Sir, In the last few 111011thS the weekly motoring journals have devoted quite a lot of spaee to readers’ descriptions of their own individual sports ears : on the whole, tImese articles and letters have been both interesting and informative, but inevitably there have been a good many performance figures claimed which, being based only on speedometer reaaliags, are of extremely doubtbd accuracy. Generally speaking. the modern speedometer is probably never more than 3 per cent. in error as far as its manufacturer is concerned, oven allowing for the effect of’ variations in tyre-pressures, and the same applies to the revaeoutiter, but deli horae errors are quite another okath-r. of recent years it has been the practice of most motor thins to specify speedometers reading 5 per cent.. 10 per (mt.. IS per cent., or even more, fast, or to (d)tain a similar effect by modifying the drive to an aceurately-calibrat ed instruniemit. W hem m this tendency began, the rev.-counter was not usually faked in this way, and the more knowledgeable motorist came to put added trust in it, but naturally it slum became standard practive to tit at rev.
eunnter with similar flatter ” to the speedometer. Most drivers are aware of this fact by now, but one still finds credulous persons who, seeing that the readings of their speedometer and rev.counter :agree, think that both are accurate, and cannot be convinced that, it is a simple matter to incorporate the same degree of deliberate error in both ifistruments. All of which led me to try to discover just what order of sustained speed it was
reasonable to expect from various vehicles, sporting and otherwise, recently on the market in this country.
It is, of course, quite impracticable for an outsider to go into all the details of a large number of different ears, to obtain exact information on bearing-loads and the like, but such information as gear ratios, wheel sizes, and general engine dimensions, is readily obtained from such sources as “The Autocar ” buyers’ guide. Also, there is a fairly widely-accepted “rule of thumb” doctrine that a normallybuilt internal combustion engine should not be expected to maintain a mean piston speed greater than 2,500 feet per minute. Like all such rules, it is far from being absolute, but it does give very reasonable results for a wide variety of engines ; it makes full allowance for the extra revs made possible by the use of a short stroke, though not allowing for the difficulty in making a poppet-valve operate at abnormally high r.p.m., and it seems well worth while to put on record the road-speeds which various cars attain at this specified piston-speed.
For purposes of comparison, I also looked up the publicly-released figures for maximum continuous power of various modern aero-engines, and allowing for the remarkably high standard of design and workmanship of these engines I found the 2,500 feet/min. rule to be very nearly correct. The Napier Dagger VIII is rated at 4,000 r.p.m., giving 2,500 feet/min.; the Rolls Royce Merlin II at 2,600 r.p.m., giving 2,600 feet/min.; the Bristol Pegasus XVIII at 2,250 r.p.m., giving 2,800 feet/min.; and the Bristol Hercules H sleeve-valve at 2,400 r.p3n., giving 2,800 feet/min. Those, then, are the figures I obtained by this simple method for the highest speed at which sundry vehicles can be expected to travel without undue wear and tear : they may displease some people, but on the whole I think they give a very good idea of the potential cruising speeds of a wide variety of cars. No account is taken of how well-made each particular type of engine is, but as all inertia loads increase in proportion to the square of the engine-speed, no very big increase in continuous piston-speed can be expected on this account : also, no account is taken of the throttle opening needed to maintain the specified speed, so that a high-geared car which might need full-throttle to hold 2,500 feet/min. piston-speed may be credited with a cruising speed at which troubles due to heat-flow would occur before mechanical trouble. With these two slight provisos I present these figures as a not altogether unreliable guide to the relative merits of the respective vehicles, sports, ersatz sports, and touring, solely from the aspect of probable high-speed reliability. If sports cars as a whole are not presented in as favourable a light as might be desired, then I can only hope that post-war models
will have a more decided advantage over their touring counterparts. I am, Yours etc.,
J. L. Loughton,