CHT vs OAT

[idahoflier]

Hi,

To the point, is the relationship to CHT vs OAT linear? In other words, for the same conditions, power setting, altitude, pressure, if the OAT increases 30° will the CHT increase a corresponding 30°?

Background/context: I'm 8.5 hrs into a newly overhauled O-360-A4M and had an engine monitor installed. Previously all I had was an EGT gauge on one cylinder. I'm a little shocked at the CHT I'm seeing, particularly on one cylinder. The last flight I took one cylinder peaked out to about 425° in the climb out. At cruise running a max power mixture at 75% power the same cylinder was running 375°. I know I may still be in break in and the CHT's should decrease about 20° or so once broken in, but even so this seems pretty hot for winter. Just wondering what this is going to mean when the temperatures start getting warmer this coming summer...

Thanks!

 

[JOhnH]

No.

 

[murphey]

CHTs during takeoff climb aren't critical (unless 450 and above, then you might have a bad sensor/probe), the EGTs are. The CHT are more important in cruise. I have the JPI installed and like you, started to freakout during climb. It's very common in the O-360 that #3 cylinder (closest to the oil cooler intake) runs hotter than the others, by the way, even in cruise.

If you don't have the Lycoming manual:

https://www.lycoming.com/sites/default/files/O-HO-IO-HIO-AIO & TIO-360 Oper Manual 60297-12.pdf

Take a look at these, if you haven't already:

https://www.lycoming.com/content/leaning-lycoming-engines

https://www.lycoming.com/content/service-instruction-no-1094d

https://www.savvyanalysis.com/articles/understanding-cht-and-egt

 

[mwagg737]

Yes, they will be warmer in the summer.

Sent from my Pixel 2 XL using Tapatalk

 

[flyingron]

To expound on John's answer. An EGT is a function of the "efficiency" of the burn. It pretty much is a function of the air and fuel delivered to the cylinders (and a tiny bit due to bad ignition or compression). CHT is a function of the heat of combustion (which is possibly related to the EGT) as well as the cooling. EGT is a fairly instantaneous read (settles out within a few seconds). CHT is much slower to respond.

 

[mwagg737]

If number three is your warm cylinder, try putting a small baffle in front of number one. That will cool down your number three.

Sent from my Pixel 2 XL using Tapatalk

 

[Stewartb]

My CHTs don't vary more than 10* or so between 70* and 0* ambient. It takes a bit longer to get fully warmed up.

Where's your oil cooler? Which cylinder is hottest?

 

[JOhnH]

To expound on John's answer. An EGT is a function of the "efficiency" of the burn. It pretty much is a function of the air and fuel delivered to the cylinders (and a tiny bit due to bad ignition or compression). CHT is a function of the heat of combustion (which is possibly related to the EGT) as well as the cooling. EGT is a fairly instantaneous read (settles out within a few seconds). CHT is much slower to respond.

Thanks. I didn't mean to be short, but I got interrupted in the middle of typing on my iphone and accidentally posted before exiting.

As Stewartb said, I have flown in outside temps varying by 70 to 80 degrees, but CHTs only varied around 10-20 degrees.
90 to 100+ F happens frequently down here in summer. That is why I desperately want an air conditioner.

 

[idahoflier]

Well that's great news, thanks!

Yes, my oil cooler intake is behind #3, but interestingly, #3 is running the coolest and #4 is the warmest. I suppose it's possible my mechanic somehow got the wiring wrong, but he's meticulous and does great work. I'm almost due for the first oil change so I'll have him take a look to verify...

Again, thanks all!

 

[idahoflier]

CHTs during takeoff climb aren't critical (unless 450 and above, then you might have a bad sensor/probe), the EGTs are. The CHT are more important in cruise. I have the JPI installed and like you, started to freakout during climb. It's very common in the O-360 that #3 cylinder (closest to the oil cooler intake) runs hotter than the others, by the way, even in cruise.

If you don't have the Lycoming manual:

https://www.lycoming.com/sites/default/files/O-HO-IO-HIO-AIO & TIO-360 Oper Manual 60297-12.pdf

Take a look at these, if you haven't already:

https://www.lycoming.com/content/leaning-lycoming-engines

https://www.lycoming.com/content/service-instruction-no-1094d

https://www.savvyanalysis.com/articles/understanding-cht-and-egt

I have reviewed all those materials, thanks!

 

[David Megginson]

CHTs during takeoff climb aren't critical (unless 450 and above, then you might have a bad sensor/probe), the EGTs are. The CHT are more important in cruise. I have the JPI installed and like you, started to freakout during climb. It's very common in the O-360 that #3 cylinder (closest to the oil cooler intake) runs hotter than the others, by the way, even in cruise.

If you don't have the Lycoming manual:

https://www.lycoming.com/sites/default/files/O-HO-IO-HIO-AIO & TIO-360 Oper Manual 60297-12.pdf

Take a look at these, if you haven't already:

https://www.lycoming.com/content/leaning-lycoming-engines

https://www.lycoming.com/content/service-instruction-no-1094d

https://www.savvyanalysis.com/articles/understanding-cht-and-egt

Good refs, but for clarification (in case anyone misunderstands), EGTs are never critical in themselves, because their absolute values are meaningless (see the last link in @murphey's post, from Savvy). What's important about EGTs is their relative value to the hottest EGT you see at any given power setting, but it doesn't matter if the actual number is 1,100°F or 1,500°F or anything in-between, because it's not really measuring a specific temperature the way your CHTs are.

The benefit of EGT is that it responds instantly to power and mixture changes, so you can use the relative value to verify a power/mixture change right away, then wait a bit for the CHTs to catch up and confirm.

 

[idahoflier]

The benefit of EGT is that it responds instantly to power and mixture changes, so you can use the relative value to verify a power/mixture change right away, then wait a bit for the CHTs to catch up and confirm.

That's been my experience so far. One thing I have found interesting is that CHT seems to rise fairly rapidly when cruising beyond 75% power.

 

[sourdough44]

I try to keep CHTs under 400, in all phases of flight. A Summer temp takeoff & climb out will often bump against that temp on #3, hottest cylinder. (Why is 3 hottest?)

Your 8.5 hours is low for the overhauled engine, though many say most break-in happens in the 1st few hours.

My temps are much more matched once leaned at a cruise altitude, #3 still slightly warmer, CHT and EGT. The CHT spread usually stabilizes near 40-45 degrees.

This is an IO-390 with about 115 hours since factory new, CGR-30P monitor.

 

[Jamie Kirk]

Being a turbo TIT and CHT are what I focus on. I try to keep my hottest cylinder under 410* in the climb (other cylinders are all 375ish) and my my TIT at 1550 (max is 1650).

In cruise CHT in the summer drives mixture, in the winter time TIT drives mixture.

My hottest cylinder in cruise May be 398* and my second cylinder is 350*. #2 is always the hottest

 

[Domenick]

CHTs during takeoff climb aren't critical (unless 450 and above, then you might have a bad sensor/probe), the EGTs are. ...

450! Yikes. 450-500F was the accepted max temp decades ago. I avoid going over 400F on any cylinder in all phases of flight. If during climb a CHT hits 390F, I shallow my climb. This with an O-320 in a Warrior.

 

[Snowmass]

So what is the correlation between OAT and CHT? Anybody got proof? When I got my STC of a cooling mod on the notorious GO-300 the the FAA said that the stock engine cooling failed in climb when their CHT readings were adjusted to a "Hot day at sea level". (It passed with my mod.)

 

[Clip4]

Hi,

To the point, is the relationship to CHT vs OAT linear? In other words, for the same conditions, power setting, altitude, pressure, if the OAT increases 30° will the CHT increase a corresponding 30°?

Background/context: I'm 8.5 hrs into a newly overhauled O-360-A4M and had an engine monitor installed. Previously all I had was an EGT gauge on one cylinder. I'm a little shocked at the CHT I'm seeing, particularly on one cylinder. The last flight I took one cylinder peaked out to about 425° in the climb out. At cruise running a max power mixture at 75% power the same cylinder was running 375°. I know I may still be in break in and the CHT's should decrease about 20° or so once broken in, but even so this seems pretty hot for winter. Just wondering what this is going to mean when the temperatures start getting warmer this coming summer...

Thanks!

Look at the performance chart for your aircraft and see how the RPM setting change. With identical RPM setting at different temps the % of power is not identical. A lot more factors go into cooling ability of the air.

 

[denverpilot]

Aluminum loses a lot of strength as you approach 400F.

One can argue the various types of strength and what might be being experienced in an engine somewhere away from the CHT probe on the outside of a cylinder, but I don’t like seeing 400 or above.

Not if the silly aircraft will climb without doing it anyway. And even then... starts to have reverse consequences for what we’re trying to accomplish, which is staying aloft. LOL.

 

[idahoflier]

Look at the performance chart for your aircraft and see how the RPM setting change. With identical RPM setting at different temps the % of power is not identical. A lot more factors go into cooling ability of the air.

Understood, to truly know power setting I would need altitude, barometric pressure, RPM, MAP and carb air inlet temperature. In my case I don't have MAP or carb air inlet temperature so it's going to be an approximation, but I'm fine with +/- 5%...

 

[Lndwarrior]

Have you double checked your baffling to make sure there is no leakage? In some cases it can be easy to get one section bent the wrong way after reinstalling the cowl.

 

[Stewartb]

Who dod the overhaul? It's very common for overhauled carbs to be leaner than they should be. The worst time for a lean carb is in winter with cool temps. Make sure you have at least 150* leaning authority in your normal (winter) temperatures.

 

[idahoflier]

Have you double checked your baffling to make sure there is no leakage? In some cases it can be easy to get one section bent the wrong way after reinstalling the cowl.

I briefly looked over the baffling the last time the cowl was off after the first flight and it appeared to be OK. The baffling was in good shape so it wasn't replaced when the engine was re-installed. Based on the feedback it sounds like cooling will be about the same between winter and summer so I can live with the temps as they are and will just have to adjust the climb out for a slower rate and faster airspeed when I'm able...

 

[idahoflier]

Who dod the overhaul? It's very common for overhauled carbs to be leaner than they should be. The worst time for a lean carb is in winter with cool temps. Make sure you have at least 150* leaning authority in your normal (winter) temperatures.

Penn Yan. I'll check that on the next flight. Thanks!

 

[Dana]

CHT should track OAT, since the engine produces a more or less constant amount of heat, heat transfer is proportional to the temperature difference, so the engine heats up until the temperature difference allows that amount of cooling. I have usually found this to be the case.

On engines with a thermostatically controlled oil valve regulating flow to the oil cooler, it's a different story.

 

[NoHeat]

I'm almost due for the first oil change

Ask the mechanic to swap two probes. If a different probe indicates the same problem, it’s real.

Also, look at the downloaded data Savvy Analysis has a great website for this. If you see the indicated CHT fluctuate by 50 F in a few seconds, that’s not real, it’s a bad connection. Metal does not heat up and cool off that fast.