26 m.p.g. driving fast but by no means in a frenzy, and 28.2 m.p.g. driving slowly like Auntie Gwen.
We used good quality, but not 100-octane, petrol; the engine was docile and well behaved and started promptly from cold. As a small sports car the Sprite is great fun and decently economical (38 m.p.g.) in unblown form. The particular one tried had a "short" in the instrument lighting and horn circuit, and the light which flashed in the driver's eyes every time he indicated a turn with the non-self-cancelling indicators would drive any night rally competitor to near-suicide. The two main instruments are before the driver, the Smiths rev.-counter reading up to 7,000 r.p.m. and the speedometer in 20-m.p.h. stages to 120 m.p.h. The needles of both were reasonably steady but the white pointers tended to conflict with the white markings. The gear-change is extremely rapid at the expense of beating the synchromesh, but the lever is rather up in the air.
The cost of the supercharger is £69 17s. 6d. complete, if fitted by the owner. This modified Sprite is great fun if you can afford the fuel bill and it is an essentially safe little car, the Girling disc brakes on the front wheels being particularly reassuring in an emergency (straying ponies on the New Forest road). The standard Sprite with this supercharger costs £872 inclusive of purchase tax. As tested, without occupants but ready for the road with an almost empty tank, the car weighed 13 cwt. 1 qtr. 0 lb. W.B.
0-50 m.p.h. 10.8 sec. (11.3 sec.)
0-60 m.p.h. 14.7 sec. (15.0 sec.)
s.s. ¼-mile 19.7 sec. (20.0 sec.)
(Figures in parentheses are mean of several runs in both directions.)