High CHTs

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Charles
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High CHTs

Post by Charles »

I've been flying my 2003 DA40 for a little over a year now and I've been following the AFM settings for engine management. I always find that my CHTs are a little on the high side though. Last weekend was a good example: 9000' OAT 9C with WOT at peak EGT, I was doing 136KTAS at a little under 8 gph. But CHT was consistently around 390F and I didn't really like that so reduced throttle a little to keep it around 380. I thought it was normal for the engine to be running hot that day since temperature on the ground was about 34C.

Then I went and looked at a few logs of fellow DA40 owners and found this one where the plane was flying in very similar conditions, but CHT was a lot lower: 9000' at an OAT of 15C, MP 21" (probably WOT at that altitude) and a fuel flow of 7.6gph, but the CHT is around 365F. (this one: http://www.diamondaviators.net/reports/flights/638 ).

I have two theories as to why the CHT was lower than mine:
- RPM was 2200 while I was running at 2400
- 7.6 gpm seems is a little lower than the 8gph I was running at (is 7.6 in LOP territory?)

What could I do to reduce the cylinder temperature? I will try running at a lower RPM on a future flight to see if it helps. Anything else I should look into? Are high CHTs a common occurence on DA40s in the summer?

Thanks,

Charles
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Robin
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Re: High CHTs

Post by Robin »

Hi Charles

I have been running WOT on some long distance trips recently. It has been hot for winter in Australia.

I have been WOT usually 24 to 25 inches.
Set the RPM to 2400 RPM.
I have been leaning to 9.5 GPH, slightly rich of peak, at lean of peak the engine has been running roughly. My CHTs have been 380s. Talking to a fellow Aussie, Mark, he aims to keep his CHT around that figure of 380 degrees. From memory this range is still well within the green range.

I am about to post my long distance flights to the centre of Australia, I shall review my CHTs.

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Robin
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Re: High CHTs

Post by Robin »

Hi Charles

Looking at one of my flights from Whyalla to Oodnadatta - trip report 655

Flying at 6000' Average TAS 146

WOT at that height MAP was 25 inches
RPM 2400
Fuel Flow 9.6 GPH - perhaps I could be leaner, but suspect I would get a lower TAS
CHTs averages 378 degrees
OAT average 12 Celsius

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Re: High CHTs

Post by Robin »

Hi Charles

Trip report 658

At 5000'

OAT 15 Celsius - so quite warm

WOT which gave MAP of 25.75 inches
RPM 2400
GPH -9.2
CHT 374
Average TAS 141

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Re: High CHTs

Post by Tommy »

Charles, it's your rpm. I never run over 2,200 rpm when I am at cruise except when I am running better than 65% and in a hurry to get somewhere at low altitude which is very rare.
Drop your rpm and you will see a substantial difference in cylinder head temps. At less than 65%, there is just not that much to gain in speed running at 2,400 rpm, at least not on my plane.
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Re: High CHTs

Post by Graham »

Hi Charles,

I noticed the same thing once I owned a DA40 and really wanted to learn what the normal numbers looked like. Also, if you read any of the John Deakin articles then you become even more sensitive to high engine temperatures. Typically I see numbers in cruise 370-380C. Yesterday I was at 4000 with an OAT of 19C and was nearly 380 at 24/2400. I had a few discussions with Diamond and the dealer/AME and they felt that was normal on this plane. Where I was most concerned was on climb with full power where you can easily climb over 400C on a hot day. Here I power back to 25/2500 for a cruise climb at about 1000AGL and climb at a higher speed to cool the engine better.

I have been up in a few other DA40's just out of the factory and they all sit in the high 300's in cruise.
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Re: High CHTs

Post by Robin »

Second the comment to watch the CHT on a climb out on a hot day. I know some are keen to lean the mixture on climb, I do, less plug fouling, but watch the CHT.

Like Graham I often reduce to 2500 RPM on climb, but leave WOT. Climbing out at 500 FPM, I am usually 110 KIAS, so plenty of cooling.

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Charles K
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Re: High CHTs

Post by Charles K »

I believe the climb checklist says to reduce to 2400 RPM at a safe altitude.

I rarely see above 375 in climbout but I climb at higher speeds and reduce to 2400 RPM. Generally at 1000 feetish I turn off the aux pump, raise flaps and prop back to 2400 which is in line with my surecheck check list.

I think you are just running your engine on the higher power side. I cruise regularly with CHT between 350-360 but I cruise at say 22 /2300 at 6000 feet. I treat my engine pretty gently and it has given me great service - I just see about 130 KTAS by doing that.

Look at your POH and use the recommended power settings and temps should be cooler.

Hope this helps a bit.
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Lance Murray
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Re: High CHTs

Post by Lance Murray »

Charles,

As long as you keep your CHT below 400 you are fine. If you want lower CHT and don't mind reducing your power try running LOP at cruise instead of closing your throttle. I find 25 LOP to be a good number. Your motor will be running much cleaner and more efficiently. You could get your fuel burn down to 8 gph or so and only lose about 5 knots.

However as long as you are below 400 you are just fine. FYI my engine has almost 2100 hrs on it and compressions are 78 or better on all cylinders. The cylinders are all original with no issues since new. I was told by Bart Lalond of Aerosport Power that the cylinders start going bad at around 430 if you operate there for sustained periods. Lycoming's limit is 500 with a caveat to run below 400 for extended service life. Lycoming also has no objection to run at peak EGT in cruise below 65% power.
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Re: High CHTs

Post by Lance Murray »

Charles,

I just noticed that you have a 2003. Check the size of your intake scoop at cylinder 2. There is an updated part number for the baffle at cylinder 2 with a larger opening allowing greater flow over the cylinders. If you call the factory I found that they didn't even realize this and there is not much documentation. Look at your intakes and then compare it to a newer model Diamond. If your intake on the number 2 cylinder is smaller then you have the superseded part number for the baffle. Order a new one and cut the intake hole on the cowl to fit the updated part.
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