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Old 10-01-2005 | 10:32 AM
  #4  
William Robison
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From: Mary Esther, Florida, FL
Default RE: Engine head temps

[b]JVB:

Using a thermocouple in contact with the cylinder head, up to 250F is safe with synthetic oils, using castor oil 325F, even 350F can be survived. Do remember, the internal temperature will always be higher than that read on the outside. And there are many other ways to detect overheating.

When you use an infra-red thermometer you are reading an average that the sensing unit sees, since the outer tips of the cooling fins will be cooled by the air stream to a greater extent than the base of the fins, the non-contact thermometer will always read a number lower than the thermocouple in direct contact at the base of the fins. The amount of the variation depends on too many things to say one temperature is OK and the next bad. The height of the fins, just the thickness of the fins will make a difference. Even in engines otherwise the same (Saito) an AAC cylinder and an ABC cylinder will give different IR temperatures while the contact thermometer reads the same.

This is not to say the IR thermometer is useless, it's great for comparison. Testing different props for example; you will see immediately when the temperature starts to spike as you run a too much prop and approach an overload on the engine. The place where your "Temp gun" really shines is balancing a multi-cylinder engine, getting all cylinders running at the same load. On "Serious" racing cars I installed on board data recorders, including the exhaust gas temperature of each cylinder. Valuable for racing, needless for normal driving or flying.

Conclusion: Unless you're running multi cylinder engines the infrared thermometer should be at or near the bottom of your list of things to buy.

Bill.