LITTLE KNOCKS ARE HARDLY BOOSTS
One of the worst things with which the autoist has to contend
mechanically is the accumulation of carbon in the cylinders, clogging
the piston rings, filling and short-circuiting the spark plugs and
causing a knock which is not only annoying, but productive of trouble
sooner or later. There are various reasons for the accumulation of
carbon, such as poor gas, defective ignition, insufficient pressure,
but few have assigned as a cause of carbon trouble and knocking a too
high cylinder compression.
Too high compression occasions much of the knock ordinarily
assigned to other causes. Engines are designed with a certain size
combustion chamber, and with a chamber of that size to get a certain
compression in pounds per square inch. The nearer we get to the point
of pre-ignition without actually reaching pre-ignition, the more
efficiently will the engine operate. Pre-ignition, of course, would
make a knock.
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.