That 1-4,xtra Ten Miles
By F. T. BERSEY,
(Mr. F. T. Bersey is the governing director of the Laystall Works).
EXCELLENT though the performance of the average
modem engine undoubtedly is, there is always the enthusiastic owner who aspires to go one better than his neighbour and who hankers after something a little above the ordinary—a very praiseworthy spirit.
One might wonder why such people do not invest in a sports model and have done with it, but it must be admitted that there is a peculiar fascination in secreting a few extra “horses” beneath the bonnet of an outwardly standard model. At any rate, the fact remains that there is an increasing tendency amongst owners of standard mass production cars to have certain radical alterations carried out to their engines in order to obtain a super-performance.
In this short chat I do not propose to deal with ordinary methods of tuning, with which most readers of MOTOR SPORT will be more or less familiar, but to go a step further and discuss some of the measures commonly employed in ” hotting-up “engines. The term” hottingup” may best be defined as the process of increasing the power output by suitably modifying existing parts or by replacing certain of them by non-standard components. This is, of course, a far more involved proposition than mere tuning, which is mainly a matter of adjusting existing parts and lies within the scope of the average owner driver. “Hotting-up,” on the other hand, is essentially a job for the specialist, demanding as it does not only advanced technical knowledge coupled with extensive experience but also a very fully equipped motor engineering works to cope with the diversified operations entailed.
The results obtainable naturally depend upon the amount of money one is prepared to lay out, but very often the difference in performance of an engine after judicious ” treatment ” is such that it must be experienced to be believed.
In favourable cases “that extra ten miles” may easily become an extra fifteen or even twenty m-Iles per hour, with acceleration and hill-climbing in proportion. In the Laystall Works, where we have a special department devoted to this class of work, it is by no means unusual to add as much as 45% or 50% to the power output of a standard engine—and this without resorting to supercharging. I do not want my remarks to savour too much of shop,” but it will be readily understood that what I write is based largely on Laystall’s practice.
Before ” hotting-up” operations are commenced it is essential that the engine should be in first-class mechanical condition, as obviously the best results cannot be obtained if power is being lost through worn bearings or badly fitting pistons. For this reason such work is usually undertaken in conjunction with a thorough overhaul unless, of course, the engine is a brand new one as is often the case.
The first step is to carry out a preliminary brake test during which the performance of the engine is closely observed throughout its speed range, particular attention being paid to its behaviour just beyond the peak of the power curve. The data thus provided form the basis of subsequent operations. One of the first requirements is to discover what particular factors are limiting the power output ; as a
simple example, it may be ascertained that at a certain speed the B.H.P. is being restricted by valve bounce, in which event an immediate increase can be obtained by fitting stronger, or double valve springs. Or again, it may be found that the compression ratio is too low to permit of the very rapid ignition necessary for extra power : this being so the compression can be raised by machining a small amount off the face of the cylinder head or block, or possibly by taking a little off each.
Other obstacles to a greater output are such factors as restricted valve ports, induction pipe or carburetter, offering undue resistance to the flow of the gases, and lack of turbulence due to roughly cast or badly designed cylinder heads. Slightly enlarging and polishing the valve ports, also polishing the cylinder heads is usually well worth the trouble, while a larger induction pipe and carburetter—or possibly two carburetters—may often be fitted with considerable advantage.
In some cases the mechanical efficiency of the engine may be so low as to oppose any increase in power owing to friction set up by inertia and lack of balance of the rotating and reciprocating parts. Such engines do not lend themselves to “hotting-up.” Balance is an important point. It may not be generally realised that, apart from the vibration set up, an unbalanced engine means a definite loss of power and it is therefore necessary to ensure that the crankshaft, flywheel, connecting rods and pistons are properly balanced. Of these, the crankshaft is perhaps the most
important, as the flywheel is unlikely to be much out if accurately machined. One has only to remember, however, the rate at which the pistons reciprocate, for instance, to realise that even a fraction of an ounce variation in their weight must produce a large unbalanced force, which increases as the square of the engine speed. The crankshaft and flywheel must be rotary balanced, i.e., they must be in correct balance at running speeds. Mere static balance on knife edges is not good enough.
Lightness of the rotating and reciprocating parts is, of course, one of the chief considerations in a ” hottingup ” job. It is not always possible, however, to lighten existing parts, especially in view of the greater loads they will be called upon to carry. Several pounds can usually be machined off the flywheel, resulting in a definite improvement in acceleration, but it is seldom that any attempt is made to lighten the crankshaft.
In fact, in certain types of engine where the standard crankshaft is already of flimsy design and perhaps prone to whip, the practice of the Laystall Works is to make and fit a special crankshaft of more robust design, possibly with larger journals and either having’extended webs to minimise whip or being of circular web construction which is extremely rigid. Such a shaft is necessarily somewhat heavier than the standard component, but the greater rigidity makes for a much sweeter-running engine and the extra weight can be compensated by further reducing that of the flywheel.
Every ounce by which the reciprocating weight of the engine can be reduced means more power at the road wheels. Pistons, if of cast-iron, should be replaced with aluminium alloy pistons of the lightest possible design consistent with strength, or if aluminium pistons are already standard it may be possible either to lighten them somewhat or to fit a lighter type. An appreciable saving can even be effected in the weight of the gudgeon pins by fitting a new set made from a high-tensile steel such as nickel-chrome instead of the conventional carbon steel. Connecting rods may or may not stand lightening, but if these are of steel a big advantage will be gained by replacing them with a set of duralumin rods. This is rather an expensive item, however, as the latter would have to be made specially. One of the most common expedients employed with excellent effect in the quest for extra ” horses ” is to put in a special camshaft designed for maximum performance at speeds over say 40 m.p.h., good deal as regards wall thickness, but it
giving overlap timing and possibly a greater valve lift than standard. Here there is, of course, great scope for the designer who, however, must not lose sight of the fact that the car may sometimes be required to run at speeds under 40 m.p.h. ! Another measure frequently adopted and which secures a direct increase of power is to bore out the cylinders to as large a diameter as safety permits. Cylinder castings vary a
is often possible to take out as much as two or three millimetres. This entails very little extra expense if the cylinders are already being reground on account of wear, but if the car is used for competition purposes the increase in cubic capacity must be borne in mind.
Adequate lubricating of a ” hotted-up” engine, particularly the crankshaft bearings, is a subject which has to receive attention, as it does not follow that the existing system will rise to the occasion with the heavier bearing loads and higher revs. In most engines to-day the main bearings are pressure fed, but where this system does not extend to the big ends it is sometimes necessary to drill the crankshaft so as to feed the big ends from the main bearings. In this case suitable oilways must be provided in the bearings and it may be necessary to increase the capacity of the oil pump. If the combustion temperature is raised appreciably the standard exhaust valves may not stand up to their job, this depending on their material. The remedy is to
replace them with valves made from a special heatresisting alloy, which incidentally may permit a slight saving in weight and thus reduce the load on the camshaft. Whilst speaking of valves—in some engines it is possible to bore out the valve ports in the vicinity of the seatings, increasing the diameter by perhaps three or four millimetres and allowing correspondingly larger valves to be fitted. The concluding stage of the work is the final tuning and testing on the bench, which is only carried out after the engine has been thoroughly run in and all signs of stiffness have disappeared. Various combinations of carburetter jets and chokes are tried ; tests are made with alternative valve timings ; different types of sparking plugs are tried to see which is most satisfactory under the new conditions and valve clearances, ignition setting, etc., are adjusted to the best advantage as indicated by that infallible criterion—actual B.H.P. de veloped. Petrol con sumption is read at
sight from a flowmeter, a direct-reading instrument which registers pints per hour. From the readings obtained the power curve can then be plotted over the speed range and at last, by direct comparison with the original curve of the standard engine, the number of extra “horses” can be counted. Readers of MOTOR SPORT will not be under the delusion that all this extra power can be conjured up for the asking, so to speak. The makers, bearing in mind that their cars will not always be driven by experts, have designed the engine to give the best possible all-round performance having regard to reliability, economy, flexibility and length of
service, which after all meets the requirements of ninetynine people out of a hundred. If therefore an engine is tampered with and made to give say 10 m.p.h., more than it was designed for, one cannot expect quite the same efficiency at the lower speeds, nor perhaps quite the same length of life. The points I have mentioned—and many others—are
considered in ” hotting-up ” an engine, but it must be remembered that every make and type is a separate study and has to be dealt with individually. Of necessity, the process is largely experimental, the various stages being closely bound up one with another and although I have by no means exhausted the subject I think I have said enough to convey, at any rate, some idea of its scope, embracing as it does practically the re-designing and re-building of the whole power unit.