Bugatti's U16

Sir,

You are wrong to suggest (Motor Sport, April 1988) that the 16-cylinder Bugatti engine was cobbled together out of old bits. The T45 was a completely new design, with more in common with the T46 and T50 than the T35 or T39, despite retaining the three-valve head.

The entire Bugatti Empire seems to have been built on the sale of the original U16 Aero-engine to the Americans during the First World War. However, Bugatti had not finished building it and clearly wanted another go with a racing engine; which he did in 1929-30.

The engineering justification for small cylinders is to allow a higher compression-ratio and specific output without getting excessively high valve and plug temperatures. Plugs were very unreliable in those days and compression ratios accordingly low.

The U16 layout with two crankshafts gives a short, compact and light design with low frontal area, and it fits into a standard-width chassis. A V16 would not have been easy to design with such close-spaced bores.

The crankshafts bolt onto the bottom of the cylinder blocks. Main bearings are spring rollers and the big ends are white metal. A plain centre main gives oil access, and there are blow-off valves at either end of the crankshafts. The two blocks are keyed onto the end gear casing which contains 15 gears. The blowers are driven off the back. The gears on the back of the cranks are centilevered, there being no outboard bearing to steady them. The whole thing is a beautifully compact unit.

The Bugatti’s engines run backwards so that the transmission could go round in the conventional way. The Maserati U16 had engines going round the conventional way, with the transmission going backwards.

This was because the Maserati engine was made from near-standard bits. The remarks about the Maserati being too fast for the chassis could be because this backwards rotation causes abrupt break-away if you lift your foot going too fast round a right-hand bend. It is possible that the later 5-litre Maserati U16 was changed to left-hand rotation because at least one engine was built to run backwards.

When Ernesto Maserati was asked about his U16 engines, he said that he was copying the Bugatti concept. It must, I think, be assumed that all the engine makers had been watching the development with interest, although very few had established eight-cylinder designs running from which to develop their own.

Bugatti probably built six engines, since he usually made half-dozens. He probably blew one up on the test bed and he also destroyed one in a car, leaving four. Ernesto Maserati said that he made three U16 cars, four V4 of 4-Litres (one of which was a sports-car for pulling the birds) and one V5 5-litre car.

The two racing cars have disappeared, but two V5 engines survive from a boat, one left-hand and one right-hand rotation. Perhaps they come from a car, because the Maseratis put two U16s into a chassis making a 32-cylinder car. They seem to have frightened themselves with the project because it was hurriedly and completely dismantled. Perhaps it was the end of the V5 racer.

If Rolls really wants something exclusive, what about a 24- or 32-cylinder motor?

J Howell

Copthorne, Sussex