Vintage postbag, July 1962

Why multi-cylinders?

Sir,

Why did designers in the Vintage period build small multi-cylinder competition engines ? The question is prompted by the Editor’s invitation to “Meet the Twins,” and by John McLellan’s remark that Salmson built a straight-eight in pursuit of greater piston area.

It seems to have become an article of faith that the straight-eights of Henry and Miller, conceived under the influence of Bugatti aero engines, pointed the only way to racing success and doomed to failure any manufacturer so conservative as to use fewer cylinders. Perhaps the time has come to examine this theory critically.

Let us consider the Salmson engines. The 4-cylinder version has 62-mm. bore, giving a piston area of 121 sq. cm., while the eight with 49.9-mm. bore boasts 157 sq. cm. The 30% increase hardly seems to justify the extra weight and complication, particularly since the same result could have been achieved by building a 4-cylinder engine with approximate dimensions 70 mm x 69 mm. Nor can the 8-cylinder engine be regarded as an attempt to obtain higher r.p.m. without unreasonable piston speed, since although the stroke is 20 mm. shorter than that of the earlier 4-cylinder engine, it is 1 mm. longer than that of the theoretical 4-cylinder unit of equivalent piston area!

A reasonable explanation for the use of a small-bore multi-cylinder engine is the reduction of inertia loadings brought about by smaller and lighter pistons. However, the almost universal retention of a relatively long stroke with the high loadings which this implies makes nonsense of such a theory. It cannot even be claimed that Vintage designers took advantage of the smaller reciprocating parts of a multi-cylinder unit in order to raise the safe maximum r.p.m., since practically no such engine was capable of substantially greater speeds than a 4-cylinder of comparable date and design. Many, in fact, have a lower safe rev. limit than a four from the same manufacturer owing to lack of torsional rigidity in the crankshaft, a sad state of affairs exemplified by a comparison between the Type 30 and Type 23 Bugatti.

The Bugatti eight is explicable as the logical extension of the corresponding four, and owes more to production convenience than to enlightened design. A large American eight is perfectly acceptable in view of the difficulty of surrounding so many litres with only four cylinder barrels. The straight-eight G.P. Delage is a high speed engine years ahead of its time, and is outside the scope of this discussion. Discounting these examples, the construction of a small eight in the Vintage period seems singularly pointless. Is it possible that designers were carried away by the undoubted aesthetic appeal of such engines, regardless of practical considerations?

Bedford. A. Skinner.