Bill, incredibly insightfull-feel like I just got out of a college
course-thanks.
Tim Schoeny
Phyllis Fields wrote:
> 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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