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POM ON THE 4 1/2-LITRE SUPERCHARGED BENTLEY PART 1
LAURENCE POMEROY, F.R.S.A., M.S.A.E., is author of The Grand Prix Car.Though not explicitly designed for Grands Prix, a 4 1/2-Litre Bentley finished second in the 1930 French Grand Prix and, thus, is included in Pom’s analyses. Here, in Parts 1 and 2 today and tomorrow, are tidbits gleaned from this erudite engineer’s comments, from L.C. Cresswell’s fascinating illustrations, and from my usual Internet sleuthing.
Setting the 1930 Stage. “In periods of technical weakness,” Pom observed, “it becomes possible for the series type high-performance car to compete with, and on occasion beat, the pure racing machine.”
He noted, “… the years 1928-31 constituted such a period, and it is, therefore, not inappropriate to include amongst the examples of Grand Prix cars a catalogue model which was originally designed for sport car racing and only found itself in a Grand Prix event by the accident of fate.”
The Le Mans Bentleys. “The 4 1/2-litre,” Pom said, “was produced largely with an eye to the Le Mans twenty-four-hour race and was entered for this event in 1927. The following year the type won the event, and in 1929 it obtained second, third and fourth places behind a 6 1/2-litre six-cylinder Bentley, which came home first by a margin of seventy-two miles.”
Sir H.R.S. (Tim) Birkin’s Suggestion—and Cash. W.O. Bentley liked the 6 1/2-litre car. But, Pom wrote, “Sir H.R.S. Birkin, advised by Mr. Amherst Villiers, was of the opinion that even better results could be obtained by supercharging the 4 1/2-litre.”
What’s more, in the winter of 1928-1929, Birkin funded the Villiers project and the supercharged Bentley made its debut in the 1929 Irish Grand Prix (a sports car event), where Bentleys finished 2nd and 3rd to an Alfa Romeo 6C.
Roots Supercharging. Pom described, “A Roots-type blower, having a capacity of 5.6 litres, was driven from the front end of the crankshaft and mounted between the front dumb-irons.”
The Roots blower gave “a boost of circa 12 lb.,” Pom wrote, “employed exceptionally wide gears and an ingenious method of scaling the rotor shafts.”
“The standard engine in unblown form” Pom said, “developed 125 h.p. at 3,500 r.p.m., and supercharging increased this by 80 per cent, so that approximately 240 h.p. was realised at 4,200 r.p.m.”
“The piston design,” Pom noted, “was, of course, determined by the sports car regulations for which the car was primarily designed. These forbade the use of alcohol fuel, and as with 12 lb. boost a 5:1 compression ratio is the equivalent of 6.5:1, from a detonation viewpoint the figure was reasonable when running on a petrol/benzole mixture.”
The Heftiest Grand Prix Car. “It will be seen,” Pom said, “on a maximum speed basis the Bentley was the equal of the racing Type 35 B and C Bugattis, but it was handicapped by great weight… The gross weight, well over 2 tons with driver and fuel aboard, must make the Bentley the heaviest car ever to compete in a Grand Prix race.”
Tomorrow in Part 2, we’ll continue Pom’s analysis and learn more of the 4 1/2 litre’s Grand Prix podium.
© Dennis Simanaitis, SimanaitisSays.com, 2021
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