When the commentator at the B.D.C. Silverstone Meeting spoke of John Goddard’s 8-litre Bentley as having exhaust-driven turbo-superchargers, there must have been those, myself included, who thought he was mistaken and was referring to twin engine-motivated blowers. Not a bit of it. One never quite knows what ” Jumbo ” Goddard will do next and, sure enough, he now has a couple of exhaust-turbo-superchargers on the Bentley.
The car is the previously-described 3-litre into which an 8-litre engine was installed. To prepare this engine for forced-induction it had to be somewhat modified. When the late Forrest Lyeett was running his well-known 8-litre he had two special crankshafts forged by Laystall in 1935, one being put in the car and the other, unmachined, kept as a spare. The spare remained on a shelf at McKenzie’s for 30 years. It was presented to John Goddard by McKenzie, machined by Laystall and put into his engine.
The crankshaft has big-end journals of 65 mm. dia., compared to 60 mm. journals of the normal 8-litre crank and, being nicely matured, was just the job for the contemplated conversion. In conjunction with this crank new con.-rods were designed by Phil Irving, forged by E. Haddaway in Melbourne, and machined by Johnsteads in London. Made of Comsteel R4 2.5 per cent. nickel-chrome steel equal or equivalent to EN25, these rods have a tensile strength of about 45 tons. Final hardening was achieved by soaking the rods at 850 C. for an hour, quenching them in oil and tempering at 550 C. The gudgeon-pin dia. is increased from 0.875 in. to an inch. The big-ends are white-metal lined with Johnstead’s ” Brigramet ” standard graphitised alloy, the small-ends phosphor-bronze. The c.r. was lowered to approx. 6.5 to 1 by inserting a 0.4375 in plate between cylinder block and crankcase, which also serves to stiffen the notoriously flexible crankcase of a vintage Bentley.
The superchargers are standard production exhaust-turbo vane compressors made by Garrett of Los Angeles in their Industrial Research Division for diesel engines. They were responsible for the exhaust-turbo-supercharger offered as an optional extra for a time by Oldsmobile. American buyers did not show sufficient enthusiasm, the project was abandoned, and Garretts were left tooled-up for unwanted equipment.
These Garrett TEO-6-series turbo-superchargers are hung from the Bentley exhaust manifolds without any other form of mounting. Each one sucks from a 2 in. dia. S.U. carburetter and feeds to the engine through a pipe curving over the top of the camshaft cover and dropping into the standard Bentley inlet manifold. At present these pipes have a flat-section to clear the bonnet and look untidy, but they will be replaced in due course with properly-fitted stove-enamelled piping. The dimensions of this remarkable induction system were calculated by Garrett’s engineers.
From each supercharger an exhaust pipe runs rearwards, either to outlets in the side of the car or at the tail, the previously-contrived side exhausts normally being closed by adapted petrol filler caps to provide rather more back-pressure for the blowers.
How does it add up, in action? Well, although Goddard does not usually exceed 4,000 engine r.p.m., the new crankshaft should enable 5,000 r.p.m. to be used safely. At tick-over the superchargers run at 1,200-2,000 r.p.m. At 3,500 r.p.m. of the Bentley engine they are revolving at 55,000-60,000 r.p.m. and at its present maximum of 4,000 r.p.m. they are doing 60,000-65,000 r.p.m. Hardly a cool 65,000, because under bonnet temperature is something of a problem, although it is now down to a maximum of 120”F., which is quite satisfactory. Plans are afoot to use a baffle to deflect hot air from the rear carburetter.
Manifold pressures are difficult to quote, due to the turbulent pulsating gas flow, but at an exhaust back pressure of 8 lb/sq. in. it is in the region of 8.5 lb/sq. in. and each blower is then giving 12-15 h.p. – 200 b.h.p. has been measured at the road wheels at 2,500 r.p.m. and zero boost.
To cool as much as lubricate them the superchargers require an oil feed of at least 10 lb/sq. in.—the supply comes off the engine filter and a tell-tale light warns the driver should the pressure drop below this figure.
This turbo-supercharged 8-litre Bentley is no freak. Since he has been back in this country from Australia Mr. Godderd has driven it 1,500 miles on the road and competed with it at Silverstone, Firle, Shelsley-Walsh, Brighton and in other sprints and drag contests. The penalty of conversion is exclusion from V.S.C.C. events. Fuel starvation was a problem at first but at Shelsley-Walsh a practice run was made in 45 sec. in spite of the rough road, and at Brighton it just broke Lycett’s record, doing the s.s-, kilo. in 27.3 sec., of course, the timing is different now and Lycett’s Bentley was not supercharged. The car runs on the best-grade Esso petrol, giving 6 m.p.g. at Silverstone, 10-12 m.p.g. on the road, and is lubricated with Castrol XL—W. B.