CHT temps too cold?

[AndreiC]

Now that winter is here I started worrying about CHTs being too cold. Is there such a thing? Plane is a 1970 M20E with the Lycoming IO-360. My single-cylinder CHT has a green range, and with outside temps in the 10-15 degrees F I barely can get the needle in the green range, if at all (in cruise). Once I begin the descent, forget about it. Combining the increased airspeed with the (eventual) decrease in engine power, can't keep it in the green, even though I don't see quick cylinder cooling off (my JPI instrument would shout at me if I had a fast cool-off).

Two questions:

1) Is there a minimum recommended operating CHT? If yes, what can be done to keep it there? I have a 4-cylinder EDM-700, but I don't know what I should try to accomplish.

2) Should I keep the cowl flaps in these temperatures even during the climb-out? 

Thanks. 

[A64Pilot]

Yes you can be too cool, you want to stay in the green. I’d start out first by ensuring the flaps fully close, usually they won’t, in warmer parts of the world you want fully closed to be at least 1/4” open, but I think in real cold you want them fully closed.

Even in Fl sometimes in Winter I have to either increase power of enrichen the mixture to stay comfortably in the green if I’m running low power. I’m thinking that maybe in Winter you may have to run ROP to keep things warm enough.

It’s very likely I think that you will have to block off airflow to some of the oil cooler, approach that slowly you don’t want to overheat by blocking too much. Many will use aluminum duct tape, the couple of times I’ve done it I use a piece of sheet metal

In real cold like where you are I’d climb with the flaps trailing open myself, I’d be hesitant to close them fully, but I don’t have any piston experience in real cold.

Watch for your oil turning milky color thats water, don’t be to concerned if you get a milky discharge from the breather tube, that’s normal in cold weather

[Shadrach]

All of the Vintage injected birds with the stock “guppy mouth” seem to have a surplus of airflow for cooling. On top of that, many of those airplanes are now running with the ignition  timing retarded to 20°, making an already cool running powerplant run even cooler. 

you don’t have a lot of options. I would definitely winterize the oil cooler. I would check the ignition timing on the data plate. If it’s 20°, I believe it’s legal to advance the timing to 25°. There is some debate as to whether you can re-stamp the data plate or must order a new one from Lycoming.

Even with 25° ignition timing and the oil cooler winterized, it’s a struggle to get #1 and #4 above 300°F this time of year. The good news is that as long as you’re not fouling plugs, it shouldn’t be an issue.

[jlunseth]

I live in MN and I have a 231 (turbo). When the temps are below freezing it is important first, to keep the cowl flaps closed even on the ground and second, to run the engine for awhile on the tarmac with the cowl flaps closed to warm the engine compartment. (Does an E have cowl flaps?) There are components that work better and/or won't work at all unless they are heated to above freezing. Without going into all the detail, I once had the air/oil separator freeze resulting in oil blowing out of any available orifice in the engine because I failed to warm the engine compartment. Now if it is below freezing, and especially below 20 dF, I warm the engine compartment and have not had the problem again. Remember that your brake master cylinder is not heated either and it probably appreciates working with unfrozen brake fluid. If you think that is a joke or not possible, you should try flying when it is in the -20 dF range or worse, which we sometimes get here in the dead of winter. Our record low here in MN is -60 dF and it happened in my lifetime.

I have had people jump in and say never run the engine with the cowl flaps closed on the ground, it will cause hot spots. I think they live on the east coast and "cold" is when you have to put on a sweater. When it is in the very cold range, from +20dF on down, you need all the heat you can get, never mind any theoretical hot spots. I also keep the cowl flaps closed in a climb during the winter cold so long as the CHTs are in the 380 range and the TIT stays under control.

My A&P made an "oil cooler block" for me years ago (actually two) , and it is about time to stuff one in the oil cooler. It is just stiff foam with tape around it that fits into the block. Very important in cold MN temps, it stays there all winter. There is a metal plate that can be installed as I understand it, but why do that when you can make one that works for next to nothing.

There is a normal operating CHT temp range in the POH for my engine. It is 250-460 dF. (That 460 maximum is silly, the temps should be around 380 and even in a high hot climb, no higher than 420 and then only for a short duration). However, there is no redline minimum CHT. There is a normal oil temp range in the POH of 100-240 dF. Although the stated OT minimum is not a redline minimum it is important not to go below 100 in a turbo because that can rob the turbine bearing of lubrication. Not your problem though.

I do everything I can to keep the CHTs in the normal operating range. But in my aircraft the front left cylinder sits right in the open cowling intake hole with no obstruction. The hole on the right side is partially obstructed by the induction air intake. The front left cylinder can be difficult to keep in the normal operating range. Although I like to run LOP, there are times in the winter when that results in CHTs that are too cool. If that happens I will go ROP and on occasion run as hot as 100 dF ROP. You could run right at peak (and there are peak settings in the POH) but the GAMI people teach that doing so is not good for the engine. They say that it is possible in very cold temps to have cool CHTs while running in the "red box," but you are overstressing the engine all the same because the peak pressure during combustion is just too high in the cylinders. 

So far as I am aware, there is no harm to the cylinders from running them under normal operating temperature. What I have read in researching the issue is that leading (lead deposits) becomes an issue, so I try to keep the cylinders warm. I have on a few occasions grit my teeth and run with the OT at 95 and the lowest CHT at 220. This typically happens when I am forced to stay at say 2,000 AGL (3,000 MSL around here) because the cooling capacity of the dense air at low altitude is too good. I don't like doing that, on the other hand the mythical shock cooling has never happened either. If you are concerned about that, Lyc recommends setting the RPMs as low as possible in order to keep engine power (and thus temperature) up. I think I might try that the next time I am stuck doing a low altitude flight in very cold weather.

Altitude can be used to fight off excess cooling, but easier in my 231 than an NA. I have been in the -54 dF range in the flight levels where it was so cold the heater and defroster could not fight off the frost. All the windows frosted over and it was one of those rare times when I have logged actual instrument time even though it was CAVU outside the aircraft. However, the engine was perfectly happy. The cooling capacity of the air up high is poor, so the engine stays warm. I use that trick when I can - go high to keep the engine happy.

Edited December 1, 2023 by jlunseth

[A64Pilot]

You don’t have redline min oil temps or cyl head temp, because if you did you couldn’t operate below those redlines, so no running the engine unless your above the min.

In Winter I’d be happy with 300ish cyl head temps, 380 is in my opinion too hot at any time, but then I don’t run mine hard either.

You need a min oil temp mostly to cook off moisture that’s a normal by-product of fossil fuel combustion, the mantra used to be mark your gauge at 180 and run 180 oil temp, but in truth you don’t need it that high, from memory the hottest place the oil gets is roughly 60 deg higher than what the gauge reads, and you don’t need the oil boiling hot to drive off moisture.

From last years IA renewal class, I was off on the temp but not by a whole lot. I don’t think my J models oil temp ever gets above 165 even in warm weather.

 

 

 

[GeeBee]

Today, I had to change from LOP to ROP on my IO-550 because I could not keep the CHT above 250. Just a fact of life in the winter. You can be too cold just as you can be too hot. Take measure in both events to keep the CHT where it belongs.

 

[dzeleski]

I adjust my cowl flaps in the winter and adjust them again once spring hits. Pretty much fully closed during the winter and 1/2 inch open during the summer. It takes 10 minutes to adjust them.

In the winter the cowl flaps should be closed on the ground and depending on ambient they are even closed during take off. Once the CHTs rise to 350 or so I put them in trail and just monitor from there but sometimes it’s so cold I never even need to touch it. I also leave them closed for approach and landing. 

[Shadrach]

2 hours ago, dzeleski said:

I adjust my cowl flaps in the winter and adjust them again once spring hits. Pretty much fully closed during the winter and 1/2 inch open during the summer. It takes 10 minutes to adjust them.

In the winter the cowl flaps should be closed on the ground and depending on ambient they are even closed during take off. Once the CHTs rise to 350 or so I put them in trail and just monitor from there but sometimes it’s so cold I never even need to touch it. I also leave them closed for approach and landing. 

I was told by a well-known APS instructor that GAMI had instrumented a Continental six with additional thermal couples to investigate differences in temperature at different parts of the cylinder. They found that the greatest delta in temperature across cylinder occurred when ground running with the cowl flaps closed. They recommended not closing the cowl flaps on the ground to expedite warm-up.

[dzeleski]

Just now, Shadrach said:

I was told by a well-known APS instructor that GAMI had instrumented a Continental six with additional thermal couples to investigate differences in temperature at different parts of the cylinder. They found that the greatest delta in temperature across cylinder occurred when ground running with the cowl flaps closed. They recommended not closing the cowl flaps on the ground to expedite warm-up.

I’m not sure that’s accurate for a Lycoming IO-360 in a J at least. My CHTs go up with them closed. That’s also with the cowl flaps adjusted closed not slightly open as normally recommended.

You happen to have a link to that test?

[Justin Schmidt]

4 hours ago, Shadrach said:

All of the Vintage injected birds with the stock “guppy mouth” seem to have a surplus of airflow for cooling. On top of that, many of those airplanes are now running with the ignition  timing retarded to 20°, making an already cool running powerplant run even cooler. 

you don’t have a lot of options. I would definitely winterize the oil cooler. I would check the ignition timing on the data plate. If it’s 20°, I believe it’s legal to advance the timing to 25°. There is some debate as to whether you can re-stamp the data plate or must order a new one from Lycoming.

Even with 25° ignition timing and the oil cooler winterized, it’s a struggle to get #1 and #4 above 300°F this time of year. The good news is that as long as you’re not fouling plugs, it shouldn’t be an issue.

It is not legal to advance to 25°, per Lycoming. They will not give a new data plate. I tried when I installed the Electroair

[Shadrach]

3 hours ago, jlunseth said:

I live in MN and I have a 231 (turbo). When the temps are below freezing it is important first, to keep the cowl flaps closed even on the ground and second, to run the engine for awhile on the tarmac with the cowl flaps closed to warm the engine compartment. (Does an E have cowl flaps?) There are components that work better and/or won't work at all unless they are heated to above freezing. Without going into all the detail, I once had the air/oil separator freeze resulting in oil blowing out of any available orifice in the engine because I failed to warm the engine compartment. Now if it is below freezing, and especially below 20 dF, I warm the engine compartment and have not had the problem again. Remember that your brake master cylinder is not heated either and it probably appreciates working with unfrozen brake fluid. If you think that is a joke or not possible, you should try flying when it is in the -20 dF range or worse, which we sometimes get here in the dead of winter. Our record low here in MN is -60 dF and it happened in my lifetime.

I have had people jump in and say never run the engine with the cowl flaps closed on the ground, it will cause hot spots. I think they live on the east coast and "cold" is when you have to put on a sweater. When it is in the very cold range, from +20dF on down, you need all the heat you can get, never mind any theoretical hot spots. I also keep the cowl flaps closed in a climb during the winter cold so long as the CHTs are in the 380 range and the TIT stays under control.

My A&P made an "oil cooler block" for me years ago (actually two) , and it is about time to stuff one in the oil cooler. It is just stiff foam with tape around it that fits into the block. Very important in cold MN temps, it stays there all winter. There is a metal plate that can be installed as I understand it, but why do that when you can make one that works for next to nothing.

There is a normal operating CHT temp range in the POH for my engine. It is 250-460 dF. (That 460 maximum is silly, the temps should be around 380 and even in a high hot climb, no higher than 420 and then only for a short duration). However, there is no redline minimum CHT. There is a normal oil temp range in the POH of 100-240 dF. Although the stated OT minimum is not a redline minimum it is important not to go below 100 in a turbo because that can rob the turbine bearing of lubrication. Not your problem though.

I do everything I can to keep the CHTs in the normal operating range. But in my aircraft the front left cylinder sits right in the open cowling intake hole with no obstruction. The hole on the right side is partially obstructed by the induction air intake. The front left cylinder can be difficult to keep in the normal operating range. Although I like to run LOP, there are times in the winter when that results in CHTs that are too cool. If that happens I will go ROP and on occasion run as hot as 100 dF ROP. You could run right at peak (and there are peak settings in the POH) but the GAMI people teach that doing so is not good for the engine. They say that it is possible in very cold temps to have cool CHTs while running in the "red box," but you are overstressing the engine all the same because the peak pressure during combustion is just too high in the cylinders. 

So far as I am aware, there is no harm to the cylinders from running them under normal operating temperature. What I have read in researching the issue is that leading (lead deposits) becomes an issue, so I try to keep the cylinders warm. I have on a few occasions grit my teeth and run with the OT at 95 and the lowest CHT at 220. This typically happens when I am forced to stay at say 2,000 AGL (3,000 MSL around here) because the cooling capacity of the dense air at low altitude is too good. I don't like doing that, on the other hand the mythical shock cooling has never happened either. If you are concerned about that, Lyc recommends setting the RPMs as low as possible in order to keep engine power (and thus temperature) up. I think I might try that the next time I am stuck doing a low altitude flight in very cold weather.

Altitude can be used to fight off excess cooling, but easier in my 231 than an NA. I have been in the -54 dF range in the flight levels where it was so cold the heater and defroster could not fight off the frost. All the windows frosted over and it was one of those rare times when I have logged actual instrument time even though it was CAVU outside the aircraft. However, the engine was perfectly happy. The cooling capacity of the air up high is poor, so the engine stays warm. I use that trick when I can - go high to keep the engine happy.

I don’t think anyone is GAMI ever said that peak EGT is a bad place to run an engine. Especially given that 100° ROP is closer to peak ICP. That being said if you’re running a turbo, Peak TIT is not an option in most cases. 
your other points are all very well taken. I would just add that the 5606 in your brake lines has a pour point of <-76°F. It’s not going to be phased by 20° temps. 

[Shadrach]

7 minutes ago, dzeleski said:

I’m not sure that’s accurate for a Lycoming IO-360 in a J at least. My CHTs go up with them closed. That’s also with the cowl flaps adjusted closed not slightly open as normally recommended.

You happen to have a link to that test?

You’re looking at a single thermocouple on one cylinder. Everyone’s  CHT readings go up when they close the cowl flaps. Walt’s (Walter Atkinson of APS) point was that the increase is less even across the cylinder when the cowl flaps are closed which leads to hot spots and the potential for thermal stress. GAMI keeps their raw data pretty close to the chest. I don’t think this was a published study.

[Shadrach]

27 minutes ago, Justin Schmidt said:

It is not legal to advance to 25°, per Lycoming. They will not give a new data plate. I tried when I installed the Electroair

 Really? Who’d you speak with? Did they tell that their word supersedes the Type Certificate Data Sheet?

 

[dzeleski]

4 minutes ago, Shadrach said:

You’re looking at a single thermocouple on one cylinder. Everyone’s  CHT readings go up when they close the cowl flaps. Walt’s (Walter Atkinson of APS) point was that the increase is less even across the cylinder when the cowl flaps are closed which leads to hot spots and the potential for thermal stress. GAMI keeps their raw data pretty close to the chest. I don’t think this was a published study.

I’ve got 4 CHT probes, all four increase. More importantly I get oil temp faster which is what actually matters for cold ops. I get what you are saying but an idling air cooled engine in freezing temps isn’t going to create enough heat to cause hot spots that actually matter. The first few inches of the prop are damn near round, there is very little airflow going through the cowl at idle even with them open, which is part of the reason for facing into the wind for a run up. The difference during taxi speeds on CHT are most noticeable, that extra airflow really drops them if they are open.

[Shadrach]

7 minutes ago, dzeleski said:

I’ve got 4 CHT probes, all four increase. More importantly I get oil temp faster which is what actually matters for cold ops. I get what you are saying but an idling air cooled engine in freezing temps isn’t going to create enough heat to cause hot spots that actually matter. The first few inches of the prop are damn near round, there is very little airflow going through the cowl at idle even with them open, which is part of the reason for facing into the wind for a run up. The difference during taxi speeds on CHT are most noticeable, that extra airflow really drops them if they are open.

You are not understanding what I am saying. I am not talking about cylinder to cylinder differences. GAMI put multiple thermocouples on all of the cylinders and found the greatest temperature variance across each cylinder (multiple points around the base, the barrel, the head, etc) occurred when ground running with the cowl flaps closed.

they also found that idling with the plane faced into the wind actually provided slightly less cooling than idling with the wind behind you.

[Justin Schmidt]

16 minutes ago, Shadrach said:

 Really? Who’d you speak with? Did they tell that their word supersedes the Type Certificate Data Sheet?

 

Lycoming engineers. The timing must match the data plate and they will not send them out anymore. The data plate is god. Remember The IO360A1A originally was 25 degrees and there was a SI that had changed them to 20 degree, which is why it is in the TCDS. There was a reason they did that, safety margin.

[Shadrach]

4 minutes ago, Justin Schmidt said:

Lycoming engineers. The timing must match the data plate and they will not send them out anymore. The data plate is god. Remember The IO360A1A originally was 25 degrees and there was a SI that had changed them to 20 degree, which is why it is in the TCDS. There was a reason they did that, safety margin.

The SI did not change the timing. The SI made made 20° optional. There is nothing in the way of written guidance that suggests Lycoming has forbidden 25°.  “Lycoming Engineers” do not supersede the TCDS.
 

[Justin Schmidt]

6 minutes ago, Shadrach said:

The SI did not change the timing. The SI made made 20° optional. There is nothing in the way of written guidance that suggests Lycoming has forbidden 25°.  “Lycoming Engineers” do not supersede the TCDS.
 

You do you, I do tend to want to keep my plane legal. FSDO and Lycoming engineers to have a say in that, and the FSDO said whatever the engineers say.

[dzeleski]

35 minutes ago, Shadrach said:

You are not understanding what I am saying. I am not talking about cylinder to cylinder differences. GAMI put multiple thermocouples on all of the cylinders and found the greatest temperature variance across each cylinder (multiple points around the base, the barrel, the head, etc) occurred when ground running with the cowl flaps closed.

they also found that idling with the plane faced into the wind actually provided slightly less cooling than idling with the wind behind you.

I fully understand what you are saying. I’m saying I don’t buy it. If the study wasn’t posted publicly it doesn’t exist.

Volkswagen had mechanisms on their air cooled engines for decades to block airflow during warmup that were thermally controlled.

[Shadrach]

30 minutes ago, Justin Schmidt said:

You do you, I do tend to want to keep my plane legal. FSDO and Lycoming engineers to have a say in that, and the FSDO said whatever the engineers say.

I’m sorry that your attempt to return your engine to its original timing configuration as outlined in the FAA TCDS was unsuccessful. You’re apparent failed attempt to make the case with your local FSDO has nothing to do with the legality of my plane (still has original data 25° plate) or anyone who has legally returned their timing to he specified setting in the TCDS.

[Shadrach]

26 minutes ago, dzeleski said:

I fully understand what you are saying. I’m saying I don’t buy it. If the study wasn’t posted publicly it doesn’t exist.

Volkswagen had mechanisms on their air cooled engines for decades to block airflow during warmup that were thermally controlled.

Fair enough. I learned a lot from Walter (RIP). He never gave me any reason to believe he would make something up, but maybe he was bullshitting me just for fun.  I was just offering what I was told.  

[dzeleski]

13 minutes ago, Shadrach said:

Fair enough. I learned a lot from Walter (RIP). He never gave me any reason to believe he would make something up, but maybe he was bullshitting me just for fun.  I was just offering what I was told.  

I completely understand and I appreciate the conversation. There are unfortunately endless old wives tales that continue to be circulated around in aviation. So I’m always very hesitant to believe anything without seeing specific data or proof.

[MikeOH]

Ok, I'm a little confused on this timing thing...

I was under the impression that 25 was on the TCDS and engine data plate, but 20 was okay, too.  Are there Mooneys with 20 degrees on their data plates?  When did that start?  And, why wasn't the TCDS updated for those models/years?

Something doesn't seem right....is it possible Lycoming built, for example, IO-360A1As with 20 degree data plates for other makes/models of aircraft..and those engines have found their way into Mooneys?

[Justin Schmidt]

11 minutes ago, MikeOH said:

Ok, I'm a little confused on this timing thing...

I was under the impression that 25 was on the TCDS and engine data plate, but 20 was okay, too.  Are there Mooneys with 20 degrees on their data plates?  When did that start?  And, why wasn't the TCDS updated for those models/years?

Something doesn't seem right....is it possible Lycoming built, for example, IO-360A1As with 20 degree data plates for other makes/models of aircraft..and those engines have found their way into Mooneys?

From the very long conversations I have with the FSDO and Lycoming, Lycoming had an SI which changed the timing from 25 degrees to 20 (We can argue the validity engineering wise, well I think everyone would be on the same page). They did it to have better detonation margin in a particular scenario. It was not mandatory, meaning it was not an AD. Everything coming from Lycoming is 20 degrees. On a field OH it would have been up to the shop to change it to 20 OR could have just been done in the field as well. Just needed a new data plate or stamped to 20 degrees and Lycoming notified.
So there will be 20 and 25 out there. Lycoming will not allow a 20 to 25. The FSDO leaves it up to the manufacture in this case since it is in the TCDS. But the data plate is god in this case.

[MikeOH]

@Justin Schmidt

So, if I understand you correctly, some engines, when overhauled, had their data plates changed to 20 degrees?

Further, are you saying Lycoming now ships new engines with 20 degree data plates??!?