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Solve High EGT with Timing

gmcjetpilot

Well Known Member
Solve High CHT with Timing

EDIT: Meant High CHT

I found this YouTube channel Savvy Aviation, Mike Busch. These are webinars hosted by the EAA over the last few years. See video link below. It's about pre-ignition but he specifically talks about RVs and timing. Every video is fantastic, they're long but chalked full of great information on engines. Starting at 58:00 in video below he starts to talk about 25 degrees BTDC, before top dead center, being a little aggressive and the cause of high CHT in Lycomings. He spacifically talks about RVs. If you go back to the beginning of the video or the beginning of the Q&A there's all kind of good information. Lycoming later issue revised timing on IO360's to 20 degrees before top dead center.

https://youtu.be/bZLxcWAk9Nc

The topic of detonation, pre ignition, and engine self-destruction is pretty fascinating. He's a proponent of setting 380?F as your yellow or warning CHT. He goes on to say for Lycoming you might push that to 400?F but anything above that is abusive. Agree that has been my policy. This is below Lycs recommend temp limit. Lycoming has a red line of 500?F. That temperature is unacceptable. If I see 400-420?F I am going to take action (enrich mixture, lower power, reduce climb or level off if climbing). He also points out that spark plug ring CHT probes read about 40?F low. All the videos are amazing.
 
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In your title, "Solve high EGT with timing" - I want to think you meant, "Solve high CHT with timing". He never mentions timing in this video. But I get what you are wondering about, in at least as it relates to perhaps electronic ignition options.

I watched your included webinar link titled, "How To Destroy Your Engine in One Minute" up to the Q&A session. Thank you for posting it. This is essentially a great in depth look at the two issues and how we need to be prepared to analyze the onset. I like his graphic illustrations so far in how he visually shows the process of moving from a normal ignition event in the cylinder along the spectrum of detonation / pre-ignition. Test cell graphs are shown in a general way early on, from George Braly's work (GAMI). I wondered if he would spent a little more time on the concept of EGT actually decreasing with rising CHT showing the onset of pre-ignition. But in his presentation, he did not. I think he considered pre-ignition as something so far down the line of "damage done" (predicted by CHT going north of 400) that by the time EGT was lowering, the action items no longer included engine analysis and it was time to "aviate" and land, then get scoped before anything else resulted.

As they say knowledge is power and I'm a visual learner. Instructional pieces like this are a good resource to keep that picture in my head gaining light and pixels along the way to a more complete understanding of what the real picture is if/when I have to deal with this in the air.

For anyone looking at this video the timer shows well beyond the presentation, the presentation itself is about 45 minutes, the rest is Q&A. The takeaways are at 39 minutes. Some of these takeaways are illuminating for me. His concept of CHT 380 degrees = do something now and 400 degrees means consider it an emergency - was a few degrees cooler than I had visualized. Yes, I keep my CHT's below 380 but on a 95 degree day I will see 385 at times before I get the engine in fast climbing air. Personal temp limits well below manufacturing limits are wise however, perhaps his numbers are more catered to a Continental engine in a Cirrus. I have to admit I did not expect his recommendation if a CHT ever reaches 400 degrees. But I'm considering lowering my yellow and red annunciations in my engine display after watching this video. I got a lot from the presentation, thanks for posting.

Fly safe to all!
 
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I wondered if he would spent a little more time on the concept of EGT actually decreasing with rising CHT showing the onset of pre-ignition. But in his presentation, he did not. I think he considered pre-ignition as something so far down the line of "damage done" (predicted by CHT going north of 400) that by the time EGT was lowering, the action items no longer included engine analysis and it was time to "aviate" and land, then get scoped before anything else resulted.

WHen it comes to timing, CHT and EGT are inversly related. As timing advances, CHTs rise and EGTs fall. Won't go into the reasons here, but suffice to say that increasing advance will cause CHT's to rise and EGT to fall simultaneously. This does not infer or identify pre-ignition or detonation. While pre-ignition may result in decreasing EGTs with rapidly increasing CHTs, it would be incorrect to state that decreasing EGTs in the presence of increasing CHTs is a universal sign of pre-ignition. Advanced timing rarely leads to pre-ignition. Only in cases where the detonation from this advance is both severe and prolonged, will it result in pre-igntion.

Also, a CHT north of 400 is NOT a unique sign of pre-ignition. Yes, CHTs will skyrocket above 400 when pre-ignition is present, but there are also many other things that will drive CHT's north of 400 in a normal engine with no pre-ignition. Engine companies routinely run their engines at max CHT (500 in most cases) for several hundred hours to prove their reliability within the specified limits.

WHile I agree that cooler temps will increase the castings longevity, I do not believe that a CHT of 400 constitutes an emergency with immediate landing. We are each afforded our own opinion on this.

Larry
 
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CHT's north of 400 do not constitute an emergency. I think we need to be careful here as there are lots of new builders and inexperienced pilots who may find themselves taking unnecessary action and maybe even creating an emergency. On first flights it is normal to see the temps in the mid 430's and then come down to 400 or less within an hour.

Lycoming cylinders are actually meant to run a little hotter than Continentals. When Lycoming had their LASAR system certified it had a CHT probe to monitor the CHT's, and as you climbed it kept the timing advanced to maintain 435 degrees. There was nothing we could do about it. And it worked just fine. My RV-6 with that system went almost 1800 hours until it had to be torn down due to a prop strike (not by me). It was clean as a whistle inside and didn't burn any oil.

When I was at the Lycoming factory I was told that we are running our cylinders too cold and it is not allowing the .005 choke at the top to work correctly.

Keep in mind that for certification they run the engines for 150 hours with all temps at maximum.

Yes, this ranks right up there with the primer wars, but just want to jump in so someone doesn't really think anything over 400 is an emergency.

Vic
 
Also, a CHT north of 400 is NOT a unique sign of pre-ignition. Yes, CHTs will skyrocket above 400 when pre-ignition is present, but there are also many other things that will drive CHT's north of 400 in a normal engine with no pre-ignition. Engine companies routinely run their engines at max CHT (500 in most cases) for several hundred hours to prove their reliability within the specified limits.

WHile I agree that cooler temps will increase the castings longevity, I do not believe that a CHT of 400 constitutes an emergency with immediate landing. We are each afforded our own opinion on this.

Larry

Excellent, thanks for your insights Larry! This jives with my previous understanding. Glad to have the additional perspective.

CHT's north of 400 do not constitute an emergency. I think we need to be careful here as there are lots of new builders and inexperienced pilots who may find themselves taking unnecessary action and maybe even creating an emergency. On first flights it is normal to see the temps in the mid 430's and then come down to 400 or less within an hour.

.....

Yes, this ranks right up there with the primer wars, but just want to jump in so someone doesn't really think anything over 400 is an emergency.

Vic

A particularly useful set of comments Vic, thank you!
 
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Timing

20 degrees is only for the angle valve 360's. Parallel valve 360 is. 25 degrees. I believe every effort should. Be made to stay below 400 degrees CHT until rings are seated. After that 425 in climb should be ok. 20-22 degrees timing until rings seat will help on any of the 0 320's or 0 360's with tight cow longs.
 
CHT's north of 400 do not constitute an emergency.

Agree.

We should note Busch merely calls it abusive. He is in the opinion business.

When I was at the Lycoming factory I was told that we are running our cylinders too cold and it is not allowing the .005 choke at the top to work correctly.

Trust but verify.

CHT alone tells nothing about required choke dimension to achieve a straight cylinder in operation. Choke is based on temperature difference between the majority of the cylinder and that portion of the cylinder at the head end. Both ends of the cylinder expand. The head end, being warmer, expands more.

Given a choke of 0.005", and an expansion coefficient for a carbon steel cylinder of 0.0000065 in/in per degree F, it would take roughly 150F difference to make the head end the same diameter as the base end.

As a practical matter, conditions which raise CHT would also tend to raise lower barrel temperature. 350 CHT and 200 lower barrel is pretty much the same as 450 CHT and 300 lower barrel. The difference is still 150F.

Other details might vary the desired deltaT. It's still going to be based on delta, not just CHT.
 
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So if you had a close personal friend who found 1 mag on a IO540 timed at 28 instead of 25 will said close personal friend see a noticeable reduction in climb CHT?s?
G
 
In your title, "Solve high EGT with timing" - I want to think you meant, "Solve high CHT with timing". He never mentions timing in this video. But I get what you are wondering about, in at least as it relates to perhaps electronic ignition options. Fly safe to all!
Dang thanks you are right. Thanks

The YELLOW line to him is 400F.... That has been my policy for decades based on Lycoming Flyer Articles. It is for long life not an absolute MAX. If you listen to his videos, he does know his stuff and backs it with DATA and Examples and Experience. No defense of Mike just saying.

http://www.alfako.be/SAFETY DOCS/lycoming---flyer_key_reprints.pdf

Page 51 (PDF page 52)

Although these are minimum and maximum
limits, the pilot should operate his or her engine at more
reasonable temperatures in order to achieve the expected
overhaul life of the powerplant. 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.

It is for long life not an absolute MAX. If you listen to his videos, he does know his stuff and backs it with DATA and Examples and Experience. No defense of Mike just saying.

My comment "He goes on to say for Lycoming you might push that to 400°F but anything above that is abusive" is being mis-understood. It is NOT an emergency. It is BEING NICE to your engine and not abusive. 500F RED LINE is abusive...

Mike also says you can not full predict or prevent pre-ignition damage. He shows CHT spike as a result of pre-ignition. However it can happen so fast you may not be able to react. The bottom line is don't ignore one jug with high CHT before takeoff. It might be nothing or it could be pre-ignition. One thing he said that was a disappointment is after the fact you can't determine the cause of pre-ignition (broke plug insulator, glowing carbon). The destruction is so bad the evidence is lost.
 
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Sorry if this is thread drift and feel free to delete if you feel so.

Regarding timing, we often hear its effect on CHT but how much does it effect oil temp. I believe the effect is much more noticeable in the angle valve engine where there are oil jets are installed.
 
Yes, this ranks right up there with the primer wars, but just want to jump in so someone doesn't really think anything over 400 is an emergency.

Vic

This is not primer wars... This is based on Lycoming and historical data. No one is saying 400F is an emergency. Vic that is a strawman argument. Really. 400F max "operational" has been a recommendation of Lycoming for many decades for long life. Of course you can fly at 430F. Lycoming says CHT 350-435F is normal. You would agree that lower CHT is better right?

Two points that got wildly lost:
Reducing your Timing (parallel valve) from 25 to say 23 can reduce CHT.
If you can keep CHT 400F or less, you will improve longevity of engine.

I don't think there is any debate about either of these, is there?
 
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Ignition timing should be set to make maximum power. Cooling should be designed to remove heat from the engine. Hobbling the engine with retarded timing to reduce power so that it will cool properly is a band aid. Period.

The Lycoming PV head has some well discussed issues but it will cool effectively if the baffling is done correctly. I'm breaking in a PV 540 right now and despite running the thing at best power mixture, high RPM, high MP, and high OAT, my cruise CHT is between 350 and 380.

Keep in mind I'm running EI (about 25 degrees at this power), the rings have NOT seated yet, and I'm doing this with less than 35 total inches of inlet area.

Lycomings WILL cool if fed properly. No ignition band aids needed.

That said, the fixed timing of a magneto IS too advanced for TO and high power ops down low. But if you pull timing to fix that, then high altitude performance suffers. That's the dilemma of fixed timing.
 
This is not primer wars... This is based on Lycoming and historical data. No one is saying 400F is an emergency. Vic that is a strawman argument. Really. 400F max "operational" has been a recommendation of Lycoming for many decades for long life. Of course you can fly at 430F. Lycoming says CHT 350-435F is normal. You would agree that lower CHT is better right?

Actually, in post number 2 it was stated that Mike's opinion was that 400 degrees was an emergency and something should be done right now.

I saw that and was trying to caution everyone. My mantra is to check your watch first before taking any emergency action. Of course there are exceptions (detonation, engine failure on takeoff, etc.), but those are memory items, right? With almost 10000 hours behind lycomings and continentals I've seen detonation and poorly timed engines (45 degrees advanced by mistake by the builder) cause an immediate need for action.

Personally, I think every engine and installation is different. My last Mattituck IO-540 brand new ran 300-330 CHT's in flight, 360 in the climbs, with oil temps never above 190 on a max hot day take off. It still needed cylinders at less than 1000 hours.

I'm not trying to argue whether temps increase or decrease longevity. I was trying to adress the reference to an emergency above 400 degrees.

Most of our fatal accidents in the AB world occur during the takeoff phase where inappropriate action can mean the difference between life and death. Just trying to help here. :)

Vic
 
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There are a lot of people who think "high" CHT's are anything over 400, yet the engine designer doesn't consider 400°F out of line.

See this tread for Lycoming's CHT recommendations.

BTW, I believe Mike was talking about the angle valve engines that Lycoming recommended reducing the timing from 25° to 20°.
 
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Not much point in arguing specific numbers, because we all do the same thing anyway...we get concerned when it starts doing something out of the ordinary.
 
There are a lot of people who think "high" CHT's are anything over 400, yet the engine designer doesn't consider 400?F out of line.

That's true - and I'm not going to argue that point - except to point out that the guys saying it's fine to run the cylinder hotter are also the same guys waiting to sell you a new cylinder... :cool:
 
That's true - and I'm not going to argue that point - except to point out that the guys saying it's fine to run the cylinder hotter are also the same guys waiting to sell you a new cylinder... :cool:

Only, I had one forum member contact me because they saw a CHT over 400? and thought he should have his cylinders removed and inspected/overhauled.

400? in climb is normal. The issue is continuous operation
 
CHTs (and cold starts....)

Yes, this ranks right up there with the primer wars, but just want to jump in so someone doesn't really think anything over 400 is an emergency.

Vic

I have far fewer credentials and vastly less expertise than Vic -- in fact, I didn't even sleep in a Holiday Inn Express last night -- but I agree.

I think Mike Busch has a ton of useful and interesting things to say (e.g. about maintenance-induced failures, compression tests, etc.). But until somebody demonstrates with actual data that engines run with CHTs above 380 do worse (in terms of longevity or whatever) than engines kept at or below 380, it all seems like speculation.

In the summer, especially after a short stop somewhere, I can see CHTs of 410 or so on the climbout. It doesn't bother me in the slightest. Just like it doesn't bother..... Lycoming.

[avoiding the primer wars, but pushing the control rods a bit deeper into the internet aviation reactor:]

Scofflaw that I am, I've also been on the road and had to start my engine when it was below 32F with (gasp) no preheat. I wasn't happy about it, but I note that I was not struck by lightning and the engine did not shatter on takeoff. Anyway, I am also extremely dubious about Busch's claim that "a single cold start without proper preheating can produce more wear on your piston aircraft engine than 500 hours of normal cruise operation." Again, I've seen precisely zero data to support that statement, and I think it can't possibly be true for the full range of temperatures that most people consider "cold." I mean, maybe you'd get 500 hours worth of wear and/or some sort of catastrophic failure cold starting at -10F (although, and I could be wrong, I don't think Busch has presented any data establishing that). But 500 hours of wear with one cold start at +25F? I see no reason to believe that.

Now, about primers...

[diving for cover]
 
I would expect that significant cummulative time above 400F is more of a concern. But if you briefly exceed 400F during initial takeoff climb but spend 99% of the time cruising at 330F CHT, it's difficult to believe an occasional brief excursion to 410F for a minute or two during takeoff is going to have a significant effect on TBO. A greater concern would be the number of thermal cycles during the life of the engine, e.g. a lot of touch and goes.

I occasionally hit 405F on a hot day after takeoff, but bring it down as quickly as possible by switching to cruise climb at the earliest opportunity.
 
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