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POM ON THE 4 1/2-LITRE SUPERCHARGED BENTLEY PART 2

THOUGH DESIGNED AS a Le Mans sports car, the 4 1/2-litre Bentley in supercharged form took to Grand Prix racing. Today, we continue Laurence Pomeroy’s analysis of this car in his classic work The Grand Prix Car.

Sports Car Heritage. Pom said in The Grand Prix Car, Volume One, “Much attention was given to reducing the weight of detail parts by drilling, but there was a large amount of heavy equipment, the large filler caps and outsize fuel tank, for instance. Inspecting of the car shows that many components are unnecessarily, even absurdly, heavy.”

This and following illustrations by L.C. Cresswell from The Grand Prix Car.

On The Other Hand. Pom observes, “At the back of the body a large fuel tank with a capacity of approximately forty-five gallons gave a range, at say, 8 m.p.g., of 360 miles without refueling.”

This implied a non-stop drive for the 1930 French Grand Prix’s total distance of 246.5 miles.

Above, the Pau circuit of the 1930 French Grand Prix. Below, the eleven finishers of 25 starters. Images from Wikipedia.

Note the long straights benefiting the Bentley’s top speed of perhaps 130 mph. Also, note the Bugatti domination. 

Tim Birkin snacks on an orange on the way to his 1930 French Grand Prix podium

Cam-type Fillers. And, as for those large filler caps, Pom observed, “It will be seen that both the fuel tank and the radiator have cam-type, quick-opening fillers, and it is worth putting on record that the Bentley Company were amongst the pioneers of these fittings, which have obvious advantages over the screw-type filler caps which had been previously employed in all racing cars. Both the radiator and the rear tank were fully protected from flying stones, a necessary feature at the time when road surfaces were liable to break up during the course of a twenty-four-hour event.”

Engine-Dictated Shape. Not unlike fighter aircraft of later eras, the Bentley’s overall shape was dictated by its powerplant. The 4 1/2-litre displacement came from four cylinders, each with a bore of 100 mm (3.9 in.) and, typical for the era, a lengthy stroke of 140 mm (5.5 in.)

The Bentley’s single overhead camshaft hardware added to its overall height. What’s more, Pom noted, “The choice by the designer of wet-sump lubrication employing large oil capacity, and, therefore, a deep base to the crankcase brought the centre line of the crank high from the ground and necessitated a wheel of 16 in. radius.”


“Thus,” Pom concluded, “the engine design is primarily responsible for the frame being 2 ft. off the ground, and for the driver’s head being 63 inches above the ground.”

Streamlining: Not a Bentley Virtue. Pom observed, “The bodies had, of course, to comply with the Le Mans and other regulations of the time governing sports car races, and thus, in conjunction with the long stroke of the engine and the high side-members of the frame, of necessity resulted in a large frontal area. Running stripped as in the French Grand Prix, the frontal area amounted to approximately 17 sq. ft., whilst with head lamps, mudguards added, this figure would increase to approximately 19 sq. ft.”

To put these frontal areas in modern perspective, they’re comparable to a Mazda Miata’s 18.4 sq. ft. On the other hand, the Miata’s Cd, coefficient of aerodynamic drag, is 0.38. By contract, a typical Cd in the 1920s was around 0.70.

Vintage Suspension. “The high centre of gravity of the Bentley,” Pom noted, “forced the use of stiff and heavily damped springs in order to prevent excessive roll on corners.”

“Stiffness of springing,” he continued, “was also imposed by the considerable unsprung masses, the rear axle in particular being abnormally large and heavy, whilst the use of large-diameter cast-iron brake drums added to the problem, which, however was mitigated by the considerable sprung mass of the car, for it must be remembered that the criterion is not the unsprung weight but the ratio of sprung to unsprung weight.” 

This erudition—described in occasionally complex syntax—is characteristic of Pom’s analyses. It also enhances the man’s vintage personality. ds

© Dennis Simanaitis, SimanaitisSays.com, 2021 

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