How hot is to hot???

  Ask 10 different experts what the limits are and I bet you will get 20 different answers. I did. When I tried to get a cylinder head temperature limit I received figures from 350 to 475 degrees. Doing a little research, wait, that's a lie, doing a LOT of research on the web I began to get some numbers to shoot for. According to Pratt and Whitney aluminum becomes brittle at 450 degrees for a casting and about 550 for a forging. This varies some with the alloy used. I was a little confused by Limbach, they have a service bulletin out that says aluminum begins to change on a molecular level at 200 degrees C ( about 390 F) yet they advertise maximum cht in their literature of 250 C ( around 480). At this stage of the game I decided I would shoot for the low end since we used to think 350 was to hot when driving down the interstate during my hot rodding days. Dune Buggies and Hot Vw's magazine has a very good article in the August 2010 edition that backs me up on this. You really need to get a copy and read this.
 On more than one occasion I was told I was sensing the temperature in the wrong location and to get the proper temperature I needed to relocate the sensor. Basically, move it until you get the reading that makes you feel good? Now...., I have a different opinion here, I feel you should sense the cht at it's hottest point since that is where the damage is going to be. I looked at several " certified " aircraft engines in the last four years while working on my A/P rating and all of them had the sensor on the bottom, some between the plug and head while others do it with a sensor screwed into the head. From what I can tell this is the hottest point on a certified head / cylinder assembly as the cooling will have picked up a large amount of heat coming down through the fins and it is also near the exhaust ports.
 I looked at where the Vw engineers put the cht sensor on the fuel injected engines and it was near the exhaust port also. I tried it in three different locations, under the plug, under one of the center head studs and under the head stud over the exhaust port. As for the plug location my testing showed the plug has an effect on the reading, a cooler plug gave a few degrees cooler cht reading. The center head stud location is the coolest part of the head as far as I can tell.  So, based on what I was seeing on the Continentals and the stock Vw head sensor location I chose the head stud over the exhaust port. All of my development work has been done with the sensors in this location so as to have a solid comparison.

The picture above shows the cht about 20 min after takeoff on a day in the forties. This is early in the test program before the fat fin mod with about 30 hours on the Hobbs. This is a cooler than normal cruise reading and it is one of the cooler running cylinders of the 4. It is below normal cruise temp because I have the power pulled way back. I was trying to get to a landing area since I have no oil pressure indicated on the gauge. Yes it was a gauge issue, the engine stayed together! I flew the plane about 60 hours with stock type heads and had constant issues with valve adjustments and head stud torque among other things. I saw it as a sure sign of overheating and I worked between every flight to get the cht lower.
 I tried all the gimmicks in the book after talking to all the experts and locals. Several told me I had to slow the air down to allow it "to pick up heat" and other wild ideas. I finally got serious and went back to my Hvac roots. I installed a air pressure manometer in the cabin with the probes in various locations in the cowling to make sure I had a pressure differential to make the air move. I found out the best I could get was about 2.5 inches and that was about all I could expect with the speed a 701 flies. Armed with that info I began looking at ways to move a larger volume of air through the head fins. I knew that heat travels from hot to cold and the larger the temperature differential the greater the heat transfer will be. Next time we'll talk about having some success!