CHT's in cruise - again

[flyboy0681]

While looking over the data download from our engine monitor, I noticed that one of the partners liked to cruise with the temps between 380 and 390. I felt it was pretty hot, since I have always been told to keep it down to around 350, and when I see it creep up to over 370, I'm immediately on it to bring it back down. When I questioned him about running it hot, he pointed to this Mike Busch article about keeping the exhaust valves clear, which involves running on the hot side. Since our Mooney has had two run-in's with sticky exhaust valves, one which was just seconds away from turning catastrophic, the subject is very near and dear to us. 

I know this question will elicit different responses and schools of thought, but is hotter better?

My partner is a really smart guy and captain for a major carrier, so I always listen to his input and what he has to say.

https://www.savvyaviation.com/why-valves-stick/

 

 

[PT20J]

Lycoming says below 400 is good. Many back that off to 380. Personally, I see no advantage to lower CHTs and Ed Kollin’s logic about valve sticking has always seemed reasonable to me. 

[EricJ]

You can go to Superior Air Parts website and download a document called "Engine Management 101", written by their VP after many years in the engine and cylinder production businesses.   I attended an AMT webinar that he presented recently and learned some new things about cylinder testing and maintenance that I've already found pretty useful.  He had good notes on engine operation to manage temperatures, etc., and also says that CHTs that are too cool can be detrimental.   

https://www.superiorairparts.com/

[DC_Brasil]

If I am not mistaken, from all that I've read, you're both correct.

Hotter is better to avoid solid deposits on the valves and cooler is better for cylinder longevity.  However, CHT alone will not determine one or the other. 

You should also consider how the engine is run in terms of power output and fuel mixture. 

The lead deposits that could bring the valves to stick tend to adhere better at lower CHT temps, but that usually happens if the burn is not as "clean" as possible. To account for that, you'll want to operate on the LOP side of mixture, where the leftover fuel and lead deposits tend to be minimal, even with lower CHTs.

If you're going to operate ROP, where you have more power output but also probably more leftover lead in the cylinder (as not 100% of the fuel will be burned at each power stroke), you'll want to run the engine hotter (closer to 380, 390 CHT) to account for that excess lead. You can do that by running higher RPMs for example at ROP.

I usually run my engine (M20J with an IO-360-A3B6D) as LOP as I can and with lower RPMs for long flights, and I get CHTs in the 350s. What I usually see at 10,000 feet is: 2200 RPM, LOP, WOT, and I get 143 Kts TAS burning 8.3Gal/h.

If I want a ROP best power cruise, I will see CHTs in the 385 range, with 2500 RPM, closer to 10 gal/h and 155 Kts TAS.

[A64Pilot]

On 4/7/2025 at 1:22 PM, EricJ said:

and also says that CHTs that are too cool can be detrimental.   

https://www.superiorairparts.com/

You know my goal is the middle of the green, what that is in degrees I have no idea, but unlike many I don’t think Lycoming are idiots, there is a reason why the green arc has a bottom to it and it doesn’t go all the way to the bottom of the gauge. I almost always run LOP and sometimes in Winter which isn’t real winter where I live I have to run richer to get temps up. I can only imagine that up in the great white north they have to run peak or ROP to keep temps up?

The only thing I differ on being in the middle of the green is oil pressure, I like it being in the higher end of the green arc myself.

[flyboy0681]

All very good points and contributions. I posed my question to seek reassurance that running toward the hotter side is OK to avoid exhaust valve issues, since our A3B6D is so damned susceptible to crud buildup.

[varlajo]

12 hours ago, DC_Brasil said:

The lead deposits that could bring the valves to stick tend to adhere better at lower CHT temps, but that usually happens if the burn is not as "clean" as possible. To account for that, you'll want to operate on the LOP side of mixture, where the leftover fuel and lead deposits tend to be minimal, even with lower CHTs.

This is spot on. I can (and regularly do) run my A1A deep LOP and observe CHTs at around 260...280 degrees at altitude in winter. Oil remains at 185 degrees. No complaints from the engine so far. 

Edited April 9 by varlajo

[DC_Brasil]

One other thing to keep in mind, in regard to sticking valves and lead deposits, is to pay extra attention to moments when power output is low, like ground operation before takeoff and after landing: lean the mixture aggressively, to the point that the engine will definitely not make meaningful takeoff power.

I usually lean until the engine runs rough and enrich slightly. If you do it like that, you'll notice the power output even to taxi out is very small and you need a lot of throttle. 

This will greatly reduce the probability of lead deposits and spark plug fouling and, since the engine will not make takeoff power, the chances that you'll takeoff with partial mixture are minimal if you forget to enrichen it before it is time to fly.

[Jetpilot86]

I have a Bravo, and if I don’t lean aggressively at SL, it won’t pass a mag check during runup. 

380° IIRC is a Conti number that has invaded the Lycoming space. I’ve spent the last year or so looking for any air leaks under the cowl and still can’t keep #3/6 under 380°-385° at Key 52 at 17k or above. 15k & 53, 360°-370° no problem. 
 

I’m at the point, after some discussions with Don Maxwell, that <400° will be ok, and even pulling the power back a little to hold that temp if I want to run hard above 15k because of less cooling air. He basically said that the Alpha engines were really just run too hard, too high because pilots thought they could be. 

As soon as I get all the post annual kinks worked out, I’ll be figuring out how fast I can run at 17-18k, max cannula altitudes, at LOP and probably 390° as my cruise CHT limit, I’ve been trying to do 52 & 380° to no avail unless I tolerate the rumble of full cowl flaps. I currently climb at WOT, Full Rich as it runs noticeably cooler than at 34/24 for the extra gallon or two it costs to climb. I’m usually going 700-1000NM any time I saddle up so I run LOP almost exclusively. I got my Bravo with 10 since SFOH, and a fresh turbo, so it will be an interesting study. 
 

I also don’t enrich for landing because of the potential fouling, but am comfortable with the extra steps in a go around to add “some” power, then full rich, then the prop and the rest of the throttle. Just can do full rich on final at 7000’