Message-ID: <375593A1.FA0E3B6A@gateway.net>
Date: Wed, 02 Jun 1999 13:27:13 -0700
From: dfs <dfs@gateway.net>
To: Lancair List <lancair.list@olsusa.com>
Subject: EGT Probes
Mime-Version: 1.0

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Answering MDS (Mike Smith was it?) regarding the first part of the
Subject about the EGT probe holes:

Actual distance (within limits) isn't too critical as long as all are as
close to the same distance from the cylinder flange as practical (+/-
1/4 inch is pretty good, closer is fine too). The numbers read out on
the EGT gauge are relatively unimportant (also within limits) but it's
nice to try to measure all the cylinders where the gases have all had a
chance to cool near the same amount.

A good starting distance is 4 to 8 inches from the flange. Any closer
and your probe life will suffer, any further and too many external
influences from place to place in the engine environment will affect the
numbers you read on the gauge.

It's not a good idea to drill thru the exhaust pipes wherever there is
one pipe fitted inside another as at a slip joint. One, you don't want
to pin the pipes together as some relative movement should be able to
take place, and two, you're correct in assuming that said relative
movement will shorten the life of your probes.

I know, Iknow, some people swear by digital EGT read-out with resolution
to a gnats a--. You're only kidding yourself (and the pilgrims who gaze
at your cockpit instrumentation and are duly impressed by things that
can be discerned to +/- 1 a gazillionth of a degree) if you think these
systems are accurate enough to meaningfully use that kind of display.

The temperature you measure and display as "EGT" is really "some gas
temperature that happens to be at this place in the exhaust pipe" which
has a relationship to the gas temperature just outboard of the exhaust
valve that you'll never know for your particular airplane. Doesn't
matter.

What's important to you is first, the numbers that your gauge displays
over time when things seem to be going rather normal, and their
relationship to one another. Once familiar with your situation, e.g.,
climb, leaned cruise, peak "EGT", etc. regardless of the actual numbers,
anything that deviates noticeably from your airplane's "Norm" should
elevate the pucker factor and persuade you to get on the ground for a
look-see.

Dan Schaefer
N235SP
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