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What is the original CHT gauge indicating?

I’m puzzling about the temperatures I see in my cockpit.

I have the EDM700 installed and religiously follow the temperatures indicated. Now this greatly limits the power I can set during LOP cruise in order to stay below 380°F. I wonder whether they show the correct temperature, or maybe show too much. Because the factory built-in analog CHT gauge shows never more than something like 220°F to maybe 250°F.

Can anyone tell me which of the CHT indications is more reliable, or more precisely, if someone knows whether JPI designed the EDM700 to indicate a higher temperature than it really is?

Of course I don’t want to blast my engine, but at the same time I also don’t want to run it too cool. So I’d be really interested in knowing what’s really going on. The oil temperature around 180-190°F in that case seems to fit more to the indications of the EDM700. @Peter, I think I had seen some stickers in fotos you posted here which turn black on reaching a temperature. Maybe I should try this to see what’s really going on. I don’t find them, where did you get them?


220 to 250 degrees seems rather low to me.

EBST, Belgium

@slowflyer yes exactly, thank you!



That is the crankcase temp, which you would expect to be regulated, via the oil cooler and oil thermostat system.

The cylinder head temp will be a lot higher, but I would think you can still get temp indicating strips for that. The problem is that the location of the CHT probe affects it a lot, and this has caught a lot of people who installed say an EDM700 and used the original CHT probe for say #1 cylinder and used new probes in the alternative location for all the other cylinders.

Probes can also go defective. Normally the engine probes are thermocouples, type J or type K and if you fit the wrong type you get a wrong reading. There are ~6 other thermocouple types out there (I have just designed an instrument which can read all of them) but not used in GA AFAIK.

If you are seeing 250F, something is reading it wrong.

If you are seeing 380F in cruise, normal OAT, check the seals. Even small gaps make a dramatic difference.

Shoreham EGKA, United Kingdom

@Peter that’s a good idea, my baffle seals really, wait, there should be seals around the baffles? ;-)

Yes maybe I should do them some good.

It still works, I’m typically doing 150 true LOP, but the limiting factor is always engine temperature.

And I’m not talking about high power operations. It’s below 50% power that I can set, and most interestingly I can always do around 150 true , be it FL60 or 160.

This, and the discrepancy to the factory installed CHT gauge made me think whether the EDM-700 in fact indicates correctly..

Last Edited by UdoR at 19 Oct 22:04

If you google for a Type J table and then google for a Type K table, look at the output voltage at say 150C, you will find the difference is about 2mV, which is worth roughly 50C. So a misconfigured thermocouple type will give you a (approx) 50C/100F error.

The EDM boxes can be configured for J or K and I guess most instruments can too.

On the cylinder head itself there will be a helluva temperature gradient, with the hottest bit being probably near the exhaust port, I would guess.

Shoreham EGKA, United Kingdom

UdoR wrote:

It still works, I’m typically doing 150 true LOP, but the limiting factor is always engine temperature.

If CHT is a limitng factor in cruise, there is something very significantly wrong with the engine cooling airflow! Even at max continous power that is allowed for LOP operation n for your engine, the cruise CHTs should be very significantly below 380F. CHT an be (and often is) a limit in climb on hot summer days, but should not be in cruise.
And I would not wait too long to get it fixed, because even though your EDM reading looks much more accurate than the orogina CHT gauge (220-250F is just too low for relevant power settings), it is only a spot measurement. The EDM gauges should be at “the hottest points of each cylinders” but that is only true with the airflow it has been designed for. so with your current configuration you can also not be sure that there are not hot spots on the cylinder that are substantially warmer than the 380F limit


Well I don’t have any CHT temp issues during ROP climb. The opposite is true, the engine would run infinitively in the climb setting (I climb on about 75% power). But running LOP in cruise the engine does get warm. However, when flying ROP on about 3 to 3.5 Gallons per hour more (so ~12.5 Gal/h) I can also have about 20 knots more, but it’s a 40% penalty in fuel burn. I don’t see no point In selecting a power level in between, so I either go LOP on about 50%, or then ROP on about 65% in cruise.

I’m not so sure that is really true that there should be no temperature problems running peak / lean of peak below 75% power. So o.k. on my next service I’ll hunt down any possible problems with the baffles. But am I really the only one having these temperatures in cruise? Don’t think so. So in the end, this leads again to the initial point of this thread, whether the EDM probes are set to indicate too high temperatures.

But I’ll continue reporting. Plan is to check next weekend how it performs in FL200. First time up there

Last Edited by UdoR at 20 Oct 16:14

You most likely have a cooling problem caused by turbulent airflow above the engine that doesn’t flow through the cylinders, but rather back out the front close to the prop.

I went through all of this, engine: IO 520 NA in a Beech F33A.
I used to have CHTs in the region of 380, often 390-400 in climb and close or above 380 in cruise.
There are various measures available, ranging from special cooling kits for single cylinders to increasing the take off full power fuel flow.
I installed, one by one, cyl kit #6 for cyl #6, then #2 etc. until cyl #4 was the hottest. Then I increased the fuel flow, not to much avail. It was frustrating.
The only measure that really and finally fixed the problem was getting the original ‘hanging baffles’ replaced with modern standing baffles around the engine, who completely seal the space above the cylinders and direct the air downwards through the cylinders. I am sure these cooling kits are available for various airplanes. The name of the brand is LiquidAir by GAMI.

I now never get over 360-370 in climb, even at 1200 ft / min, and as soon as I go into cruise the temps go down to the 340-350 it’s as they should.

The 380 degrees F shown by the EDM 700 are most likely correct. The original CHT gauge doesn’t show the hottest cylinder and isn’t attached at the hottest point. So I wouldn’t even try to search for the problem there.

Hope that helps.

Last Edited by EuroFlyer at 20 Oct 19:22
Safe landings !
EDLN, Germany
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