Group,
The P & G Company made the first cylinder displacement testers. The tester
measures the volume of air displaced by a cylinder.
Since the air entering the engine being tested is at ambient temperature,
it will expand when it comes into contact with a hot cylinder, head
piston or valve. Thus the volume of air pushed back into the tester will be
greater then it was when it was inducted into the cylinder. In order
to get a reasonable measurement it is necessary to know both the ambient air
temperature and the temperature of the heated air when it is
pushed back into the tester. Fuel residue in a cylinder may also cause the
reading to be inaccurate because the vapor pressure of the sample
is different than a plain air sample.Cold motors can also present problems.
Cold engines tend to have more ring leakage. Valve leakage can
also cause errors in the measurement. Normally an inspector should take the
measurement several times to insure that he is getting the right
measurement. The condition of the tester is also important. As a matter of fact
I just purchased a new tester for use at Maxton this year. I
have played with it a little at the shop and find that it usually repeats
values within 1/2 %. Due to the variations in engine condition and
temperature & pressure measurement as well as a slight variation in inertia of
the air, due to varied cranking speeds of the motor, the tester is
not used to certify measurements within 3%.
Most engine builders do not take special precautions to make displacement
calculations at a specific temperature. I usually measure bore
in a race motor at 175 degrees. My hone is equipped with an oil heater and the
block and torque plates are allowed to temperature stabalize
prior to the honing operation, but most of the other measurements are taken at
shop temperature. Measuring the bore at 175 degrees
requires good gloves and a lon handle on the bore gauge.
If the engine has to be subjected to a physical measurement then that
measurement should be made at the ambient temperature. Unbolting
the head from a hot motor can cause a lot of unnecessary warpage, and it is not
exactly comfortable for a mechanic to disassemble a hot
motor.
Many racers chose to have the engine sealed and torn down at a later time.
Since these engines are usually measured cold, then in all
fairness all engines should be measured cold. Also if you have inspected
engines you know that everybody wants to run all day and have the
inspection done at 6 PM. Since the inspector can't do every engine at once some
engines will again have to be measured cold. If you want to
get really close in inspections we could set a temperature standard and factor
in the thermal expansion rate of the material being measured,
but is that really necessary.
Many round de round racers use cylinders of different sizes. Some short
track engines are build with larger right side cylinders since
centrifical force tends to throw the fuel mixture to the outside of the motor
when cornering. Other long track motors are build with larger
center cylinders since the four barrel manifold usually feeds those cylinders
better. Other builders make 5 & 7 smaller since they are next to
each other in the firing order and draw from the same area of the manifold. But
the inspector gets to choose the cylinders he wants to
inspect.
As far as the point in the cylinder that is measured for bore, I think that
it should be at the top above the ring wear area. This portion of
the cylinder is the same size as it was when the engine was assembled. If the
rest of the cylinder has worn more than several thousands of an
inch the motor probably won't be in the impound area anyhow!
Lastly I would like to mention Dan Warner, I have been present when he
measured engines. Dan used good equipment in a very
professional mannner. I only hope to do as good a job as Dan does.
Bill Mackey
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