THERE has recently been a revival of interest in the problem of upper cylinder lubrication by means of fluids mixed with the fuel, and we have carried out a prolonged test of the substance known as Speedoil, marketed for this purpose. Before dealing with the result of our findings it will be as well to consider some of the problems arising in attempting to lubricate valves, guides, and the upper part of the cylinder by substance in the fuel.

The parts required to be lubricated fall under two main headings. First such parts as come in contact with the lubricant before combustion and secondly those which only come in contact with the burnt or burning gases.

Under the first heading come the inlet valve stems and guides, and these will be to some extent lubricated by the old method of mixing an ordinary mineral oil with the petrol. Such a practice has, however, a certain definite disadvantage in the fact that such an oil is bound to be completely burnt in the combustion chamber, which means a definite increase in carbon formation, and the resulting burnt product is liable to cause gumming up of the exhaust valve stems, and the consequent delayed operation of same will be detrimental to the exhaust valve seats.

It is the claim of the manufacturers of Speedoil that no normal oil can remain unchanged under the heat of combustion, and they make no such claim for their own product. This on the other hand works on quite a different principle. It is a chemical compound whose nature is naturally secret, but which, in addition to being a lubricant in its normal state, forms, when decomposed by the heat of combustion, a greasy compound which acts as a lubricant, and so attends to those parts which only come in contact with the products of combustion.

The engine selected for test, was one of which the performance and habits were already well-known, one of them being that of badly burning its exhaust valves. The engine was cleaned out and then run with Speedoil in use for just under 4,000 miles. This engine was sufficiently worn to need decarbonising rather frequently, so after this period, as compression on one cylinder had become poor, it was taken down for examination. No adjustments had been made during the test, and the loss of compression was due to an exhaust valve tappet which was not allowing the valve to close fully.

The actual effect on the performance of the car was hard to define exactly, and there was no increase in maximum speed.

Starting in cold, weather seemed to be slightly improved owing no doubt to the improved lubrication of cylinders, and sealing effect on the valve guides, but it was in the effect on the mechanical condition of the engine that the results were most surprising. The carbon, usually rather hard in this engine, was very soft and could be wiped off the head and pistons. The valve seats, with the exception of the one which was held up, were bright and clean. The stems of both inlets and, exhausts were clean and slightly oily. It was specially noticeable that the burnt valve, in spite of the fact that the burning gases had been impinging fully on the stem, had remained clean and well lubricated in this department, which goes to indicate that the products of combustion of Speedoil fulfil the makers' claims. The standard grade of Speedoil must not be used, with alcohol fuels but the makers supply a special "

brew" for this purpose.

Many famous continental firms use this compound on their cars, including Mercedes-Benz, Rudolph Caracciola being one of their drivers who employs it consistently. The suppliers in this country are the Speedoil Co., 28, Basinghall Street, E.C2.