CHT - What's too hot?

[PT20J]

5 hours ago, MikeOH said:

Hmm, yet, supposedly, Mike Busch is the one who said 450 during break-in is okay?!?  Which is full circle back to my original question: How is 450 degrees during break-in any less damaging than 450 during climb out?

Yes, I get it that higher temps should be avoided...but, by that logic and the strength of aluminum vs. temp graphs, we should be operating below 300 F (approx. 25% loss of strength).

You ask a couple of very reasonable questions. First, when you break in a new cylinder on the airplane (i.e., no run in a test cell) there is a lot of extra friction as the rings mate to the cylinder walls. This creates the high CHT. You want a rich mixture, open cowl flaps and high airspeed to keep the CHT as low as possible while keeping power at around 75% so as to keep combustion pressures high to seat the rings. The high CHTs should drop after a few hours, and this is an indication that the break in is largely complete.

Second, the engineers designed the heads with a lot of extra metal. A few hours at 450 deg isn’t going to hurt anything. It is long-term running at high power and elevated temperature that causes problems. Every combustion event causes a peak pressure much higher than the BMEP, and that causes the head to expand and then contract. When it is hotter, it expands and contracts more. Just like bending a piece of aluminum sheet back and forth repeatedly, it will eventually fatigue and fail. Heads do wear out, and that’s why it is best to use new cylinders at overhaul. So, keep temps below 400 most of the time and don’t sweat a few minutes over 400 during a climb if that’s necessary for a safe climb out. 

Skip

 

[Cruiser]

5 hours ago, MikeOH said:

LOL!

Boy, ain't that the truth!  I've been stressing over my above 400F CHTs since I've owned my plane.

Thanks to this thread I'm going to relax a bit and accept my 410F climb CHTs and mid-300s cruise CHTs.  It was really bugging me to pull back power on hot day climbs just to keep CHTs under 400.

you should continue to do that.........

[MikeOH]

1 hour ago, PT20J said:

You ask a couple of very reasonable questions. First, when you break in a new cylinder on the airplane (i.e., no run in a test cell) there is a lot of extra friction as the rings mate to the cylinder walls. This creates the high CHT. You want a rich mixture, open cowl flaps and high airspeed to keep the CHT as low as possible while keeping power at around 75% so as to keep combustion pressures high to seat the rings. The high CHTs should drop after a few hours, and this is an indication that the break in is largely complete.

Second, the engineers designed the heads with a lot of extra metal. A few hours at 450 deg isn’t going to hurt anything. It is long-term running at high power and elevated temperature that causes problems. Every combustion event causes a peak pressure much higher than the BMEP, and that causes the head to expand and then contract. When it is hotter, it expands and contracts more. Just like bending a piece of aluminum sheet back and forth repeatedly, it will eventually fatigue and fail. Heads do wear out, and that’s why it is best to use new cylinders at overhaul. So, keep temps below 400 most of the time and don’t sweat a few minutes over 400 during a climb if that’s necessary for a safe climb out. 

Skip

 

Nice summary, Skip.  That's pretty much the conclusion I was coming to.

Aluminum has no fatigue life limit, so ANY repetitive stress will eventually cause a fatigue failure.  It's worse, for just the reason you stated, at elevated temps.  The question becomes one of time (how many repetitive stress events, i.e. combustion events) and temperature curve to failure.

Based on material FACTS,  it's pretty clear that 25% of aluminum's room temp strength is gone at only 300F; yet, no one worries about that.  And, at 400F 75% of that yield strength is gone! Yet, 'keeping it below' 400F is somehow considered 'safe.'  As you say, the engineers have designed to account for this.

I think your previous comment about CHTs being the latest LOP and shock cooling sacred cow is spot on

 

[MikeOH]

15 minutes ago, Cruiser said:

you should continue to do that.........

Uh, NO I am not.

Rather than follow presently accepted witchcraft I am going to go with Lycoming's advice (posted by G Miller earlier in the thread):

       From a Lycoming Publication.....

       In our many years of building engines, the engines have benefited during continuous operation by keeping CHT below 400 F in order to achieve best life and wear of the powerplant. In general, it would be normal during all year operations, in climb and cruise to see head temperatures in the range of 350 F to 435 F.

I have no problem keeping CHTs in the mid-350s during cruise, and will continue to do so.  I am, however, not going to sweat a 10-15 minute climb to altitude in the summer with a 410F CHT.

Nevertheless, I am going to look into my full power fuel flow as it does seem low both anecdotally, and from Savvy cohort data.

[HRM]

When in doubt, consult with the master: Mike Busch

[0TreeLemur]

10 hours ago, HRM said:

When in doubt, consult with the master: Mike Busch

Mike Busch says we should strive to keep the CHT in Lycomings below 410F.   Looking at the programmable settings in the JPI 900 menu, I don't see a "warning" CHT setting.  Of course the 500F redline setting is there.   I wish we could set a warning level (e.g. 408) that would turn the CHT bar yellow and help show that the  temp is getting high.    Is there such a setting and I just missed it?   I need to look at the manual- I don't think there is a warning setting.   Anyone have any experience communicating with JPI?  I've heard rumors that their attention can be difficult to obtain....

[EricJ]

14 minutes ago, 0TreeLemur said:

Mike Busch says we should strive to keep the CHT in Lycomings below 410F.   Looking at the programmable settings in the JPI 900 menu, I don't see a "warning" CHT setting.  Of course the 500F redline setting is there.   I wish we could set a warning level (e.g. 408) that would turn the CHT bar yellow and help show that the  temp is getting high.    Is there such a setting and I just missed it?   I need to look at the manual- I don't think there is a warning setting.   Anyone have any experience communicating with JPI?  I've heard rumors that their attention can be difficult to obtain....

I think there is a CHT temp warning level, because my JPI-900 went off once when I forgot to open the cowl flaps on takeoff after a taxi-back landing.   The AC-17 Bitchin' Betty paid for herself that day.   I don't remember what the temp was, but it was well below 500F, I think maybe 435 or something.

 

 

[HRM]

29 minutes ago, 0TreeLemur said:

Looking at the programmable settings in the JPI 900 menu, I don't see a "warning" CHT setting. 

These are the settings that EI put in my MVP-50 (IO-360-A1A). Savvy flags high CHT at 450º.

[Skates97]

From the Lycoming O-360 Operator's Manual. It doesn't note that it is for moments during climb, etc... it is talking about cruise operation. I'm not recommending that people run that hot in cruise, but as some others have stated I'm not going to stress too much about seeing numbers over 400° while climbing out.

"For maximum service life, cylinder head temperatures should be maintained below 435°F (224°C) during high performance cruise operation and below 400°F (205°C) for economy cruise powers."

 

O-360 Operator Manual.pdf

[Rick Junkin]

1 hour ago, 0TreeLemur said:

I wish we could set a warning level (e.g. 408) that would turn the CHT bar yellow and help show that the  temp is getting high.    Is there such a setting and I just missed it?

There is - its a "pre-warning' setting if I recall correctly, and I have it set to 390 on my JPI 900. It gives a flashing warning at the bottom of the display. I won't have my JPI manuals available for about a week, but I'm sure someone else can give you a point-out before then as to where exactly in the setup menus you can find it.

Cheers,
Rick

[Ragsf15e]

21 minutes ago, Junkman said:

There is - its a "pre-warning' setting if I recall correctly, and I have it set to 390 on my JPI 900. It gives a flashing warning at the bottom of the display. I won't have my JPI manuals available for about a week, but I'm sure someone else can give you a point-out before then as to where exactly in the setup menus you can find it.

Cheers,
Rick

Yeah I have a 930, but that’s how I remember it too.  There are a bunch of red lines and limits that are preprogrammed and you can’t change because they are manufacturers limits, but there are a few pre warnings you can set.  Cht, and fuel level among them.  It definitely took the manual and some trial/error to get it set.

[Rick Junkin]

41 minutes ago, Junkman said:

2 hours ago, 0TreeLemur said:

I wish we could set a warning level (e.g. 408) that would turn the CHT bar yellow and help show that the  temp is getting high.    Is there such a setting and I just missed it?

There is - its a "pre-warning' setting if I recall correctly, and I have it set to 390 on my JPI 900. It gives a flashing warning at the bottom of the display. I won't have my JPI manuals available for about a week, but I'm sure someone else can give you a point-out before then as to where exactly in the setup menus you can find it.

Cheers,
Rick

@0TreeLemur- Found it! Page 44 of the EDM 900 Pilot Guide. Here's the link to the document on the JPI website - Pilots Guide

Cheers,
Rick

[0TreeLemur]

48 minutes ago, Junkman said:

@0TreeLemur- Found it! Page 44 of the EDM 900 Pilot Guide. Here's the link to the document on the JPI website - Pilots Guide

Cheers,
Rick

That's awesome!   Thanks @Junkman!  I'll work on getting that set.  Having a visual warning will really help.    Looking at my data, most flights before I started really watching it, CHT4 went over 410F, usually by a few degrees, seldom more.  I put that limit into the Savvy Analysis online software, and having the ability to stop it from happening so often will hopefully maximize my engine life.   With his ability to perform analytics on all those EDM data, plus the experience of matching damage histories with CHTs, when Mike Busch gives a limit I think there is nothing wrong with trying to do as he suggests and keep the CHT below 410F.

[Raptor05121]

On 2/29/2020 at 6:57 PM, MikeOH said:

@Ragsf15e

Yes, I have looked at sea-level full rich fuel flow; mine runs about 16.5 gph.

I'm not an A&P but that sounds low for a fuel-injected 200hp engine. I've always heard the "rule of thumb" for fuel flow is (rated HP / 10). So a 180hp engine should be 16-18gph, 200hp engine 18-20gph.

[0TreeLemur]

After work today I changed the JPI EDM-900 pre-alarm setting for the CHT from 460 to 405 and took her up to see how that worked.   I was hoping that the CHT bar chart would turn yellow like the EGT bar graph does.  Nope. As I recall, the warning is a text string in the rolling window that said something like "CHT4=405F" in a blue font.  Not a real attention grabber.  Anything I can do or am I S.O.L.?

[HRM]

9 minutes ago, 0TreeLemur said:

Anything I can do or am I S.O.L.?

Install this.

[EricJ]

19 minutes ago, 0TreeLemur said:

After work today I changed the JPI EDM-900 pre-alarm setting for the CHT from 460 to 405 and took her up to see how that worked.   I was hoping that the CHT bar chart would turn yellow like the EGT bar graph does.  Nope. As I recall, the warning is a text string in the rolling window that said something like "CHT4=405F" in a blue font.  Not a real attention grabber.  Anything I can do or am I S.O.L.?

I have an AV-17 voice annunciator connected to the JPI.   Since there's only one, universal, alarm signal coming out of the JPI, all she says is "Engine monitor," but it makes you look at the display to see what caused the alarm.    I've found that to be very useful and so far unambiguous as to what caused the warning.

 

[ArtVandelay]

After work today I changed the JPI EDM-900 pre-alarm setting for the CHT from 460 to 405 and took her up to see how that worked.   I was hoping that the CHT bar chart would turn yellow like the EGT bar graph does.  Nope. As I recall, the warning is a text string in the rolling window that said something like "CHT4=405F" in a blue font.  Not a real attention grabber.  Anything I can do or am I S.O.L.?

The LED alarm should light up (yellow).

[0TreeLemur]

5 hours ago, ArtVandelay said:

The LED alarm should light up (yellow).

Either I missed it, or it hadn't happened.   When the CHT exceeded 405 and got up to 407 I was pretty intently watching the EDM900.  I didn't notice the caution LED, although it would have been hard to miss being right in front.  The CHT was above 405 for only less than 1 minute.  I'll have to deliberately keep an eye on it next time.    It did light up as usual for the high fuel pressure alarm on takeoff.  Indicating  7.4 psi which is within Lycoming's 8 psi spec but above the limit that is, in my opinion, incorrectly low in the Mooney POH.

[ArtVandelay]

Either I missed it, or it hadn't happened.   When the CHT exceeded 405 and got up to 407 I was pretty intently watching the EDM900.  I didn't notice the caution LED, although it would have been hard to miss being right in front.  The CHT was above 405 for only less than 1 minute.  I'll have to deliberately keep an eye on it next time.    It did light up as usual for the high fuel pressure alarm on takeoff.  Indicating  7.4 psi which is within Lycoming's 8 psi spec but above the limit that is, in my opinion, incorrectly low in the Mooney POH.

When the 900 boots up, the LED should flash (yellow, then red IIRC), check this first.

[EricJ]

2 hours ago, 0TreeLemur said:

Either I missed it, or it hadn't happened.   When the CHT exceeded 405 and got up to 407 I was pretty intently watching the EDM900.  I didn't notice the caution LED, although it would have been hard to miss being right in front.  The CHT was above 405 for only less than 1 minute.  I'll have to deliberately keep an eye on it next time.    It did light up as usual for the high fuel pressure alarm on takeoff.  Indicating  7.4 psi which is within Lycoming's 8 psi spec but above the limit that is, in my opinion, incorrectly low in the Mooney POH.

407F may not be hot enough to trigger the alarm.  I think there's a way to check where it is set withing the JPI via some of the service menus.

I think the LED drive voltage is also the engine monitor alarm input for my AV-17, or if you have an audio panel that can accept some warning inputs (like a PMA450) you could get a voice alarm that way, too, or drive some other kind of audible alert with it if that helps.

 

[0TreeLemur]

1 hour ago, EricJ said:

407F may not be hot enough to trigger the alarm.  I think there's a way to check where it is set withing the JPI via some of the service menus.

I think the LED drive voltage is also the engine monitor alarm input for my AV-17, or if you have an audio panel that can accept some warning inputs (like a PMA450) you could get a voice alarm that way, too, or drive some other kind of audible alert with it if that helps.

 

It did light up as usual for the high fuel pressure alarm on takeoff. 

 

[0TreeLemur]

1 hour ago, EricJ said:

407F may not be hot enough to trigger the alarm.  I think there's a way to check where it is set withing the JPI via some of the service menus.

I think the LED drive voltage is also the engine monitor alarm input for my AV-17, or if you have an audio panel that can accept some warning inputs (like a PMA450) you could get a voice alarm that way, too, or drive some other kind of audible alert with it if that helps.

 

After work today I changed the JPI EDM-900 pre-alarm setting for the CHT from 460 to 405.

[Sandman993]

On 2/27/2020 at 10:55 PM, MikeOH said:

Familiar with 1 through 13...doesn't really address my confusion, I'm afraid.  Not worried about 'wearing a step' due to not changing RPM.

Trying to understand how much time at elevated temps is going to appreciably affect cylinder life.

Put another way: How many hours at elevated (400s kind of temp) does it take before a cylinder is broken in?  How are those hours any different, as far as affecting longevity, than operating at elevated temperatures after break-in?

I've fought moderately high temps since I bought my M20F over two years ago.  #2 would run up to 425 on climb out on a hot day.  With new baffles that number is down to just over 400 when it's hot out.  I'll pull power to keep the temps down, but someday I may need full-power and backing off won't be an option.  The way everyone talks, it sounds like running at 425 for even a few minutes is HORRIFIC  I'm NOT talking about cruising for hours at high CHTs.

Now, I'm beginning to wonder how true that really is...  if running for HOURS at 450 during break-in is okay and, presumably, doesn't destroy cylinder life, why is running 400-410 for a 15 minute climb on the occasional hot day such a problem?

Running 450 is not ok, even at break in. 
when an engine is new, it creates heat simply because of all the friction, while the new parts learn to play nice together. Avoid 400 if possible... when my engine was brand new recently, I literally watched the cylinders break in one at a time on the jpi... you could see the temps fall into a less hostile environment. Oil should be below 240 degrees... around 190 to 200 is better.

Try pitching the airplane for a higher indicated airspeed before you settle into the climb. Mine does well about 120 and that means around 360’s and 70’s at climb power. Do inspect your baffling and while you do, look for holes and gaps where air can bypass. I read a while back how a one inch hole can raise engine temperature by as much as 50 degrees. I’ll wager the gap around your alternator is probably big enough to stick several fingers in. 

Edited March 4, 2020 by mooniac58
offensive