A TECHNICAL TALK ON—
The Humble Sparking Plug
Its Value as an Indicator of Engine Condition and the Use of the Various Types
II would be as well to make it clear at the start that I do not propose to discuss sparking plugs (hereinafter referred to as ” plugs,” as they say in official documents) from the point of view of their design and manufacture. This can be very well left to the well known forms whose experience of racing and other engines give them invaluable experience. It might well be said that plugs are one of the few components, of the modern car engine in which there is very little room indeed for improvement. This may seem to be a strange statement to many who have memories of changing plugs which have given up work for various reasons, but it is due to the fact that the plug itself is rarely at fault, but to the way in which it is used.
It is for this reason and because of the many fallacies which exist with regard to them that I propose to talk of the way in which they may be employed to the best advantage ; the rules governing the use of the various types, and how these are graded ; and also how intelligent examination of the plug after use. may give valuable imformation when an engine is being tuned for speed and power.
li;ven at the risk of insulting the intelligence of the more technical readers of MOTOR SPORT it will be a good thing to set down immediately the limitations of the ordinary plug. The chief of these is the fart that no plug can be made which will stand both excessive oil and excessive heat. There is nothing to be wondered at in this, as the way of keeping it free from oil is to design it so that it will keep hot enough to burn away the oil that collects on it, while the way to make it stand great heat in running, is to arrange for such a rapid flow of heat away from the ” business end,” that it will keep cool.
I have often been asked by owners in trouble with plugs for advice, and the first question I put to them is ” What type of plug are you using ? ” In many cases the reply gives the make of the plug, which does not help in the least. In Shooting various kinds of game the maker of the ammunition, provided that it is some first class firm, is of little importance in comparison to the type of cartridge and size of shot, which must vary in different cases. In just the same way the makers of the plug, as long as they are one of the big firms who really know their job, do not matter.
Types and Troubles.
The important thing is the type of plug, and what conditions it is meant to cope with. In a low efficiency touring engine, the mechanical condition has a relatively small effect upon plug performance. If wear of cylinder bores and pistons is causing a lot of free oil in the combustion chamber, oiled plugs may to some extent be avoided by the use of a ” soft ” type, that is a plug with a large internal gas space, and thin electrodes. As soon as the compression ratio is increased, upon which the theoretical efficiency of an engine depends, the mechanical condition of the engine becomes more important, while in a very high-speed, high efficiency engine, the mechanical condition must be maintained as nearly perfect as possible to avoid plug trouble. As it must also be good for every other reason connected with getting maximum power, this is as it should be.
The reason is plain enough. As soon as the compression ratio is raised, the combustion temperature is also raised, and the ” soft ” plug will no longer be able to get rid of the surplus heat, and will “burn out.” That is to say the insulation will be destroyed by the heat, and the overheated electrodes will also cause pre-ignition. It is surprising what a lot of drivers do not seem to know the difference between a burnt out plug and one that has failed through merely getting dirty, while still more fail to separate the latter category into “oiling up” and “sooting up.”
” Oiling up.”
In the case of really bad oiling up, due either to excessive oil, or a plug of hopelessly unsuitable type, the plug will be merely soaked in liquid oil, and the trouble obvious. If, however, the process is slower and the conditions more nearly correct, the oil will actually burn away to some extent, but not entirely, resulting in a gradual. formation of carbon over the insulation and a consequent short circuit. It is when the plug is carboned up that care is needed in diagnosis, and many people try to cure the trouble by attention to the type of plug and the oiling system when neither are at fault. An over-rich mixture will also cause carboning of the plugs, and if this is the cause of the
trouble no plug on earth will stand it, and immediate attention to the carburation is indicated. It is easy in most cases of carboned plugs to distinguish the cause. If oil is the trouble the carbon will be hard and coarse, and probably slightly sticky. If excess of fuel is the trouble the carbon will be fine and -dry— exactly like soot in fact. In both cases it is the carbon inside the plug, that is on the insulation and the inside of the plug body, that must be examined.
No notice must be taken of a thin film of soot on the end of the plug in this case, as we shall see later that examination of this part of the plug will be useful otherwise.
The result of the above diagnosis will tell what type of plug is required. If the insulation has that unmistakable, dry, white look, and bits of it are starting to flake off, it will show that the plug is too ” soft,” even if signs of pre-ignition have not already given notice of this fact. Therefore a change is indicated to a type with larger electrodes (to get the heat away) and a smaller gas space.
Checking Carburation.
Here another word of warning is necessary. Just as a rich mixture may be confused with too much oil, so may a weak mixture be the cause of too much heat. It is here that the condition of the end of the plug is a guide. For sustained high speed it is advisable to have the full throttle mixture very slightly on the rich side, as this will do a great deal towards keeping the engine cool, and so avoid more serious troubles such as burnt pistons, etc. This, of course, does not mean that the mixture should be over rich all the way up the scale, as clean and snappy acceleration is then impossible. Therefore the way to examine the plug is not to go for a ” blind ” and then potter home and take the plugs out, but to keep the motor all out for as long as you wish –or can, more likely–and then slip into neutral, switch off, and coast to a standstill. Remove a plug and examine it for signs of heat. If there is a slight film of soot on the end of the plug, but nowhere else, the mixture is somewhere near being correct. if, in spite of this the insulation is showing signs of giving in, it means that a plug capable of standing more heat must be used. If, however, in addition to the plug being burnt, the end of the plug is also burnt
clean and dry, it means that the mixture is too wea,k, and before that type of plug is condemned, it should be given a chance to see how it stands up when a richer allout setting of the carburettor is used.
It is very important to examine the plugs at once after a spell of full throttle, as otherwise a short period of slower running might ” soot ” the end of the plug again, and so give a misleading idea of the state of affairs.
As plugs in general, and racing plugs in particular, are by no means cheap, it is obviously advisable to narrow the issue down as much as possible, and so increase the chance of hitting right away on the correct type. Although this article is not supposed to deal with carburation, it has already been shown that a correctly tuned carburettor is absolutely necessary before satisfactory experiments can be made with plugs, and the best course is to get in touch with the manufacturers of the instrument if there is any doubt on the tuning of the same. At the same time explain carefully for what purpose the instrument is being tuned, as otherwise they will probably send a setting specially suitable for “all-round economical running” or some such expression, which apart from giving a relatively weak performance, may cause bother through overheating at full throttle.
Having got the rest of the engine in sound condition the plug manufacturers should be consulted, and here it is more than ever important to give them accurate information about the engine and the conditions under which it will be working. If the compression ratio is high enough to require special fuels, state what fuel is to be used, and with what compression ratio.
If you have an engine which has had about I of an inch rubbed off the cylinder head, or its maximum revs pushed up about 1,000 r.p.m. above standard by various alterations it is quite useless to give the make and type of engine only, or to consult one of the maker’s charts. They will merely recommend the type of plug most suitable for normal work in a standard edition of the engine in question. These will merely burn out at the first prolonged spell of full throttle and may cause serious damage to the engine.
Tell the Makers. .
It is no good blaming the makers, as it is not their fault, and they will point out, with perfect truth, that the plugs they recommended would work perfectly in the engine you mentioned, but not in the very different engine you are using ! Conversely it is no good telling them that you have a very special model capable of some wonderful maximum speed, when the engine is actually quite standard and its owner’s happy hunting ground ; the local boulevardes. In such a case the makers will probably recommend the owner to fit some rather expensive special
plugs, quite suitable for the engine he has described, but which in his actual vehicle will persistently and constantly oil-upand it will serve him right.
I recently came across a refreshing example of the exact opposite of the breed of ” promenade motorist,” when after a stop for lunch on a long run, I was looking round one or two cars outside the said house of call. One car, of very touring and rather well-worn appearance, attracted my attention by one or two details of unnoticeable but important equipment. As I was looking at it the owner appeared, and raising the bonnet, disclosed that the very normal looking engine boasted a set of racing plugs of a fairly ” warm ” type. These certainly did not fit in with the appearance of the vehicle, or with the commendably subdued exhaust note when he started, so that when he moved off on the road I was due to take, I followed. The first few miles gave an inkling that the special plugs were not put in for fun. Although the car I was driving would normally have been some 15 m.p.h. faster than the one I was following, the latter was so much faster than it had any right to be that I could only just hold on to it. A few miles later we both stopped by mutual consent for a smoke (no, they were closed by this time I), and chatted about the respective merits of our vehicles. I remarked on his car’s performance and also on the plugs he was using, and made tentative enquiries about the modifications in his engine.
He was genial but not communicative, and confined his information to such remarks as ” only doing ordinary tuning,— the usual alterations, you know,” and mumbled almost apologetically that he “had to cover a lot of country,” before bidding me farewell. Would there were more of his kind.
However, returning to more serious matters, we come to various special cases where one type of plug is not alone sufficient for all purposes. These cases are almost entirely confined to genuine racing engines, where tuning has been carried to such an extent that for full throttle work a very special plug is required, something like a K.L.G. 341. Not that this is anything like the “hottest ” of their range, but it is well up the scale, and used in many highly supercharged engines. Plugs in this category are almost always non-detachable, as the detachable variety is not so well able to withstand the terrific pressures involved. This is a very strong reason for avoiding oiling up, as they are very difficult to clean properly and also rather costly. When soaked in oil, Discol is quite effective for washing them out when petrol or benzol will not.
Taming Racing Engines.
When a racing engine of this type is used for normal running, on the road (and many owners use competition cars for ordinary work), it is necessary to use a much “softer “plug than for racing, as the conditions are so different that only
an engine in exceptional mechanical condition, with a very well thought out lubrication system, and with perfect carburation, will abstain from oiling up a racing plug at low speed.
This is especially true of engines tuned for sprint work, where piston clearances are sometimes rather abnormal and piston rings reduced to a minimum number to avoid friction. In these cases great care must be taken to avoid ” blinding ” the motor at all without first changing to more suitable plugs, as in addition to the danger of destroying the plugs, there is also the likelihood on a really supertuned engine, of cracking or distorting the cylinder head and seriously damaging the engine in other ways.
” Special ” Fuel Problems.
There are bound to be many drivers whose enthusiasm outruns their mechanical knowledge and experience, and who get hold of a very special model, probably an ex-racing car. Failing to observe the injunctions of the makers, and unable to feel the symptoms when driving, they may do a great deal of harm to a good car, and unfairly blame the car or its makers for what is their own fault. The use of alcohol fuels, such as Discol, introduces one or two fresh problems, especially in the way of starting from cold and running light. In a very high-compression engine the use of this fuel has many advantages over the more normal types. Compression ratios up to 8 or 10 to 1 and even higher can be used with a suitable blend of alcohol fuel, but some of the very properties which make this possible, make special attention necessary in using it.,
The latent heat of evaporation of alcohol is between 400 and 500 British Thermal Units per lb., while that of the paraffinbenzone-napthene mixture which constitutes a normal ” petrol ” is about 1 50 B T .s. per lb. Also, a combustible mixture of air and alcohol must contain nearly twice the proportion of fuel that is required in a petrol-air mixture. This means that the heat required to vaporise each charge of alcohol is about 6 times that required per charge of petrol, and in consequence, the mixture when cold will consist of a lot of liquid fuel which will soon ” drown ” the plugs. Hence the need for starting up on alcohol with an old set of touring plugs with good ” spidery ” electrodes, and running the engine on these until it is thoroughly warm, when the fuel will be sufficiently pre-heated to run normally with complete vaporisation.
There is no room in this article to discuss the numerous advantages of alcohol for racing, such as the greater volumetric efficiency, lower running temperature, and higher power output obtainable, and I have only mentioned it as one of the many instances where a little knowledge of the use of sparking plugs suitable to the special conditions in each engine will enable the owner of a sports car to get the best out of his engine and an added interest from his clrivh.g.
—B.
