Message-ID: <19981111191450.27228.qmail@www04.netaddress.usa.net>
Date: Wed, 11 Nov 1998 19:14:49
From: Dan Schaefer <dfschaefer@usa.net>
To: lancair.list@olsusa.com
Subject: EGT probes
Mime-Version: 1.0

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Chris Moore was asking about placement of his EGT probes and the 
effects of distance from the exhaust flange.

Some years back, so I would know what to do on the Lancair when the 
time came, I bought a great book from Kas Thomas, currently editor of 
TBO Advisor, (a take no prisoners because we take no advertisement 
magazine) called "EGT Systems". I based my installation on much of
what I gleaned from his book.

Choosing the probe distance from the flange is a compromise between 
sensing accuracy and probe life. Close doesn't give the hot gasses 
time to lose much heat thru the exhaust pipe walls, so close should be 
better for accuracy. But too close will burn your probe tips right 
off. As I remember it, the recommendation was to try for 4 to 6 inches
from the flange. Too much further downstream than that and not only 
will the gas temperature have dropped significantly (losses thru the 
pipe walls and gas expansion) but may be influenced by other ambient 
conditions that can vary between cylinders.

As for varying the distance from flange to the different cylinder
probes, it's not an optimum set-up, to be sure, but everything we do
on an experimantal is probably some sort of compromise. You do what 
you have to do, I guess, and then learn how to deal with the results.
Consider that most spam cans, if they have EGT instrumented at all, 
usually only have a single probe system - supposedly measuring the 
*hottest cylinder* - which it probably isn't, anyway.

Fortunately, the absolute value of the EGT isn't as important as how 
it changes from what you've determined to be normal. A couple of 
inches difference will make a difference in the measured temperature, 
even if the *real* temperature in two different pipes started out 
exactly the same.

Once you get used to whatever is *normal* EGT readings for your 
engine, (even if the measurement points are a couple hundred degrees 
different) with some experimentation, you should be able to set the 
mixture as required and spot any adverse trends.

One last thing, I doubt if the stainless clamps will wear thru your 
pipes at a bend, as song as they're really stainless. They have to
be quite tight to insure a good seal at the probe penetration, and if
the clamp and pipe have nearly the same coefficient of expansion 
(both stainless), there should be no relative motion to wear thru
the pipe. If you're still worried, you could put a small cushion of
waste Fiberfrax (sp?) from the firewall between the clamp and pipe - 
might be enough.

Dan Schaefer
N235SP


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