Piston speed

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Sir,
In writing that “the traditional way of making an engine run faster is to make one with more small cylinders” (Formula One Engine Design Trends, Motor Sport, January 1989), I was not stating that this was the only (or even the best) approach for producing a powerful or successful engine, but simply that this has been the traditional approach to high specific power; and It is an approach that continues to appeal to many designers today, for example Ferrari, Lamborghini, Life, MGN, Honda, Alfa Romeo and Renault.

Secondly, when writing, I was aware that piston-acceleration, not just mean piston-speed, is an important factor limiting engine speed. Pomeroy himself was familiar with this view (as is evident in his book), apparently having read Mr JL Hepworth’s article, “Piston Speed and All That” (Motor, June 17, 1953). However, like Pomeroy (and other writers on engines, Setright or Cameron for example) I continue to employ mean piston-speed as one of several limiting criteria.

In doing so one is simplifying; the use of different materials and methods of machining and processing also influence the safety factor. But like many simplifications, mean piston-speed remains a useful and valid measure. However neither piston-speed nor acceleration, as Prof Dr Ing R Eberan von Eberhorst noted in a reply to Hepworth, should be used exclusively to judge the safety factor of an engine.

Regarding the 1.5-litre V16 BRM engine,! doubt whether piston acceleration was the factor which limited the success of this ambitious machine, even though it was unusually high by the standards of the day. Moreover, and contrary to the popular impression, the V16 produced a substantial amount of torque, probably as much as or more than the opposition. The torque curve of the BRM may have climbed more steeply than those of its competitors, however, but this was not caused by the number of cylinders. It was primarily a consequence of the centrifugal supercharger which was used without the cones-throttling vanes of the original design.

Today the sort of power developed by the BRM would cause few problems. Given a suitable multi-speed transmission and adequate suspension and tyres to absorb the power, an engine with a powerband steeper than the BRM can be a race-winner. Witness the 1.5-litre 4-cylinder BMW engine raced recently; the shape of its power-curve is very dramatic, with power tripling between 6000 and 9000 rpm!

David D Hebb, Institute of Historical Research, London WC1