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Date: Tue, 21 Mar 2006 10:45:41 -0500
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From: "Johannes Schredl" <jschredl@web.de>
Subject: high CHT
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Hello Scott!
Hello to everybody interested in my problem!

Thank you very much for your comments, suggestions and advices!

I try to give some more explanations and remraks to your mails:

1. Calibration of probes: use 4 pc. EGT and CHT thermocouples with the
EPI800 system from Vision Micro. I checked one probe years ago with
confidential results. But I agree, I have to re-check them all again to be
on the save side.

2. During a normal flight I have following results (+-10°F):
   2500 RPM; 23,3"Hg; OAT=72°F; at 4500ft with 185MPH Indicated Airspeed;
Oil-Temp=193°F; Fuel = 7,5 gal/hour; (Temperatures converted from °C to °F)
        CHT       EGT
Cyl. 1  390°F     1400°F
Cyl. 2  410°F     1436°F
Cyl. 3  410°F     1418°F
Cyl. 4  390°F     1382°F

I have a standard IO320 Lycoming (new when installed) with standard Bendix
injector, Slick magnetos and standard pistons.
I notice Cyl. 2+3 have slight higher EGT and same to CHT.

3. Measuring the pressure conditions was my first attempt to get more
information about this problem.
I have to add I used the static pressure of the cockpit, not from the static
ports. Now you tell me there might be a considerable differenc I have to
repeat measurement using the exact static pressure!
The reason why I used a airspeed indicator was I can compare it better:
Aircraft speed v: thats the maximum of pressure I can ever get (knowing
p-dynamic = 1/2 * rho * v * v) -> max ram air
The "air-speed" at the upper cowling never can be higher than v! In reality
less than v because the "upper cowling" is not perfect sealed and air is
going down through the cylinder fins. My feeling 75% of v (100%) is a good
value.
Very supprising to me is a pressure of about 40% below the cylinder. My
first thought was: this is much to high to have a good pressure difference
between upper and lower plenum! Here is the biggest potential ot improve
cooling by reducing this pressure.
Reading all your mail show me that´s "normal"...
By the way: the airspeed indicator (I use for "pressure-measurement") needle
is vibrating about +-3knots and gives a not too bad reading. However using
some kind of damping should improve this instrument

4. On the backside of my baffles I have holes for the oil cooler (as
described in lancair manual) and cabin air. Both holes are about 2"
diameter. Variating the hole for the oil cooler do not have a signigicant
change to CHT but of course to the oil temperature (I use different cut out
sizes for summer /winter time)

5. I building a pressure cowling which seals the baffles perfect. At the air
inlet section I tied to have only very small gaps (about 1/8"). Right now I
did not consider to have a perfect air flow design with smout ramps less
than 7° angle (will be improved).

Conclusion:
I see the aim to get a lower CHT is not that simple (as I already know
before). But when measuring a "40%-Pressure" below the cylinder I thought
that´s the reason! But when the air outlet area from the lower cowling is
big enoth and is acting like a venturi system there should be a much lower
pressure...
Maybe there are turbulences or other reasons (like Gary Casey mentioned )
for this high pressure!

Will continue to work on this problem!

Thanks to all
Johannes