From: greg@saltydog.dpsi.com (Gregory R. Travis)
Subject: Re: Temperatures after overhaul
Organization: Data Parallel Systems, Inc
Date: Sat, 2 Jan 1993 21:34:59 GMT

In <1hss00INN8fo@transfer.stratus.com> lee@deneb.sw.stratus.com (Lee Cooprider) writes:

[That he sees WIDE variations in EGTs between the cylinders of his O-470]
>I have an engine monitor that registers 6 EGT's and 2 CHT's.  The latter are
>nominal (340 and 360) in cruise, but the EGT's seem odd.  During idle and
>runup they are all together and normal, but after climbing to altitude I get

>	1470  1550  1380  1525  1330  1250

Lee, I'm going to assume that you had the multi-probe EGT installed at
overhaul time (i.e. that EGTs were not all even before your overhaul.

If so, what you're seeing is perfectly normal for a carbureted engine.
Mixture distribution is very uneven in a carbureted engine
and will be affected by things like:

	Throttle setting (meaning throttle butterfly position)
	Outside Temperature
	Power levels
	Altitude
	RPMs

2-300 degree spreads are not at all uncommon.  You mentioned that EGTs
were all close together at idle - this is a good indication that you just
have a bad distribution at power.  It's also common for EGTs to be together
during a full-power climb.

Your #2 and #4 cylinders are probably your "leanest" - although they may
also be your strongest.  What you should do is to lean until the FIRST
cylinder peaks.  Do not try and get an "average" - it's unfortunate, but you
will end up running with a few cylinders quite rich.  Whatever you do, do
NOT try and lean until all the cylinders are about equal if that means leaning
beyond peak on some cylinders!

If it really bothers you, try different altitudes and/or throttle settings.
Sometimes partial carb heat can help promote good mixture distribution.



Some data:

	My O-320 tends to run lean on the rear two cylinders (#3 and #4) with
EGTs in the 1470-1550 range (higher in winter because of higher power developed
by the engine) with the front two run about 1200-1300.  The problem is
especially bad at moderate power settings at low (< 6000') altitudes.  I find
that a full-throttle cruise at around 7-9000' gives me excellent distribution
and leaning peaks all cylinders simultaneously.

Normal operation has me lean by the #3 cylinder as it is USUALLY the first
one to peak.  However...

Very cold days will reverse the relationship - i.e. the front two "peak"
well before the rear two cylinders.  #2 is usually the leanest when it's
very cold outside.

Again, I always lean ONLY until the LEANEST cylinder peaks.



A couple of other things that can cause bad distribution:

Mags:  If you're really worried, take your bird up to cruise and shut off
one mag.  Note the EGT rise on all cylinders.  Lean to peak.  Note which
cylinder peaks first and at what temperature.  Repeat the experiment with the
other mag (disclaimer: shut off only one mag at a time!).  Temperature
rises should be roughly equal.  If you find that EGTs are much closer together
with one mag than the other, your problem may be a mag problem.

But I think you've really got nothing to worry about.  The Penn-Yan "lifter"
explanation sounds extraordinarily fishy.


When Cessna certified your 182 with the O-470 they did not, nor were they
required to, instrument all cylinders for EGT.  Subtle differences in airframes
can produce remarkable differences in mixture distribution.

Greg

--
Gregory Reed Travis				D  P  S  I
Data Parallel Systems Incorporated   greg@dpsi.com (For MX mailers only!)
Bloomington, IN			     greg@indiana.edu (For the others)