A Coatalen conundrum

It is well known that while Louis Coatalen was responsible for the engineering affairs and racing and record-breaking activities of the Sunbeam Motor Company of Wolverhampton, in the years 1909 to 1929, he was father to a very wide range of diverse engines. Indeed, the number of Sunbeam cars of catalogue-type alone is very complex. This was sorted out for the first time to the present generation of enthusiasts through the erudite article by John Wyer and John Coombes that appeared in Motor Sport after the war in 1948/49, and which helped to spark-off the interest that resulted in the formation of the now well-established Sunbeam (now STD) Register.

There may well be more than one difficult to solve conundrum posed by all these many Sunbeam car and aviation engines. But one in particular concerns me at the moment. It relates to the rare single-overhead-camshaft, four-valves-per-cylinder sports engines announced by the Sunbeam Company early in August 1921. At that time all the Sunbeam touring cars had side-valve power units. The introduction, so soon after the partial recovery of the Motor Industry from wartime disruption and the post-strife strikes and financial setbacks, of these advanced ohc engines, should have been of some significance, especially as no other well-established production car except the Brescia Bugatti had four valves for each of its cylinders. However, exciting as these Type-OV Sunbeam sports engines were, I doubt whether many were made, or if they ever made any impact on the competition scene. In fact, I do not think more than a handful of people, apart from Coatalen and those responsible to him, really knew much of the engines’ existence.

I must now make a confession. When I was confronted, in 1934 or thereabouts, with a slim, aluminium-bodied Special constructed by a Mr Lambert. who had a garage in Hampstead, and I wrote a letter about it to Motor Sport, long before I became its editor, I had been told that the engine in that car was believed to be a Sunbeam. This I fear I stoutly denied in print, never having at that time heard of STD power units with other than twin-cam, push-rod-ohv, or L-head valve-gear. I blush now at my dogmatic refusal to believe the car’s builder; but I have an idea that I was by no means alone among those who had forgotten the very existence of Coatalen’s Type-CV sports engines, let alone seen one.

At the time when they were announced, some 13 years before I happened to stumble on one in this other chassis, the Sunbeam Company was busy with many other things as well. By the time of the 1922 Motor Show all the former side-valve Sunbeams had been given ordinary, push-rod, overhead-valve engines, with the valves vertically in line, work on which must have started around the time when these rare OV designs were pending, if not before, and, anyway, tuned or sports versions of the side-valve 16 hp and 24 hp Sunbeams were available. The preparation of the new aluminium Sunbeam Fourteen engine that made its debut at this 1922 Olympia Show must also have been occupying the Drawing Office and the Foundry at Wolverhampton, apart from all the racing cars Coatalen was having built in the years 1921-23. So why did Coatalen bother to rush out these far more complicated OV engines? Although they were disclosed to the motoring public three months before the 1922 Olympia Show, they were not displayed thereat, nor, I believe, was there any mention of them in the 1922/23 Sunbeam catalogues. If they ever helped Sunbeam’s to a slice more competition prestige, this has gone unrecorded: as the engines were intended to fit snugly beneath the bonnets of the normally side-valved 16 hp and 24 hp Sunbeam cars, there would be no outward way of knowing they had been so used. As far as I can discover, no extra price was ever quoted for them. Where Mr. Lambert found his I have no idea.

Yet these were interesting engines. They had exactly the same bore and stroke as the two side-valve models, namely 80 x 150 mm., giving a swept-volume of 3,016 cc in the case of that with four cylinders, 4,524 cc from the six-cylinder version. The camshaft ran in three bearings and was driven by a vertical shaft and bevel gears at the front of the engine, enclosed by the crankcase, cylinder block and cylinder-head castings. The valves, of tulip shape, with multiple springs, were slightly inclined and operated neatly by bell-crank rockers bearing on the sides of the cams, so that the camshaft was higher than these rockers. Tappet adjustment was by threaded buttons in the rocker extremities bearing on the valve stems.

A cross-shaft, skew-gear-driven from the vertical timing shaft, operated a water-pump at one end, two vee-mounted ignition distributors for the coil ignition of two plugs per cylinder at the opposite end. The crankshaft ran in three plain bearings, block and detachable head were of cast-iron, on which the valves seated, and care had been taken over water flow. The pistons were aluminium, of semi-slipper type, and an interesting feature was a camshaft-damper consisting of a sort of pressure-release valve in the form of a spur-gear pump on the rear of the camshaft with restricted outlets for the oil fed into it, thus acting as a sort of hydraulic damper for camshaft fluctuations — Coatalen had discovered the need for some form of damping here on a pre-war Brooklands ohc engine. Rather as on a 12/50 Alvis, the head gasket had water passages only at the rear. Inspite of the sporting nature of this OV Sunbeam engine the exhaust manifold was very undistinguished and it had a central down-pipe, presumably because these engines were intended to go straight into the same chassis as the side-valve power units, on the same mountings and picking-up the same piping.

Power output for the 3-litre OV engine was quoted as over 60 bhp at under 3,000 rpm and a high axle-ratio of 3.59 to 1 instead of 4.0 to 1 was available for use in conjunction with it — this compares with 50 bhp at 2,000 rpm developed by the side-valve 41/2-litre engine and an optimistic 40 bhp from the push-rod 3-litre engine.

The aforesaid Wyer/Coombes article in Motor Sport refers to these OV engines “as showing Henri influence,” but this is stretching credulity rather far. I had wondered at first whether Coatalen, who had a propensity for making good use of old designs and components, had based these engines on that of the very successful 1911 Sunbeam racing car “Toadies II”. which had a four-cylinder 16-valve, overhead-camshaft engine of 80 160 mm, the camshaft being chain-driven, especially as another engine with shaft-driven oh-camshaft was built in 1912. Then I realised that in these racing Sunbeam engines the valves were inclined at 100deg and that there had been provision for driving a magneto. So perhaps the OV design was new after all; in 1919 that is, for that is apparently when they were first laid out.

It seems odd that these engines were introduced when Sunbeam’s had so many other things up their sleeve and that they were apparently undeveloped. I will venture to offer two possible solutions to this Coatalen conundrum. Louis Coatalen was well known for saying that “the racing engine of today was the production power .it of tomorrow” and since 1914 he had been racing cars with engines using four valves per cylinder, admittedly actuated by twin oh-camshafts, a crib of the classic 1912 Peugeot GP engine of course. He may have felt it advisable to have a four-valve-per-cylinder design available to the customers, but was not yet ready to provide them with a complex twin-cam model. He did this eventually, in May 1924, with the famous six-cylinder twin-cam 3-litre Sunbeams, which had two valves per cylinder, which, however, the current (Fiat-based) Sunbeam GP cars had by then adopted. The 3-litre Sunbeam, which for some reason had a touring-type chassis with cantilever rear springing, did not really get off the ground until 1925, when it gave Coatalen second place behind a push-rod-ohv Lorraine at Le Mans.

That is the one possible explanation of the OV conundrum. Another is that perhaps Coatalen did it to provoke WO Bentley. The 3-litre Bentley chassis, with its four-cylinder 16-valve overhead-camshaft engine, had made its debut at Olympia in 1919 and had been shown again in 1920. But had not been available to buyers until around October 1921. When it was announced Coatalen had only his pre-war side-valve designs to offer. He might well have been prompted to lay down the OV alternative, as a quick and simple answer to the similar Bentley engine, then quoted as giving 65 bhp at 2.500 r.p.m. When the Bentley failed to get into production for nearly two years Coatalen might still have thought it worthwhile to bring out his two alternative OV models a few months beforehand, if only to tease his worthy opponent. An opponent with whom he was to enter into public correspondence in 1924 in which he chided WO for using an antiquated pre-1914-type purely racing engine in his sports cars!

These are only my personal theories and it would be very interesting to hear from anyone who had first-hand experience of those little-known 1921 Sunbeam sports engines.  W.B.