Random thoughts about o.h.c.
The Editor looks at the past and present position of the overhead camshaft engine
“Every one of these new engines, except the Viva, has a single overhead camshaft, or, in the case of the Mercedes and the flat-six 2-litre Porsche, a single overhead camshaft to each bank. This is a development of which John Bond [of Road & Track] would not approve because of the complexity of the camshaft-drive and because cylinder-head removal becomes a more difficult job. Certainly the Mercedes timing chain layout is very elaborate, but Rover and N.S.U. (Prinz 1000), as well as the three already mentioned [Hillman Imp. Mercedes, Porsche], all feel that a very rigid valve train with excellent dynamics and a long service life make this worth while.” – Charles Bulmer, B.SC., A.F.R. AE.S., in last year’s Show Review issue of The Motor.
I observed the other day that German engineers have a predilection for transverse leaf springs with their i.f.s. Even more noticeable is their belief in the overhead camshaft. Mercedes-Benz, B.M.W., Glas, N.S.U. and Porsche operate o.h. valves in this way. Admittedly the o.h.c. engine has had new adherents recently in other parts of the World, the Rover 2000 and Hillman Imp making use of it in this country, for example. Although it is not essential to put the prodder upstairs to actuate valves inclined in a hemispherical combustion chamber, as I showed in a special article published in Motor Sport in March 1960, it is a neat method of operating vertical or inclined overhead valves. It was while I was pondering on Germany’s enthusiasm for this system that I came upon some words of wisdom written by my friend Laurence Pomeroy, M.S.A.E., apropos the Frankfurt Show whereby he endorsed o.h.c. valve gear as giving symmetry and freedom from cylinder block distortion, a low-inertia valve train and the ability to run for tens of thousands of miles without the need for tappet-clearance adjustment. Pomeroy has, indeed, been advocating this form of o.h. valve gear for more than a quarter of a century.
Perhaps the low-inertia aspect is the least of the o.h. camshaft’s merits, if we judge by the remarkable safe rates of revolution achieved by modern production push-rod o.h.v. power units, such as the Ford 122E GT engine, which has a top limit of 6,000-7,000 r.p.m. Nor can it be denied that placing the “prodder” above the “pots” induces certain complexities not present when the designer is content with a fistful of push-rods. For example, more positive lubrication is called for and when this is taken upstairs leaks used to be difficult to control, so that some of us remember Morris Minors and M.G.s that dribbled down their dynamos, and earlier engines which lost more lubricant down their valve stems than was altogether good for their sparking plugs.
Mainly, however, the most pertinent problem is that of driving a camshaft somewhat far removed from the crankshaft. To arrange this drive so that it is neither too noisy nor too costly yet remains unaffected by different rates of heat expansion between crankcase, cylinder block and cylinder head is quite a poser, apart from the need to obviate oil leaks from the drive and camshaft covers, retain reasonable accessibility of tappet adjustment, and obviate loss of valve timing when the head is lifted for routine decarbonisation or more serious inspection.
These problems notwithstanding, the o.h.c. engine has had many adherents down the years. There was, and still is, some prestige associated with o.h.c., and more particularly with twincam valve gear.
Whereas the single o.h. camshaft was used by Maudslay in veteran days, the twin-cam head was evolved by Ernest Henry of the Peugeot racing division expressly for racing purposes and was a revolutionary feature of his 1912 G.P. Peugeot power units, a development so revolutionary that it set the fashion for racing engines from that day hence, right to the present day. But for ordinary engines such valve gear was cumbersome, apt to be noisier than a single o.h.c., and sometimes necessitated such a bulky cylinder head that the designer felt it better to dispense with detachability, thereby obliging the luckless mechanics who had to decoke it to lift the entire cylinder block, with consequent heavy bills to the customer, as fond owners of 3-litre Sunbeams may recall.
Although after the convincing successes of the 1912 G.P. Peugeots most designers of racing engines went twin-cam, it must not be forgotten that the team of Mercedes which finished in the first three places in the 1914 French Grand Prix had single overhead camshafts operating slightly-inclined valves via rockers. So even the single o.h.c. engine has a racing pedigree and much prestige, enhanced by Dr. Porsche’s “Prince Henry” Austro-Daimler in trials and handed on to the present day by the 3- and 4 1/2-litre Bentleys of the vintage era in sports-car races. Currently, Cosworth are happy with a single o.h. camshaft for their new F.2 racing engine.
The o.h.c. engine was, however, found in cars far removed from the scene, or hint, of competition motoring. After the First World War it found favour with engineers looking to the luxury car market. While the Rolls-Royce “Silver Ghost” still had its poppets discreetly on one side, the 37.2 h.p. Hispano-Suiza used an o.h. camshaft, driven by a vertical shaft, operating the valves without intervention of rocking levers. Lanchester, too, adopted the overhead camshaft for his Forty and later for his 21 h.p. and 23 h.p. cars, achieving the required standard of mechanical quietness by the use of worm-gears for the vertical drive shaft. Indeed, I believe it was the whirring of the epicyclic gearbox of the Lanchester Forty and not its o.h.c. that caused unfavourable noise-comparisons with the current 40/50 Rolls-Royce. The less refined 40/50 Napier had a shaft-driven o.h. camshaft, Rowledge having obviously been influenced in its adoption by his war-time aero-engine associations, as had Marc Birkigt when he planned his beautiful Hispano-Suiza. Lancia used an o.h. camshaft for his 1919 V12 and Trikappa models. Parry Thomas not only shut the o.h. valves of his luxury Leyland Eight by means of leaf springs and opened them through the medium of substantial girder-like rockers but actuated them by means of an o.h. camshaft. He was clearly concerned about the noise of a gear-train or gear-and-shaft drive and, emulating earlier marine practice, coupled crankshaft to camshaft by means of a triple-eccentric drive. I am not sure whether Thomas can be said to have pioneered this form of drive or whether the late L. H. Pomeroy and Daimler-Benz beat him to it, respectively for an experimental Vauxhall o.h.c. engine that was to have replaced the push-rod 30/98 and for Mercedes-Benz marine engines.
Other luxury, or near-luxury, cars to have o.h.c. engines in the years following the Armistice included the Ensign, Farman and Straker-Squire, while Hotchkiss complicated the issue by having an o.h. crankshaft coupled to the valves, to give smooth opening and closing. The fact that it was not until 1925 that Rolls-Royce discarded side-by-side valves and never used anything but pushrods and rockers for o.h.v. actuation can be taken either as timid conservatism or stubborn adherence to pre-war precepts, although R.-R. fanatics may find consolation in the fact that Sunbeam, although builders of both single and twin o.h.c. racing-car and aero-engines, kept to push-rods for its 6-cylinder and straight-eight luxury models. Ettore Bugatti always used an o.h. camshaft engine, whether for racing, sports or boulevard model, Lancia adopted it, with exquisite rocker gear, for the Lambda, and for many years Alfa Romeo have made nothing else. Indeed, there have been too many single o.h.c. engines to list, but Delaunay-Belleville, f.w.d. Alvis, Beardmore and Ballot spring to mind as typical vintage examples.
Nor was o.h.c. valve gear the prerogative of expensive cars. The Wolseley Ten of 1921 had it, but gained no noticeable speed from it, and so did the Dawson light-car of 1919. In the case of Wolseley, undoubtedly the Company’s task of manufacturing a better V8 Hispano-Suiza aeroplane engine during the 1914-18 war than the Hispano Company made themselves, influenced the decision to have a shaft-drive o.h.c. for this sluggish little Wolseley Ten, and subsequent, larger, Wolseley car engines, the valve gear and valve covers of which were unmistakably reminiscent of those of the Hispano-Suiza. Years later the Wolseley Hornet small-six had an o.h. camshaft power unit. On the other hand, the Wolseley Ten had rocking levers interposed between cams and valves and a patent drive in which the silent chain for the auxiliaries was used to cushion the flexible-blade vertical drive to the front of the o.h. camshaft, whereas the Dawson had direct actuation much closer to Hispano-Suiza aero-engine practice but modified for quieter tappet action. The Rhode was another inexpensive o.h.c. design, notable for very crude rocker gear and oiling arrangements.
In the later ‘twenties such inexpensive small cars as the Morris Minor, from which stemmed the M-type M.G., Singer Junior, and Fiat 509, had “upstairs” camshafts, which rather confirms what Sir William Lyons has always known (even when duplicating it) that o.h.c. valve gear is neither unduly costly nor impossible to volume-produce. The major problem of how to drive an o.h. camshaft has been met in a number of ways. Gear trains (as on the Twin-Cam 3-litre Sunbeam) and shafts-and-gears, even when the latter were skew or worm gears, were apt to be noisy, expensive, heavy and complicated to lubricate. Indeed, a Mr. Vesey Eyre, who had owned a 6-cylinder Maudslay and had suffered undue wear after a few thousand miles on the small gears revolving at high speed in the drive to the o.h. camshaft, felt great mistrust of what he chose to call the new post-war “semi-aeroplane” cars, even when the gears were case-hardened, and he went to the 1919 Olympia Show to confront the salesmen with his anxieties. However suitable in aero-engines, Mr. Eyre felt that continual bumps from the road put a strain on such delicate timing gears, by which he meant, presumably, that the undamped back axle hopping up and down would cause intermittent back wheel adhesion and allow hunting right along the transmission-line to the engine crankshaft and timing gears. Later chain drive became extremely popular, a single-step gear drive between crankshaft and lower sprocket enabling the chain to run at half engine speed. The Weller-tensioner had made such drives quiet and practical and figured successfully on the 1919 4-cylinder, and later 6-cylinder, A.C. light-alloy power units, the latter persisting into the post-World War Two years. The Singer Junior and Nines had means whereby the top sprocket could be hung up conveniently before the cylinder head was removed, as you hang up a hat.
Makers of Cars for Top People were worried by the noise of o.h.c. valve gear. As I have said, Parry Thomas overcame this with his triple eccentric drive, and when W. O. Bentley cocked-a-snoot at the Rolls-Royce “New Phantom” with his 1925 Big Six Bentley he used a similar system of triple connecting rods to eliminate gears and chains (except for the half-time pinions) from his camshaft drive, which was used also for the later 8-litre Bentley. Both systems were retained for the racing Leyland-Thomas and Bentley cars. The short-lived twin-cam 2-litre Maudslay had another version of this silent o.h. camshaft drive, as has the N.S.U. Prinz today, the last-named adopted in 1952/3 when chains from German suppliers were of poor quality. When chain drive was used, two chains were sometimes employed to shorten the distance between sprockets.
Today the o.h.c. engine can be made as quiet or quieter than a push-rod engine and lubrication and oil-sealing present no problems. The method of drive is universally by chain, with two notable exceptions. N.S.U. continues to use the aforesaid eccentric drive for the overhead camshaft of the Prinz’s vertical-twin engine and Hans Glas defies conventional engineering precepts by driving the o.h. camshaft of his 1,289-c.c. GT and 1,500 Glas engines by means of an external internally-cogged synthetic rubber belt at the front of the crankshaft, which is inexpensive, simple, and ensures complete silence, with stretch presumably controlled so that no variation of the timing is to be feared. As I have said, the o.h.c. engine is more popular in Germany than elsewhere, Mercedes-Benz using a single camshaft above each cylinder block of the V8 600 power unit (for their V8, Rolls-Royce adhere to push-rods), N.S.U. having a chain-driven o.h. camshaft for its new transversely-mounted air-cooled 4-cylinder engine, and Porsche going over to this form of valve operation for the new 901.
Until you can look scornfully at mere poppet-valve cars and say truthfully that you have a Wankel rotary or Chrysler or Rover turbine engine under your bonnet, you can seek consolation from the overhead-camshaft power unit, which has prestige value dating back to pre-war races and, more to the point, is sound engineering practice. Georges Roesch might say it is quite unnecessary, but then the push-rod valve gear of his Talbots was sophisticated, with specialised attention to detail design. In England Rootes’ Hillman Imp and the Lotus Elite share such valve gear of the Coventry-Climax design, and the Rover 2000 has an o.h.c. France eschews the idea entirely, nor does America court upstairs camshafts, but in Italy Ferrari uses a camshaft over each bank of cylinders on his fabulous V12 power units.
Curiously, away from Germany, the simple single o.h.c. has been overtaken by the twin o.h.c. head, that classic racing-car layout used on catalogue cars before the war by a surprising number of makers – Amilcar, Lagonda, Sunbeam, Alta, Squire, Alfa Romeo, Maudslay, Lea-Francis, Vulcan, Singer, H.R.G., Triumph, Ballot, M.G., British Salmson, Bugatti, Facellia, Beverley-Barnes, Rally, Lombard, Salmson, Stutz, Duesenberg, Invicta, and Frazer Nash, to quote only from memory – there were others, and any you can think of you are welcome to. For 1964 this classic and covetous valve gear is favoured in this country by Aston Martin and Lagonda (but the V12 Lagonda had single o.h. camshafts), for all Jaguar models, for the Lotus version of the Ford Cortina and the Ford-engined Lotus Elan. Maserati in Italy remains staunch to the twin-cam engine, and Fiat lists the 1600S in this form, while Abarth uses several such Fiat-base engines, twin overhead camshafts are universal for the Alfa Romeo range, and Lancia has them on the Fulvia. From Germany Porsche enjoys the best of all Worlds, with push-rods for the 1600C, a single camshaft above each bank of air-cooled cylinders in the Type 901, but two camshafts over each head on the Carrera and the new Type 904GT. This form of valve actuation is the ultimate in the operation of inclined o.h. poppet valves for high-speed engines but we are primarily concerned with the single o.h.c. engine, because it is more logical to make the plea that of changing from antiquated push-rods to single o.h. camshaft (push-rod engines appear, so far as purely German cars are concerned, only in the B.M.W. 700 and Glas “cyclecars,” the big 6-cylinder B.M.W.s which are ancient designs, and the VWs, D.K.W., Auto-Union and Goggi being valveless) than to ask that the designer should embark on the complex task of design and manufacture of a twin-cam head, which even Sir Henry Royce tried and found too complicated for his 20-h.p. Rolls-Royce of 1920. – W. B.