Traditionally, the 2,500 feet per minute maximum mean piston speed, equivalent to 12.7 meters per second, has been used by Road & Track to establish the maximum cruising speed of an automobile in the Maserati GT heyday of the Fifties and Sixties, as the stress level that the average engine will tolerate for prolonged running. Needless to say, this number is only a heuristic average, and specific engines may well thrive on much higher speeds, or expire well below that limit. Also, owing to the geometrical fact that, for any given mean piston speed, the rate of piston acceleration is usually lower in the long-stroke design, long-stroke undersquare engines such as the Maserati I-6 tend to accept higher piston speeds than those designed with an oversquare bore to stroke ratio of a Maserati V-8.
Let us consider the exemplary performance of highly supercharged, all roller-bearing, long stroke I-8 Grands Prix engines. The 1928 Bugatti 35C with its SOHC I-8 engine breathing through 2 inlet and 1 exhaust valves per cylinder displaced 1.955 liters via its 60×88 mm bore and stroke, at a 1.47 stroke/bore ratio, and produced 135 bhp at 5,300 rpm at 9.6 kg/cm2 mean pressure and 16.8 mps mean piston speed, reached 190 km/hour, and was considered mechanically safe to 17.5 mps. Its famous competitor, the all-winning DOHC five-speed 1927 Delage, which displaced 1.488 l via its 55.8×76 mm bore and stroke, at a slightly shorter 1.36 stroke/bore ratio, produced 170 bhp at 8,000 rpm at 12.6 kg/cm2 mean pressure and 22.00 mps mean piston speed, and was good for 206 km/hour. This GP bolide, which won 5 of the most important Grands Prix of its time and remained competitive until 1939, was built in a production run of four without any regard for expense by Louis Delâge. It was motivated by a remarkable engine designed by Albert Lory to be safe up to a staggering 22.00 mps. For comparison, the 1949 Maserati 4CLT 1.492 l I-4, with its 78×78 mm bore and stroke, at a square 1.00 stroke/bore ratio, yielded 260 bhp at 7,000 rpm at 21.50 kg/cm2 mean pressure, ran at 18.20 mps mean piston speed, up to 270 km/hour. Whereas the 1950 Maserati Milan in the same configuration produced 315 bhp at 7,000rpm, and achieved 285 km/hour with a taller final drive ratio, thanks to its higher 27.80 kg/cm2 mean pressure.
Consider that a piston moves up and down the length of its stroke for each revolution of the crankshaft. Accordingly, mean piston speed can be readily calculated as twice the stroke length times the number of crankshaft revolutions per minute (RPM). Let us calculate mean piston speed in meters per second for some familiar Maserati engines:
|bore x stroke
Going up to 7,500 rpm pushes mean piston speed of a 5000GT from 16.2 mps to 20.25 mps; a Bora V-8, from 17.0 mps to 21.25 mps; and a Ghibli SS, from 16.3 mps to 22.25 mps.
Going up to 7,000 rpm pushes mean piston speed of a 3500I from 19.33 mps to 23.33 mps; of a Sebring I-6, from 18.37 mps to 24.73 mps; and of a Mistral I-6, from 20.17 mps to 25.67 mps (!).
A modern 16-Valve DOHC I-4 engine propelling Honda S2000, displaces 1,997 liters via a 87×84 mm bore and stroke, and produces 240 bhp at 8300 rpm, revving safely to 9,000 rpm, thereby improving upon the Delage GP figures at 23.24 mps and 25.2 mps mean piston speeds. In achieving these remarkable speeds, it is greatly aided by its engine block construction in aluminum alloy with fiber-reinforced cylinder walls. Even once the ponderous Maserati pistons have been replaced by lightweight modern slipper two-ring pistons, and the crankshaft has been rebalanced to suit, the vastly disparate thermal expansion rate of its cast iron cylinder liners would require much greater piston clearances and ring gaps than those made possible with a modern alloy liner. Accordingly, I seriously doubt that comparable piston speeds can be sustained in a Mistral or a Bora. The most likely candidate for hot-rodding is the mighty 5000GT with its 98.5×81 mm bore and stroke amounting to the most oversquare ratio of the lot at 0.82, and already the most powerful in stock tune. But with only thirty-two of them ever made, you might want to think twice about choosing one as a project car platform.
(According to the official factory word on the matter, only the first 2 or 3 specimens of the 5000GT had the 4938 cc (98,5x81mm) engine; the other cars were equipped with a tamer 4941cc (84x89mm) engine that made the car easier to handle. But Italian GT automobile manufacture being what it is, I would not be surprised if some 450S racing bottom ends had made it into the later 5000GT production run. Indeed, I would be surprised if they did not. Waste not, want not.)
(Originally posted on 17 February 2000.)