TURBOCHARGIN( on the increase, no doubt about that. An interesting letter came in from a reader in Seattle the other day, in which he wondered whether vacuum-servo braking is quite the correct thing on a Turbocharged car. My remarks about the “sudden” feel of the Audi 200 Turbo’.’, retardation and the power-braking of the Audi Quattro prompted the thought, cautiously put. Well, a turbocharger is only a supercharger driven by an exhaust-gas turbo and if it, the supercharger, sucks mixture from the carburetter as it did on most engine-driven supercharger layouts, the effect on vacuum-servo braking must be the saute. However, on the Renault 18 Turbo the blower feeds air into a pressurised carburetter as Mercedes-Benz did in the old days of supercharging. In the past we have had supercharged cars with suction-servo braking and we have seen proprietary supercharging sets put onto ordinary cars, some of which must have used vacuum braking-assistance. On brusher hand, it does seem that raising inlet manifold pressure by supercharging (or Turbocharging) is not the best ‘nay of humouring a brake servo which is actuated by manifold suction. As I discovered before the war when driving a 61/2-litre Bentley, the suction is essential, for having switched off the big engine to conserve petrol I could scarcely afford, I had a shock when I tried to stop the heavy car and
found absolutely no help from the brake pedal. Only hauling on the hand-brake saved the situation. And in recent times I have been astonished to find how many cars have very little reserve “vacuum” indeed, by which I mean that if you coast with the engine stationary, after one or two applications of the brakes all servo assistance has gone, whereas the older cars had reasonable-sized reservoirs to store the “vacuum” for just this contingency. However, presumably, properly-conceived vacuum-servos function all right with Turbocharging, because Saab. Audi on the 2WD car, Lotus, Peugeot, and Ford Mustang all use
THE vet–v neat turbocharger Installation for the Renault S Turbo, the brakes of which are vacuum servo-assrsted.
vacuum brake-assistance. Indeed, you only fiat sophisticated power-braking on Rolls-Royce Bentley, Comm and Audi Quattro Suction-braking works for diesel engined cars and the Mazda rotary-engined RX-7, incidentally. When loan taking-away the road-test Audi 2. .5T I asked at what speed the Turbocharge revolved. No-one knew. When I was a Rolls-Royce trying the new Silver Spirit they tolt me they were experimenting with Turbochargim (for their expected smaller-engined car?). So I pir the question again. No-one knew. But Rolls Royce Motors Ltd. has since written to tell me They say that Turbochargers fitted to cars like du Audi and Saab run up to 120,000 r.p.m. and tha larger Turbochargers, like the Garret Air-Research model which R-R have been playinj with, have a maximum speed of between 60,001 and 70,000 r.p.m.
That’s a lot of speed) Back to our Seattle correspondent. He makes the point that in the aeronautical world the rim of a turbine-disc goes through compressive and tensile stress-cycle due to cornbinations of heating /cooling anc increasingidecreasing speeds, so that aircraft turbomachinery is subject to very definite life-restrictions, to safeguard against low-cycle fatigue. When failures do occur they are. he says spectacularly destructive. Our correspondent wonders if everyone who is on the automobile Turbocharging band-wagon is aware of this? So do we. — W.B.