Corvair CHT, letters and notes.


We picked up several letters from friends on this topic. Let me share some of them and offer a follow up on the story.

Zenith 601XL builder and flyer Ron Lendon wrote:

“And after all that reading I’ll be dipped if I can find your recommendations for the temperature range your followers should be shooting for.  Try putting those details in too, OK?”

Ron, for a guy who made his own cowl and experimented with carbs, I would have guessed that you would have liked reading the information. The topic was the measurement location and test tools, not so much the values. For people with busy schedules and short attention spans, The numbers:

GM Factory limit, measured on bottom of head: = 575F

Highest temp I have personally seen in a Car, measured on bottom, without engine damage = 575F

Highest sustained temp I have personal seen on a CHT in a plane without any damage to the engine = 480F, measured on bottom of engine

Highest temp on bottom of engine  that builders should consider tolerable before stopping to rework cooling system = 420F

A reasonable goal temp in a hard climb that will still have a very wide margin of safety = 380F, measured on bottom of engine, easily achievable with our existing cowls and cooling designs, even in large planes.

Highest Temp that Dan Weseman saw on bottom of engine in full air combat maneuvers in 3100cc “Wicked Cleanex” = 375F

Typical Max climb/ 5 gal/hr cruise temp numbers in well prepped Zenith 601on 80F day = 375F/320F

My target goal, measured on bottom of engine, for gross weight Vx climb in 3000cc Wagabond, 100F day, 5″ inlets, 60mph, 1,600 lbs = 350F.

Lowest cruise temp recorded for a plane with a full cowl, measured on bottom of engine, Chris Smith 3100cc “Son of Cleanex” = well below 300F.

Lowest cruise temp recorded for a plane with a J-3 cowl, Jim Weseman, 3100cc Celibrity biplane = 250F


Above, Phil Maxon’s 601XL, finished in our hangar in 2006, flies over the Atlantic Ocean near Ponce Inlet FL. This aircraft was one of the first 4 Corvair powered Zeniths, yet is has never had any cooling issues. Corvair Cooling, especially on airframes like Zenith XL’s is not a mystery. Anyone who chose to follow our directions and used our designs did not have to be a pioneer. If you want your plane to work, just make it a clone of successful aircraft.


For readers with a little more time, get a look at the following links to stories I have written. I found them all simply by searching “Cowling” , “Cooling”, and “CHT” in the search block at the top right side of this page. If you are only going to read one story for right now, make it the last one, because it highlights the difference between measuring the CHT under the plugs and below the engine:


Cooling with J-3 style cowls. (Pietenpols, Cubs, Biplanes, etc)


Corvair Cooling, Three 2007 examples from our hangar.


Corvair Cooling, something of a human issue…..


Corvair Cooling



Measuring Cylinder Head Temps on Corvairs.


Right here and now I am going to make a very important point: I could put an engine on the run stand, hook up two of the finest CHTs, fuel the engine with pure ethanol free 94 octane gas, have it be ice cold and get ready to start it.  Then I could reset the timing from 30 degrees total to 44 degrees, (a number that people without timing lights often think “sounds good”) Start the engine, and take it to 2,700rpm.

It would take more than 3 minutes for the CHT to exceed 400F on the gauge. Long before this, the detonation would begin, and within 30 seconds of it really knocking, (which would be hard to hear over the prop,) the engine would blow out at least one head gasket, and crush down the ring lands on several pistons.

I could then honestly report “The engine had never exceeded 400F.” I could give many examples like this, and then we could have a lot of internet people offer opinions about the cooling system design we use, and some car guy is going to offer the “expert opinion” that the cowl needs 6″ inlets.  If you want to learn something in aviation, the discussion is going to have to be held at a higher level than that.

CHT is not something that can be discussed as an issues divorced from other operational parameters. I have seen people waste incredible amounts of time trying to solve a “cooling problem” who have not bothered to check any of the basics.

I have been doing this for a long time, and I still have no idea why some people get to the point of having 5 years of work and $30,000 in their aircraft project, but they refuse to drive down to their local auto parts store and buy a $39 timing light.

About a year ago, a guy contacted me who bought an engine I had built for someone else in 2005. He had it ground running on his plane. When I asked him what the timing was set at, he said he had no idea, he just assumed, without a single word to confirm this, that I had set the timing before mailing it to the original owner six or seven years earlier.

I explained that this was a very poor assumption, that even if I had, there is no telling that it wasn’t changed by the first owner, or bumped as it went in or out of the two airframes and was shipped around the country. I pointed out that if I said I unloaded a firearm before selling it to someone else 7 years ago, I would hope that no one would never say to me “I can point it at people today because you unloaded it half a decade and three owners ago.”

About a month ago I heard from the same guy, The engine still was not running evenly on both ignitions but he still had not checked it with a light. If someone presented me an engine that had a years ground running on it, but had never had the timing checked, I am going to flat out say that I would not fly it without tearing it down. Why? because you can check things the easy way with a timing light right away, or you can check things later the hard way with wrenches. Or of course there is the third option, which is just say “I am sure it didn’t hurt anything, It will be alright”

‘Sprint’ builder Joe Goldman wrote:

“William I as am not building my engine I would feel comfortable with 6 cht’ too know what is going on with each cylinder. Does the stock location average temps or would it indicate a particular cylinder over heating. You would not know which one, then how do you locate the trouble.”

Joe, we are friends and you know I think you are great, but lets get a little more thoughtful on this one. First, why are you not building your engine? If the goal of asking me a question is to learn something, then let me offer that building your own engine is the best way to learn many things. Honestly, what are 6 chts going to tell you that 2 are not? Lets say that flying along at 5,000′ and the CHT reads very hot. What are you going to do? Land, that’s what. If your right side CHT indicates an excessive number, please explain to me what knowing if it was #1 #3 or #5 is going to do for you at 5,000′. I have said it many times and many ways, but please know this well; You detect issues in the air, you land. Diagnosis is always done on the ground. The only guy who argues against this is the one who is going to rationalize some reason to keep flying. Each CHT sender cost more tan a Raytech thermometer. If the plane isn’t running right, land and use tools on the ground to look into the problem.

Besides, if you build and equip your engine correctly, you are not going to be flying around looking at issues in the air. Stop for a minute and honestly tell me what is going to happen to an engine, in flight which is going to make an individual cylinder suddenly go up in temp? If you were looking to tell which individual cylinder was causing an issue if your engine started running rough, you are a poor decision maker, and you need a better flight instructor. Any skilled pilot will tell you that the first thing you look for when your aircraft runs rough is the nearest airport. -ww

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