In this extremely erudite and absorbing article Cecil Clutton does his share of gazing into the future – a habit which excellent war-news encourages. His suggested designs have their claims carefully explained, and Clutton also offers us a new basis of comparing existing sports cars. – Ed.
I spent a day of my last leave stirring up my motoring file with a slide rule, with a view to ascertaining what is likely to count for sports-car performance in the next few years. Some of my conclusions may be worth writing down, if only as a basis for discussion.
As a prelude, I made a graph of existing performances, and I can very well recommend anyone sufficiently interested to do the same for himself. No reliable curve can be made of maximum speeds and high-speed acceleration, because streamlining and other factors are so upsetting; but I take it that 0-50 and 0-60 m.p.h. acceleration is mainly a function of b.h.p., weight ratio, although small irregularities will occur owing to improper gear ratios and slow gear changes. To construct the graph, you take several cars of which you know the b.h.p. and weight. You then divide the b.h.p. by the weight in hundredweights, and these figures are plotted on the horizontal axis of the graph. You then look up the 0-50 and 0-60 figures and plot them against the vertical axis, which should be divided in seconds. Having done this for several cars two curves can be interpolated with considerable accuracy, and the results are very interesting. From it, knowing any two of the three factors of weight, brake-horse-power and acceleration of any car, the third can be determined within fairly close limits. One can also test the veracity of claimed horse-powers, and the like. For instance, I had always wondered why the touring Darracq only claimed to give 115 b.h.p. from its 4-litre engine at 4,200 r.p.m., while the no faster competition Delahaye offered 120 b.h.p. from a 3 1/2-litre engine at 3,850 r.p.m. Well, the graph supplies the answer – it doesn’t. The V.12 Lagonda has a similar claimed b.h.p./weight ratio to the Delahaye, and very similar performance figures, so that it, too, is down on schedule. This, I think, is a fairly natural consequence of a high-speed, short-stroke, unblown engine, which is not at its best at low speeds. That is, the Lagonda probably wasted a lot of time reaching 20 m.p.h. The 4 1/2-litre Bentley is another car which does not show up to the best advantage, while the 4.3-litre Alvis is two seconds faster than it should be on its claimed b.h.p. of 137, and a weight of 35 cwt. Conversely, it is interesting to note that a “Phantom III” Rolls-Royce probably puts out about 170 b.h.p.
Here are a few cars which run very true to figures:
Unfortunately, I know of no reliable figures for the 57 S.C. Bugatti (220 b.h.p.) and the 2.9-litre Alfa Romeo (180 b.h.p.), both of which generally weigh about 25 or 26 cwt. (chassis 19 cwt.). What is rather humiliating is to recollect that Lycett’s Bentley does 0-50 in about five seconds, and 0-60 in about seven seconds. The graph becomes rather vague for figures of this kind, but suggests an output around 320 b.h.p., and certainly indicates that even the 57 S.C. would have to get up uncommonly early to rival it, while the Alfa-Romeo might as well save itself the trouble, and stay in bed.
Taken as a whole, the figures are not particularly inspiring, and one is led to wonder what standards of performance might be expected if an intelligent manufacturer really applied his mind to manufacturing a sports car. There are, of course, two ways in which he can improve performance. First, by hitching more horses to his buggy and, secondly, by streamlining it. For many years, advances in horse-power have been slight; streamlining was just coming into its own before the war, particularly in the later Le Mans and 1940 Mille miglia racers. As a pointer to performance we have the 328 B.M.W., which many people regard as the most advanced sports-car of the day. It achieved a very fine performance on a modest output of 80 b.h.p. In the 1940 Mille Miglia it was giving some 135 b.h.p. (still unblown) and this was enough to drive its streamlined saloon along at 135 m.p.h. A similar output would not move an unstreamlined saloon of similar frontal area at more than 100 m.p.h. What exact relationship this engine had to the 328 I do not know, but I have heard from what should have heen an entirely reliable source that B.M.W.’s were working on a twin-cam edition before the war, and crankshaft speeds in excess of 7,000 r.p.m. were mentioned.
In trying to assess power outputs of the future it is perhaps before anything else necessary to consider what sort of crankshaft speeds are likely to become current in the near future. Everyone knows, of course, that the i.c. engine, even with poppet valves, can operate satisfactorily at 10,000 r.p.m., but I suggest that a number of factors are likely to make much more than 6,000 r.p.m. unusual-in production motor-cars.
In the first place, very high engine speeds call for very small cylinders and very short strokes. These may be compatible with sports-car practice up to 1 1/2 litres, but above that, the complications. attendant upon upwards of eight cylinders would not, I think, find favour in any but the largest sorts of car. Furthermore, to fill the cylinders at such high speeds a very large valve overlap is necessary, and this produces intractable running at low speeds. Equally, a large overlap leads to fuel wastage, which is necessary as an internal coolant, to remove some of the excessive heat generated at such speeds, but which would not be tolerable in a road car. I suggest that about 45º is the greatest overlap which is likely to be practicable in a sports car, and I believe that neither the Bugatti nor Alfa-Romeo has anything like as much.
Valves are wonderfully reliable things, and in the future it is to be anticipated that they will have sodium cooling to help them. Hairpin valve-springs are not beloved of motor-cyclists without good reasons, but perhaps there would not be room to fit them in on a multi-cylinder engine. Of course, if rotary valves come into their own, the r.p.m. question at once has to be reconsidered, since many of the objections to high r.p.m. then disappear, and it is worthy of note that a single-cylinder, 350-c.c., touring type, unblown Aspin engine has developed over 30 b.h.p. at 8,000 r.p.m. This figure is equivalent to 90 b.h.p. per litre, and 143 b.m.e.p., and it has been exceeded in racing form. Even so, such speeds raise the problem of high piston speed, and although these may be indulged in by racing engines, they are wasteful, demand expensive construction, and are not conducive to long life. This again, coupled with the objection to the very small cylinders compatible with racing practice, suggests a maximum of some 6,000 r.p.m. for sports cars. It may, of course, be that the development of light alloy pistons and connecting rods may promote a second revision of existing ideas about piston speeds, similar to that which was brought about when aluminium ousted the cast-iron piston, but that seems to be looking rather unnecessarily far into the unknown.
Reverting to piston speed, a maximum of 3,500 f.p.m. seems to be an extreme figure, since, if it is exceeded, the cruising speed has to be so very far below the maximum. This means that top gear has to be arranged so that nothing like peak revs. can be attained in it, while the power available at cruising revs, is undesirably low.
Assuming, therefore, a maximum of 6,000 r.p.m. we are tied down to a stroke of not more than 85 mm. Cruising revs. can then be about 4,400 r.p.m., which is certainly a useful speed. So far as the bore is concerned, too wide a piston is difficult to cool, and it is also heavy. Current practice suggests that 75 mm. is to be regarded as a maximum for high efficiency engines. We therefore find ourselves with a 75 x 85 cylinder, of 375 c.c. capacity which, incidentally, are the exact proportions used in the V12 Lagonda. They give us a 4-cylinder 1 1/2-litre, 6-cylinder 2 1/4-litre, or 8-cylinder 3-litre engine, which is convenient from a class competition point of view.
Now, the more or less square engine, especially with inclined valves (as any really high efficiency engine must have from a cooling point of view, alone) has very large valves, relative to the swept volume, and the gas speed is therefore low. This apparently means poor turbulence at low speeds, and a rather bad falling off in power at the bottom end of the range. Equally, with a short-stroke engine, it is impossible to obtain an even moderately high compression ratio.
It therefore seems essential that the engine should be supercharged in order to make the most of itself; and because it also needs assistance at low speeds it seems that a Rootes blower, running at 1 1/2 to 2 times engine speed, is the most suitable kind. A touring machine which is lacking in punch at low revs. is a maddening thing to drive for long. This is of importance, because a Rootes blower is not practicable at more than 15 lb. pressure, and probably around 12 lb. per sq. in. is the most at which it can profitably operate. This means to say that we can look for roughly a 70 per cent. increase upon whatever power output we can obtain with atmospheric induction, and this is the next thing to estimate.
It is difficult to find an unblown production engine which produces 50 b.h.p. per litre; probably the 57S Bugatti is the only one which does so, though the V12 Lagonda reached this figure in its slightly-tuned Le Mans form. An experimental 6-cylinder 2-litre Scott two-stroke engine has attained 50 b.h.p. per litre at only 4,500 r.p.m., which is equivalent to 145 b.m.e.p., as against about 115 or 120 for the Bugatti. Even the Bugatti cannot be used as a direct analogy, since the compression ratio had to be reduced by two atmospheres before a blower could be fitted, which would bring the power down to about 43 or 44 per litre.
At the same time, a glance at racing practice suggests that these figures are not as good as they well might be. Very comparable is the excellent output of the unblown 1923 G.P. Sunbeam which gave 108 b.h.p. and 140 b.m.e.p. at 5,000 r.p.m. running on straight petrol (which then meant a very low octane value) and about 7.4 to 1 compression ratio. These engines had by no means excessive valve timing. Let us also look at the 1927 1 1/2-litre G.P. Delage engine, which is, to this day, probably the most efficient engine that has ever been made. What makes it particularly comparable is that the valve overlap is only 43º, which is within the limits of sports car practice. As run by Seaman it had a compression ratio of 7 1/2 to 1 and a blower pressure of 7 1/2 lb. per square inch. After deducting the effect of the blower this leaves a b.m.e.p. of 135 lb. per square inch at the peak speed of 8,000 r.p.m. Now, even allowing for the extraordinarily high mechanical efficiency of this engine, which might be expected to give it a 10 per cent. advantage over a plain bearing production job, the inference is that the breathing arrangements of this 16-year-old design are far in advance of anything which has been done since. I have not seen the Delage power curve, but it can be assumed that if it shows 135 b.m.e.p. at 8,000 r.p.m., it will be at least 150 b.m.e.p. at 5,500 r,p.m., at which our hypothetical engine may be expected to peak. From this must be deducted 10 per cent., to bring it down to the mechanical efficiency of a plain bearing production engine, leaving us at 135 b.m.e.p.
A further deduction would have to be made to allow for a reduced compression ratio, at which the engine would operate on pump fuel; but having regard to the very moderate boost and compression ratio, and the improvements in cooling and post-war high octane fuels, which, alone, are likely to bring about a 10 per cent. increase in power output, this factor can probably be ignored.
Now, 135 b.m.e.p. at 5,500 r.p.m. means 57 b.h.p. per litre, to which we have to add a 70 per cent. bonus as a result of our intended 12 lb. boost; which reaches a total of 95 per litre. This is certainly a 50 per cent. increase over anything at present available, but unless there is some very bad flaw in my argument it does not seem an unreasonable figure.
It is, of course, allowed that a 7 1/2 to 1 compression ratio and 12 lb. per square inch boost would without doubt call for an alcohol fuel (such as 50 per cent. alcohol and 50 per cent. benzole) if the car were to be used for racing, or any other purpose requiring prolonged use of full throttle. But my satisfactory experiences of driving such supercharged cars as the 1924 G.P. Sunbeam, 57 S.C. Bugatti 2.9 Alfa-Romeo and blown “Phantom I” Rolls-Royce, all on pool petrol, lead me to suggest that, on an 85 octane fuel (such as we may apparently expect in commercial production after the war) such a compression ratio and boost would be perfectly satisfactory for all normal running.
One must also remember that even the most advanced of our current machines, namely, the 57SC Bugatti and “2.9” Alfa, are based on the 1928-1934 era of Grand Prix racing design, which was a very long way behind the standards of design during the previous 2- and 1 1/2-litre formulae.
We have now got a 1 1/2-litre engine developing some 135 b.h.p., and we must see how fast it will go when fitted into a properly streamlined outfit. Fortunately, this happens to be exactly the same b.h.p. as the Mille Miglia B.M.W., and as that body was basically a perfectly practicable one for regular road use it seems not unreasonable to expect a speed of 135 m.p.h. or, say, 130, to allow for contingencies. By applying the cube root rule we are also able to calculate the maxima for the 6- and 8-cylinder editions, since these need have no greater frontal area. Furthermore, assuming that the 1 1/2-litre car would weigh 19 cwt., which is not unreasonable, and that the 6- and 8cylinder versions would weigh 22 and 25 cwt. respectively, we can assume that the 1 1/2-litre would be a little inferior to Lycett’s Bentley on low speed acceleration, and that the other two would be a little better. The maxima would be as follows:–
1 1/2 4-cylinder, 135 b.h.p., 130 m.p.h.
2 1/4-litre 6-cylinder, 210 b.h.p., 150 m.p.h.
3-litre 8-cylinder, 280 b.h.p., 165 m.p.h.
These figures certainly sound quite staggering judged by existing standards, but there does not seem to be any reason why they should not be attained in the light of existing knowledge. They are arrived at solely with regard to the limitations of fuel, fuel consumption, flexibility and mechanical complication necessary in a car intended for regular road work. There seems no reason why the 8-cylinder car should be more expensive than a 57SC Bugatti (chassis £1,070 with the then favourable rate of exchange) though the chassis layout would be different, a de Dion rear axle, i.f.s. (probably of the hydraulo-pneumatic kind which will undoubtedly be in prominence) and tubular chassis being a likely arrangement.
This is certainly talking in terms of money for most of us, and one’s mind next turns to the sort of sports car which will be available for the pecuniary “polloi.” Much depends on the standards of road-holding which become prevalent on production cars, and the extent to which streamlining becomes popular.
Performance is always expensive, and while there is a certain number of people who are willing to put up with bad roadholding if they can have high speed, I think that the discriminating enthusiast places the greater insistence on proper handling. For such people there was really quite a wide choice, even before the war. In the cheapest range, the D.K.W., Citroen “Light Fifteen,” Peugeots, and any car in the Fiat range offered respectable performance (according to size) and first-rate road-holding; while in the under £500 class came Lancia, Type 320 and Type 55 B.M.W., H.R.G., and the 16- and 20-h.p. Rovers, which were notable value for money, and an excellent example of what can be done with non-independent suspension if properly designed. Much the same could be said of the 2-litre Triumph.
Any of these cars, if fitted with streamlined coachwork, could have given an outstanding performance, and if streamlining does not meet with popular acceptance after the war, it seems apparent that there will be a good opening for anyone who markets any of this type of machine with a standardised streamlined body at a reasonable price.
Probably the first manufacturers to do so were the Fiat people, and their streamlined 1,100 would undoubtedly have met with a riotous reception from enthusiasts had not the war intervened. For details, readers are referred to the magnificent road test by Gordon Wilkins in the Motor for July 25th, 1939, but some details may be recorded here, as a pointer to the type of performance which may be brought within the purse range of a very large number of people, and as an example of what may be done with a light, simple, lightly stressed, short-stroke engine.
In standard form, the 1,100 Fiat developed a very modest 32 b.h.p. at 4,000 r.p.m., had a 70 m.p.h. maximum, and sold in saloon form at £198. The weight was 16 1/2 cwt., and the cylinder dimensions were 68 x 75 mm., which meant that the engine could cruise at practically any speed it was likely to attain. In point of fact, it must have been capable of roughly 5,500 r.p.m., and its ability to maintain such speeds undoubtedly contributed largely to its racing successes in the hands of Gordini and others. The streamlined car developed 42 b.h.p. on the moderate compression ratio of 7:1, probably weighed about the same as the standard car, and sold at £375. This is a lot of money compared with £198, and there seems no valid reason why the model should not have sold for well inside £300 when it got into serious production.
Within very small limits this machine has the same b.h.p./weight ratio as an “Aprilia,” but its 0-60 acceleration is no less than six seconds faster than the very good Lancia which, itself, is by no means aerodynamically inefficient. Incidentally, the Fiat seems to have been geared to give about 16 m.p.h. per 1,000 r.p.m., having 15″ x 5″ wheels, and the excellent overall ratios of about 3.5, 5.1, 8.1 and 13 to 1. This means that it must have reached about 5,500 r.p.m. at its maximum of 90 m.p.h. The 0-50 acceleration time was 13 seconds; 0-60, 19.1 seconds; standing quarter mile 21 seconds (an amazing figure). Petrol consumption, 40 m.p.g. at 60 m.p.h. (even more amazing).
It certainly seems that, coupled with the already excellent handling of the Fiat, the ordinary enthusiast could not reasonably ask for more, though if the car could be improved without additional expense, it would be by manufacturing it with front wheel drive, whereby the road-holding would be even further improved, and the body space increased without increasing the overall height.
Considering the popularity of the “500” and “1,100” Fiats in this country I have often wondered why the “1,500 ” model did not sell in larger numbers, for it is a really excellent car. In particular, it is rather more solid in construction, and the general finish is much superior to the smaller models. Possibly the 15-h.p. R.A.C. rating operated too much against it. The 65 x 75 engine developed 45 b.h.p. at 4,400 r.p.m., and ministrations similar to those accorded to the “1,100” streamlined car should, therefore, bring the output up to 60 b.h.p. which, in streamlined form, would be ample to secure a maximum of 100 m.p.h. The price, in standard saloon form, was £298, or £215 for the chassis.
Finally, if the horse-power tax is removed, one wonders if there might not still be a future for the “husky” type of vintage car, coupled with moderate streamlining and modern suspension. It may be remembered that I outlined the form such a machine might be likely to take, in an article in the Motor of February 11th, 1942. I still believe that there is a tremendous thrill to be had from cylinders of not less than 1 litre each in capacity, though clearly they are not practicable in closed bodies, and will only appeal to the genuine enthusiast. What I do dislike are the highly-tuned engines with cylinders around the 600 and 700 c.c. capacity, which are now so popular. They have neither the smoothness of the high-revving engine with cylinders not larger than 400 c.c. each, nor the virile punch of the big four, with not less than 1,000 c.c. per pot. Experience certainly suggests that these engines are cheap to make; exceedingly durable, and very easy to maintain.
As a slight extension of this type, I have sometimes wondered if a magnificent, small production, fast tourer could not be made round such an engine as the de Havilland “Gipsy Major,” which powers the Tiger-Moth and so many other light civilian aircraft.
This superb, engine has a capacity of 6,124 c.c. (roughly 117 x 140), develops 180 b.h.p. on 73 octane fuel, has a maximum of 2,350 r.p.m. and cruises up to 2,100 r.p.m. Cruising at about 80-90 m.p.h. air speed they are expected to run 1,300 hours without even a top end overhaul, and I believe they have gone as much as 1,600 hours without an overhaul. Just imagine a car that ran 100,000 miles without even having to be decarbonised! It is, of course, difficult to tell in an aircraft whether the engine would be sufficiently flexible for road use, but it gives no indication to the contrary. The fact that it is air-cooled need be no disadvantage in a car, as we know from the Franklin, or K.D.F., and the lubrication would need very little attention to make it work the other way up (the engine sits upside down in an aeroplane). With an increased compression ratio and four carburetters (the separate cylinders just beg for a carburetter apiece) the power output could certainly be raised to between 150 and 160 b.h.p., and as the whole outfit only weighs 310 lb. it could be fitted into a car of not more than a ton total weight. The overall dimensions of the engine are perfectly reasonable, i.e. approximately 3′ 4″ x 1′ 7″ x2′ 5″, or about 1′ 8″ x 1′ 2″ at the bearers. With moderate streamlining this would be good enough to give us 115 m.p.h. and a cruising speed of 100 m.p.h. at 2,000 r.p.m. With 32″ wheels suitable overall ratios would be roughly 2, 2.6, 4 and 7 to 1. This would combine the thrill of high-powered Edwardian motoring with modern brakes and bodywork, and I believe that it would appeal very strongly to any true enthusiast. I have certainly never taken anyone in the Itala who was not immediately captivated by the savage punch of the huge, slow-speed engine, coupled with really big performance. I should expect the “Gipsy” engine to give at least 13 m.p.g. on the road; probably more.
Much of this article is necessarily of a highly speculative nature; some of the reasoning and assumptions in it may be quite fallacious – perhaps other readers may be able to throw more authoritative light on the matters discussed.