Valve Multiples

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In commenting last Show-time on the trend of design, Autocar pointed to the recent re-introduction of multiple valves as an aid to engine efficiency. Thus we have seen the latest Jaguar AJ6, Mercedes-Benz 190 Sport, BMW M635CSi, Toyota Corolla, VW Scirocco, Nissan Silvia RS, Audi Quattro Sport, Peugeot 205 Turbo 16, and Saab adopting four-valve-per-cylinder engines, and Honda is using three valves per cylinder, once a legacy of old-type racing power units.

In discussing this trend, Autocar’s Technical Editor instanced the 1912 Grand Prix Peugeot as usually credited with being the first of the four-valve-per-cylinder breed, also noting that all proper Bentleys and, much more recently, the Ford BDA, the late-lamented Triumph Dolomite Sprint, Lotus and Ferrari have followed this multiple-valve pattern. He remitted the Brescia Bugatti, another notable exponent of the theme.

Although Peugeot and M. Ernest Henry are rightly quoted as the initiators of the 16-valve four-cylinder engine, it was the method of actuating those valves which was the true pioneering aspect of the 1912 GP Peugeot engine, its twin overhead camshafts operating the valves in the pent roof cylinder heads without the intervention of rockers, at a time when crankshaft speed was exceeding a dizzy 2,000 rpm. Nor should it be overlooked that, whereas the present reversion to multiple valves is intended to promote better breathing and controlled gas-flow, before the First World War designers who used more than a pair of valves per cylinder tended to do so for other reasons.

It was mainly the unreliability of the valves themselves, in those days, together with the fact that valve springs could quickly lose their tension, rockers break, and so forth, which prompted a doubling up of the valves, on the “belt and braces” principle of security. If the valve springs stood up to the demands of a racing engine, and the rocker gear could be induced not to break up, or the valve collets or cotters not come adrift (in which case a valve would fall into the cylinder, and that was usually that!), the valves were not unknown to distort and leak, or develop other maladies. If more than one valve per “pot” were used, at least the engine might continue to run, but at reduced power. Moreover, with the big-bore cylinders and large capacity swept volumes of pre-war days, large valves were required if the cylinders were to be adequately filled, and their very weight imposed severe loadings on valve springs and operating gear that could be beneficially reduced by using more than one, smaller, valve.

So the racing engines built by Peugeot, Ballot, Fiat, Delage, Humber, Mercedes, Aston-Martin and Sunbeam, etc, between 1911 and 1921, Mercedes with their convincing 1, 2, 3 victory in the 1914 French Grand Prix demonstrating that a single-overhead-camshaft would still suffice, used four valves for each cylinder.

After the war it can be assumed that valve steels and valve springs had been so improved by aero-engine development (for although such engines ran comparatively slowly, failure could be terminal) that multiple valves were needed to improve breathing efficiency more than for reliability purposes, and with the arrival of supercharging there was no longer any call for them. So it is interesting that, in spite’ of the availability of downdraught carburetters, fuel-injection and turbocharging a number of engine designers have gone back to doubling up on valves, a lead taken in this country by Jaguar.

At one time the need for more than one inlet and one exhaust valve per cylinder was regarded as unhappily archaic, as came out during 1924 in the controversy between Louis Coatalen of Sunbeam and W. 0. Bentley. I quote: Coatalen-“One of my points is that, in essentials, the present Bentley car is a faithful copy of the design of the more prominent racing cars of 1913 and 1914”. Bentley— “Multiple valves call for no special pleading. They are obviously better from every standpoint than single valves, being lighter in reciprocating weight and giving better cooling for a given port opening. For valves, even in the cars we have raced, we use ordinary stainless steel. It never gives any trouble, because we like small valves, two of each”.

If you think that Walter Bentley was labouring the case for his antiquated engine, the fact is that he remained faithful to the layout he had finalised in 1919 for his famous 3-litre Bentley for all his subsequent —engines, over a period of 11 years, up to the 8-litre. That does not disguise the fact that post-war production-car engines with multiple valves stemmed from pre-1915 racing-car practice. Nevertheless, long after Bentley had decided on four valves per cylinder, two great American companies followed suit, Duesenberg for their twin-cam Model-J and Stutz for their swan-song twin-cam DV (for dualvalve) 32.

Ettore Bugatti built those four-cylinder 16-valve Bugattis after the Armistice, but they had been laid down before the war. Even before that, around 1912, Bugatti had used three valves per cylinder for larger engines, and he retained this layout for his war-time straight-eight aero-engine, later doubled-up as a bi-block 16-cylinder. If it wasn’t until he had studied a twin-cam Miller in 1929 that he was wooed away from the triple-valve configuration, at least he had introduced America to the eight-cylinder inline power unit, and after Duesenberg had built the Bugatti aero-engine under licence they adopted the triple-valve set-up for their racing straight-eight that won the 1921 French Grand Prix.

However, whereas Bugatti always used a single exhaust valve and two inlet valves per cylinder, with induction advantages in mind, the Duesenberg racing engine had two exhaust valves and one inlet valve per “pot”, presumably for more expedient cooling of the hotter poppets.

For a few years after the Armistice four-valve layouts continued to appear, in Aston-Martin and Sunbeam racing cars and in Sunbeam’s rare “sports-conversion” power units, for instance, but the need for them was over and they were a throwback to pre-war thinking. Even in Edwardian days, though, the multiplicity of valves may not always have been on account of metallurgical limitations, as for instance when Peugeot, presumably to improve breathing, used three inlet valves and three exhaust valves for the single 1,957 cc cylinder of their 1909 voiturette!

A production-car engine that followed the pattern of the 1921 racing Duesenberg was that of the 20 hp 3½-litre Nazzaro of 1922, the cast-iron head of which had two small exhaust valves and a much larger inlet valve per cylinder. The maker’s reasons for using multiple valves, even in a post-war engine, bear out my theories: “If only two valves of adequate size had been used they would have had to be of such magnitude as to make their reliability a matter of doubt in an engine doing nearly 4,000 rpm, while the head (90 mm bore) did not allow four”. The valves were vertical, prodded by a single oh-camshaft and rockers, the much greater lift of the inlet valve being achieved by making its rocker of unusual length on the valve side of the axis. Nor was that the end of Nazzaro novelty, because one exhaust valve of a pair was made to open considerably in advance of its fellow. . . .

Now we have come full circle, with the new generation of “four-valve” engines and the latest Honda range having three valves per cylinder. Honda use two inlets and one exhaust, with staggered inlet valve opening, Mitsubishi also use this arrangement for their turbocharged Sirius range, but with one small “regular” inlet valve and a larger inlet valve which opens when the engine is turning at over 2,500 rpm. Thus the engineering of earlier decades is vindicated and there is the happy thought that if a 12-valve or 24-valve Bugatti now costs tens of thousands of pounds, those who are convinced Ettore was correct in using this valve layout can indulge their whims for only a few thousands, by buying the latest Honda Prelude! — W.B.