CHTs - What is acceptable?

TripFarmer wrote:
I just don't buy that "airframe" stuff. A cylinder head can only take
so much temperature for so long before it gives in. 400 degrees is
400 degrees no matter what airframe it's in. If I'm wrong please tell
me why.........

I agree the temperature a cylinder can tolerate probably doesn't depend on the
airframe, but the method of measurement might. Where is the CHT probe on the
cylinder? Don't know, but maybe Piper puts the probe in one hole, and Cessna in
another.

Dave Butler wrote:

I agree the temperature a cylinder can tolerate probably doesn't depend on the
airframe, but the method of measurement might. Where is the CHT probe on the
cylinder? Don't know, but maybe Piper puts the probe in one hole, and Cessna in
another.

In a sense, you're probably right. There's a threaded hole in the cylinder head. The
probe goes there, no matter what the airframe. The only way the airframe could make a
difference is if a) they have the probe in only one cylinder and b) the
temperatures on the other cylinders are known to be significantly different.

So, if Piper puts the probe in cylinder #1 for one aircraft and cylinder #4 in
another, the allowable max temperatures are likely to be different for the two.

George Patterson
If you want to know God's opinion of money, just look at the people
he gives it to.

"G.R. Patterson III" wrote in message ...

. The only way the airframe could make a
difference is if a) they have the probe in only one cylinder and b) the
temperatures on the other cylinders are known to be significantly different.

And both of the above are the case. Unless you have an engine analyzer
the CHT is only measuring one cylinder. They usually try to put it in the
hottest cylinder (one of the back ones), but that varies based on how the
engine cooling air is designed on the airframe.

The oil temperature temps have an even greater variation.

In a sense, you're probably right. There's a threaded hole in the
cylinder head. The probe goes there, no matter what the airframe. The
only way the airframe could make a difference is if a) they have the
probe in only one cylinder and b) the temperatures on the other
cylinders are known to be significantly different.

Actually, it can make a difference even beyond that. The fine folks at
GAMI (as part of their liquid air project) completely instrumented a few
cylinders (dozens of probes each) and went flying. They found 80
degree differences from one side of some cylinders to the other side.
So even a threaded CHT probe may or may not be telling you the true
temperature of even THAT cylinder.

FWIW, GAMI theorizes that this uneven cooling air around each cylinder
may be a significant cause of the cylinder operating "out of round" and
causing significant scuffing and wear.