What CHT requires cowl flaps in cruise?

[201er]

Hello, I'm a new owner of a '78 201. One question that I can't seem to resolve is what the actual acceptable range for CHT is. Although red line is at 475F, I've read plenty of people panicking when CHTs cross 400F or even 380F. So to get a better idea of what CHT is too high, I would like to know at what CHT (on hottest cylinder) in cruise would you begin opening cowl flaps?

While #1 and #2 cylinders seem to stay around 330, #3 is typically higher toward 360 and #4 is consistently higher at 370-400F. I'm using an EDM-700. Any suggestions? Is it typical to see higher CHTs on the back cylinders?

What's better? 75% power with cowl flaps partially open or 65% power (which results in less heat) and cowl flaps closed? Or is it ok to let the CHT get to around 400 so that Cowl flaps can be kept closed at 75%? Thanks for your help with all my questions.

[Cruiser]

There are some engineering types that will be able to give you the technical reasons for the limits on CHT but from several sources it seems anything up to 380f is acceptable for cruise. IF you are above 380f immediate action should be taken to reduce the CHT. I think up to 400f for short periods is tolerated such as in a climb, but for extended time (cruise) it is 380f.

Find out which cylinder has the OEM CHT probe attached, that one will most likely read 30-60f different from the others. but the remaining three should be close. Your spread seems to be high. Have you checked the baffles and any blockage on #4?

As for cowl flaps opened or closed, try trailing them rather than completely opening. You should get some increased cooling that way. You have listed a few combinations. Speed and fuel would be my guiding factors.

[Jeff_S]

Also, it depends on whether you're running ROP or LOP. I won't debate the merits of either here...take your pick and fly how you want. But ROP will show higher CHTs especially in these hot summer months.

In climb, it's not uncommon for #2 and #4 to show hotter simply because there's less airflow back there, but once in cruise that shouldn't really be a factor. So the fact that #4 stays hotter does indicate something's different...maybe baffling? Or an injector issue?

[Becca]

Its funny, I just went to a flight safety seminar taught by a guy who's worked on engine monitors for 30 years for ALCOR and JPI.  We definitely talked this topic a lot.  He said that over 380 starts to expect the lifespace of the cylinder and over 400 should definitely be avoided.  Interestingly he said when you start to get up towards the redline at past at about 500 degrees its hot enough to melt your engine instrumentation.  Ideally you want to run between 300-330, because that provides the best temperature for having a clean fit between your cylinder, piston, and valves.  We think ours is running a little hot at 360, but our baffling is in pretty bad shape, so we're going to put jetdriven to work fixing it up

Before opening your cowl flaps, I would definitely consider your leaning.  Are you ROP or LOP?  Your CHTs peak about 50 degrees before your EGT, so 50 degrees ROP is probably the hottest place for your CHT.  I would richen your mixture more if you are running ROP, or maybe lean a little more if you are running LOP and see how the temps do.  I guess I would adjust my mixture before putting the cowl flaps in trail to control CHT.  But my hard limit is 380, if I see it running above that I take some sort of action.  I ran right at 370-380 the whole time I did the air race, WOT, full rich, even the slightest lean would bring it above that, I needed all the fuel to cool the engine, which was sad because it was 18 gph.  When I go to LOP, I can get in the 8-9 gph range.

Also, he pointed us to this very useful flyer he helped write, see page 11, on troubleshooting engine problems based on what you are seeing on the engine monitor: www.alcorinc.com/PDF/EGT_Nutshell_06_14_2010.pdf

Another interesting tidbit I picked up from the seminar was that he doesn't believe shock cooling alters the life of the engine.  They can pull hundreds (thousands) of hours of data off these engine monitors and ones that have been shocked cooled seem to have no shorter lifespan than ones that haven't.  He points out things like skydiving planes, which climb hot, then dump all their power and dive 10,000 feet towards the ground as fast as they can shock cool multiples times a day, and they seem to be getting the same life out of their cylinders as more traditional engine users.  Just a data point.

If you PM me I'll give you his email address, he might be willing to share the slides he presented at the air safety seminar with you electronically.

[jetdriven]

I'm in the same camp.,  you should never have to open your cowl flaps in cruise.  The problem is either improper leaning (too rich, or too near peak at to high of a power setting)  or your baffling is leaking.   We are starting with baffling this weekend.

[The-sky-captain]

My #2 runs a little hotter than the other three in cruise. At 100 ROP it runs around 385-390 at 8k with ground temps of 90 plus degrees. The engine is a couple months old and only has 60 or so hours on it since OH. The shop owner said that is just due to everything being tighter than before with the added friction. The others are in the 340-370 range. We'll see....

[GTWreck]

I recently found that one of my CHT probes connected to the engine monitor was not a cylinder well probe, but was a probe under the spark plug.  The cylinder well probe for that cylinder was connected to the CHT guage above my 6pack.  Thusly, one CHT reads about 50 higher than the rest on the engine monitor. 

I have plans to replace my engine monitor soon, so I'll swap them back so they are consistant.

[danb35]

GTWreck, unless you replace your engine monitor with one that's STC'd to replace the engine primary gauges, you're legally required to keep the factory CHT gauge operational.  EI sells a probe that will piggyback on the factory CHT probe (rather than piggyback on a spark plug), but in my installation, that one reads ~100 deg F below the opposite-side cylinder.  I don't know if it's something unique to my installation, or maybe if something was done wrong when installing it (though it looks like it matches the instructions), but I'm having trouble believing that reading.

[jetdriven]

I have the factory well probe on #3 and this replaced my crappy spark plug washer CHT thermocouple with this http://www.overnitesupply.com/wtk38-2424typekwasherstylethermocouplegrounded38studsize24stainlesssteelbraidedleadssplitwithspadelugs.aspx

 

 I simply installed it under the factory CHT well probe.  It now reads within 5-10 degrees of all the bayonet probes.   It is just a standard "Type K" thermocouple.  a 10$ solution.

[jetdriven]

if your injectors are dirty the mixture distribution wont be even.  Run a GAMI test, and clean them if necesary.  Perhaps your #3 and #4 are at peak when you are trying to run ROP.

[201er]

I doubt that since I'm leaning 25F ROP based on the leanest cylinder. Seems to me that 25F ROP is actually a little cooler than 50F ROP. LOP isn't an option for me right now. 100F ROP doesn't seem any cooler either really.

Also, is it just me or does it seem that regardless of what power setting you use, CHTs are almost the same. It's like 75% runs it pretty hot but also provides a lot of cooling from airspeed. 55% produces less heat but the reduction in speed also provides less cooling. It seems like only fuel consumption rather than heat/engine life are considerations for how much power to use for cruise. Agree or disagree?

[KSMooniac]

25 or 50 ROP are terrible, terrible places to be if you're running at 75% power.  You won't be able to get the CHTs much hotter than that, short of detonation.  100 ROP will be much cooler, as would peak or 10 LOP or 20 LOP.  You're also not considering the peak pressure differences inside the cylinders due to mixture setting, and those are every bit as important as fuel consumption and temps.

If you haven't already, you should learn all about engine management... I cannot recommend highly enough the Advanced Pilot Seminar (www.advancedpilot.com) if you want the most effective and efficient way to learn.  Otherwise, you can search a lot of past threads here and on the AOPA Forum, and read all of the Pelican's Perch columns by John Deakin on www.avweb.com

[jetdriven]

Agree with KSMooniac.   

25 ROP is right square in the middle of the "red box" taught by Deakin and the folks at APS.  Thats the last place you want to be.  Its just about at hot from 50 ROP to 10 LOP the curve is pretty flat there.  Figure out why you can't run LOP and fix it. Its costing you 1-2 GPH.

Either run 10-50 LOP, or 75 ROP at 75% power.  At 65% power you can run at peak.  If you can't, check bafflling and injectors.  CHT at 65% power will be lower that at 75% power.  The airspeed isnt all that much less, maybe 10 knots.  but heat is less. Ours at 50% power are in the high 200s to 320.   65%, 350.  75% is 360-370.  We can gain 20 degrees from new baffle seals, they go in this weekend.

[danb35]

201er, how you operate your engine is up to you, but how do you determine that "LOP is not an option"?  Most Lyc IO-360s run just fine LOP without GAMIs or anything else special.

[201er]

The EGT/CHT temp spread between cylinders is pretty high as it is. Even if doesn't run rough without GAMIs, some cylinders will end up too rich or too lean that way. What's the issue with running at peak anyway? Shouldn't we expect the CHTs to be lower at peak than 50/25 ROP anyway? I'm sure the exhaust manifold can handle those kind of temps just as well, right? Or is the issue ICP? How do we go about measuring that anyway?

[Shadrach]

201er,

Your looking at this wrong.  Your EGT temp spread is meaningless in cruise. EGT number valuse are meaningless in cruise. Find the richest cylinder (last to peak) and see if you can run leaner than peak on that one. How far you can go LOP will determine your LOP options. My IO360 (67M20F) with stock injectors will go to nearly 100LOP before it stumbles. Ram air will give me an additional -15-20df EGT on the lean side. I can and do run WOTRAOLOP right down on the deck and keep all chts below 350 easily. MY EGT number values very by as much as 160df...but that's not relevant.  Also, My CHTs tighten up nicely on the lean side. Short hop at 3500ft (cruise DA around 4400) on Tue CHTs were :

30LOP 151kts  

#1-295df

#2-331df

#3-327df

#4-322df

Whom ever told you that the 5-10 knot speed speed differential running LOP has a measurable effect on cooling at cruise speeds was full of it...  There are O360s hung on FG aircraft that cruise 50kts slower than your AC at the same power level and they manage to do so with the cowl flaps closed without overheating... 

[Shadrach]

For some reason I cannot quote you.

WRT ICPs, the only indication we have of ICP in flight is CHT... 

40-45df ROP is the hottest place to run an engine there is ZERO utility in running an engine in that range. It does not produce the most power, just the most heat.  Any setting Richer or Leaner than the 40-45ROP range will produce cooler CHTs. How much cooler depends on how far rich or lean of that point you are.  

 

 

[201er]

Ok, I'll play around with it next flight and see how she handles LOP and what effect it takes on CHT. How far LOP should you be on the richest cyinder and what determines this?

Still, I don't think anyone answered my question of what the real CHT redline should be, what normal operating CHT should be, etc. Cause 475F according to the POH just seems ridiculous. There's no yellow range either... they mark it green all the way to 475F as though it's ok to be anywhere in that. Is it? If you could make a green, yellow, and red area for CHT for an M20J, what numbers would you set those at?

[Shadrach]

 

I try to never exceed 400 and I would have to be pretty ham fisted to get there... at crossing 380 I'll start to take remedial action by enrichening or leaning depending on the circumstances... See Pratt & Whitney graph below...

 

[KSMooniac]

380 is the best number to use as a "soft" red line if you want your cylinders to last a long time.  If your IO-360 won't run LOP smoothly, then you most likely have an induction leak, a clogged fuel injector, or a weak ignition system on one or more cylinders.  Almost all of these engines will run smoothly LOP on stock injectors.

[danb35]

Quote's still broken...  To address some of your other questions/concerns, if the engine runs smoothly LOP (say 20-30 LOP), there isn't any significant mixture imbalance.  If there were, particularly if some cylinders were still ROP, those cylinders would be producing quite a bit more power than the cylinders that were leaner, and you'd feel that as engine roughness.

"How far LOP" depends on your power setting; at 75% LOP I'd probably want to be at least 50 LOP, at 65%, 20 LOP or so.  When LOP, power depends entirely on your fuel flow--take your fuel flow in gph, multiply it by 15, and that's your power output.  So, a fuel flow of 10 gph corresponds to 75%.

If I were marking the guage, I'd put a yellow arc from 380-420, and redline at 420.

[Magnum]

> What's the issue with running at peak anyway? Shouldn't we expect the CHTs to be lower at peak than 50/25 ROP anyway? I'm sure the exhaust manifold can handle those kind of temps just as well, right?

I ran my engine on peak for some time and my exhaust burned away (flame tubes in the muffler). Some others on the forum fly peak, too, and don't have a problem with the exhaust. So I can't recommend nor disadvice it. Lycoming recommends peak EGT for best economy.

The CHT's are lower than 50/25 ROP, and you have power than going LOP.

[201er]

I tried LOP today just to see how it goes and surprisingly I was getting 20F cooler CHT on hottest cylinder, cowl flaps closed, and still running smooth. The spread isn't perfect but reasonable without gamis! Didn't get rough till about 60 LOP but I wasn't too exact about it.

So in this case, can you tell me what the target LOP temp is? Does it vary by %HP? Do you just go for the leanest LOP that you can get before the engine turns rough?

[Shadrach]

Maybe you were surprised it was 20df cooler, but I'm not.  Your assigned reading is below... It's best you understand what's happening when you turn the mixture knob so that you can make an educated decision about how to run your engine depending on your mission... 

Pelican's Perch #63: Where Should I Run My Engine? (Part 1)

Pelican's Perch #64: Where Should I Run My Engine? (Part 2 — The Climb)

Pelican's Perch #65: Where Should I Run My Engine? (Part 3 -- Cruise)

Pelican's Perch #66: Where Should I Run My Engine? (Part 4 -- Descent)

[201er]

Already read all that. What I'm not liking is that pretty much the entire source of info about LOP operations comes down to the same group of people (GAMI and APS). If it's so great, then I don't understand why the manufacturers aren't recommending it or at least other pilot groups that reach the same conclusion independently. Also, most of the info about it seems to always point to continental.

Anyone know of any good info for LOP on Lycoming in Mooney? Some power setting tables for LOP operations, graphs, etc? Cause I've seen nothing but continental based stuff. Yet it seems that the Lycoming has a better shot at running LOP with stock injectors.

Anyone here gone TBO to TBO flying nothing but LOP?

If I'm having these problems with CHT from 25-75 ROP, how did folks fly Mooneys for the last thiry years without engine analyzers or LOP?