A good many manufacturers make the compression figure just as high as they dare, with the result that, when the carbon forms, the size of the combustion chamber is reduced and the pressure is raised to such a degree that it will cause pre-ignition and its resultant knock. When a manufacturer tells the buyer that his engine is proof against carbon and the knocking occasioned thereby, he probably is trying to offset more serious “knocks” the car is receiving from disgruntled users.

This is an instance which illustrates the point:

A friend of mine has a four-cylinder engine in one of the later models of a well-known car of high speed and power. On several occasions we have been driving in and near the city and, after about 125 miles, we seemed always to have trouble with knocking in climbing hills. On one trip my friend had the carbon burned out carefully before starting. About the time we reached the end of the trip the engine began to knock on the hills from the collection of carbon. On our return he had the carbon burned out again and the knock ceased.

I advised him to raise the cylinders one-fourth of an inch by a fiber gasket under each cylinder casting, thus increasing the size of the combustion chamber and naturally lessening the compression. He also had to adjust the water connection and raise the valve push rods, and a few things of that sort. He ran the car upwards of 2000 miles after that before it began to show any signs of knocking under severe conditions, indicating that the cylinders needed to have the carbon removed.

Where the knock is caused in this way by a slight compression increase, it indicates that the manufacturer has put the pressure as high as the engine will stand, and the only way to cure it is by raising the cylinders or lowering the pistons. The gasket is the simpler method.

This trouble was very obvious in one model of car used for road instruction at the Automobile School. No amount of ordinary adjustment and cleaning out of the carbon would keep the car from knocking after very short service. It was taken to the service station several times and returned with the remark that it would “be all right now.” It was not all right. Finally the request was made to let the car remain at the station several days and the experts would see what could be done. When it was returned the trouble was cured. But when the expert was asked what had been done he replied: “Nothing much.” He admitted cleaning out the carbon and adjusting the carburetor. But a still hunt was made for the corrective cause and it was discovered that fiber gaskets had been put under the cylinders. They were camouflaged with enamel to conceal their presence, their existence was denied, and they were like the man without a country, “unhonored and unsung,” but they did the trick, and until the car was retired because of old age and decrepitude the gaskets stood between the engine and the knock. Perhaps if that agent picks up this volume one day he will be surprised to find that his subterfuge was discovered. It may have been his little secret.

The owner who learns this remedy for knocking due to carbon and high compression will be saved a lot of worry and be enabled to cure the engine’s ills, or have it done at the shop. But take it from the writer that carbon accumulation will cause any of the high compression engines to knock, and the only way to cure it is to lessen the compression or continually clean out carbon. Also the only simple way to lower the compression is to raise the cylinder with a fiber gasket.

CHAPTER XXXIX
SOME OTHER CAUSES OF KNOCKING

The motorist must not imagine that all knocks come from too great compression, however, for there are “fifty-seven other varieties” of knock to be taken into consideration. He must not take it for granted that the cylinders are filling up with carbon if the engine starts knocking while out on the road, nor is it a foregone conclusion that the main bearings are loose.

The knock may be from a totally different source. In fact there are so many different kinds of knocks that even an expert cannot always tell just where one comes from without totally dis-assembling the engine. Even then it sometimes puzzles him a lot by its elusiveness.