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Bryan Cotton wrote:

BRS wrote:Bryan, I've always wondered how you keep your cyl so cool. Mine (R2300) tend to run about 390 on climb and 360 in cruise (2700-2900 rpm). Oil is around 160. Where are your CHT probes located? My baffles are well sealed and cylinders all wrapped.

Under the top spark plugs.

This might explain a bit of difference. My CHT probes are mounted in the head just above the exhaust manifold.

That´s a new one! Never seen anything close to that good. It seems that a hole has been drilled in the head coming out just above the head bolt stem. Then they stick in the cht sensor and secure it with a setscrew. Well out of any contact with fridgit cooling air! And the need to replace a ring sensor on more or less a regular schedule because the spark plug pipe killed it when the plug had to come out, is a thing of the past! I speculate this is Mr Horvat´s brainchild?

BRS wrote:This might explain a bit of difference. My CHT probes are mounted in the head just above the exhaust manifold.

http://www.aeroconversions.com/support/ ... nstall.pdf

Sonex instruction sheet says under the plug or close to it.

Bryan Cotton wrote:

BRS wrote:This might explain a bit of difference. My CHT probes are mounted in the head just above the exhaust manifold.

http://www.aeroconversions.com/support/ ... nstall.pdf

Sonex instruction sheet says under the plug or close to it.

Yup! Revmaster vs Aerovee design choices. I'll probably not take the time but it would be interesting to mount a temporary probe like in the aerovee instructions just to see how different the two locations are.

Hi Brock,

BRS wrote:Yup! Revmaster vs Aerovee design choices. I'll probably not take the time but it would be interesting to mount a temporary probe like in the aerovee instructions just to see how different the two locations are.

I know it's a big ask, but if you get a chance please do. It would be valuable information.

Also, you wouldn't happen to have a pic of the thermocouple tip. Just wondering how the junction is made. Cr/Al or Copper Con?

Edit: What is that black adhesive used to secure the T/C wires?

Thanks,

Wes

Location---Location---Location.
This is where the probe was located on the Xenos prototype. The owner was always bragging about how low his CHTs were in a protracted climb. Typically in the 175deg range.
Funny thing......he's had to replace a cylinder head every 75 hours or so. .

Jester504 wrote:NACA did at least two studies in the 40s investigating the relationship between CHT, EGT, valve temperature, and valve condition. They had to create custom valves with thermocouples in them. The results proved that valve temperature correlates nicely with EGT (but 150-200 F lower) and not CHT as many think.

Here's a link to the first: https://www.abbottaerospace.com/downloa ... rmocouple/

Far be it from it that I etc etc………. , but what did I tell you all those years ago?? Unfortunately it is not known where exactly in the exhaust pipe NACA would have located an EGT probe, so we are still more or less in the dark. In my work with gas temperatures and pressure there were a lot of rules defining the location to ensure uniform and comparable readings: downstream at least 3 pipe diameter lengths from any elbow, and upstream about 5 diameter lengths from an elbow- to mention a few. Not so easy in the cramped exhaust piping of a VW-1. if anybody has any good ideas- please come forward!

Kai wrote:Unfortunately it is not known where exactly in the exhaust pipe NACA would have located an EGT probe, so we are still more or less in the dark.

Whether they put them at the top or the bottom of the exhaust or put each one in a different spot, it wouldn't change the resulting curve which correlates with valve temperature and not CHT. I agree that it would change the absolute EGT measured for a given cylinder, but the correlation is what's interesting, not the specific temperature.

WesRagle wrote:Agreed. But... at that one point, stoichiometric/peak, EGT has special value. If you can use EGT to tune an EFI unit such that EGTs all peak at the same mixture setting, that's pretty cool. EGT then get's demoted to a cross check parameter and from that point forward you can operate your engine by mixture according to flight regime. I think that, along with an EGT limit set below spec, will help me manage/keep me from abusing the engine.

Agreed! Still not convinced any EGT limit is anything more than shooting in the dark, since the measurements are so variable across installations, but I'm with you on the general idea of using it as a cross-check and always setting your mixture setting relative to peak EGT.

Good thoughts here concerning EGTs and the life of an exhaust valve. Still I hope that the effect of high CHTs are considered as most of the cooling of the exhaust valve is done by the valve being in contact with the head and still more through the valve stem. (I've heard different numbers but most give an 80% or more through the valve seat and 15% or less through the stem). Cooling the exhaust valves by enrichening the mixture can only go so far. If the heads are running redline CHTs (or nearly so) then something needs to be done.

In the report cited earlier it was noted, “…. relatively large increases in over-all cylinder-head cooling are required to maintain constant valve temperature.” When CHTs are high the valve cannot be cooled as well as with lower temps. High CHTs weaken the aluminum and cause seat loosening and recession and misalignment. When the valve and seat can't seal and hot exhaust gas escapes past the valve the burning begins.

A good review is found here with many references (including the earlier study): https://www.csobeech.com/exhaust-valves.html

“Variation in fuel-air ratio (leaning) had a relatively great effect on valve temperature.”

Many were surprised when Jabiru gave such low max EGT numbers for their aircooled engines along with lower max CHT numbers also. Some of that may be due to the alloy used in their heads. VWs heads are not healthy when run over 400-450ºF while Corvair turned on the idiot light at 575ºF. No one wants to run their heads that hot.

FWIW, I generally see 225-250ºF CHTs in the summer with EGTs running 1225-1250ºF. When I had an Aerovee Sonex the best I could do with head cooling in cruise was 385ºF on the left rear (all others were lower) while the left front was slobbering rich. I was reading accurate temps taken under the plugs.

Charlie Radford did a test back in the days of the old Yahoo group where he compared the under the plug sensor with the smaller probes mounted next to the plug per the Sonex instructions. IIRC I believe the difference was 20-30ºF cooler for the remote mounted sensor. There were a few that mounted a CHT ring probe under the head bolt near the exhaust valve. That was said to be 70ºF lower than a plug mounted sensor. Some were claiming amazing CHT numbers but hadn't bothered to calibrate thier system and/or mounted their probes in places where the cooling air was cooling the probes.

So the joke was to mount the probes where they made you feel the best and just go fly the thing! 8~)

Dale
3.0 Corvair/Tailwheel