Confusion about low CHTs

[MV Aviation]

Hello,

I recently installed an EDM-830 in my E65. Before that I had the single factory CHT (cyl. no. 3), which almost never reached the green arc from below. Only on hot days and during climb the needle struggled into the green. I deemed the analog gauge to be fairly inaccurate.

Now that I have digital readings of all four cylinders, I thought the picture might change. However, the EDM confirms the reading of cyl. no. 3 and the other cylinders show similar values. In cruise, on a normal day with cowl flaps closed I get in the area of 270-280°F. Isn't that way too low? When I read articles about LOP-flying, people are concerned to not get above 380-400°F?!? Three days ago I took off from Athens, Greece at about 27°C and during climb-out CHTs barely climbed above 320°F.

What am I mistaking here? By the way, the ac does have no cowl enclosure mod or anything.

Thanks Marco

[carusoam]

Hey Marco,

What's your OAT lately?  °F (preferred) 

In Cruise 280° - 320°F are what I remember seeing flying LOP, winter - summer,  with the IO550, no cowl flaps...10k', 13gph, 

This would be a good sign of good air cooling.

With that fancy new JPI, have you checked your gami spread yet?  This will give a good indication of how well your fuel injectors are working, distribution wise...

a close gami spread, and even airflow through the cowl, will lead to close CHTs...  other things that are nice to have.

CHTs below 380°F are good...

there is a lower limit for CHT, based on the lead content of the 100LL... but I don't remember that number off the top of my head...

Best regards,

-a-

Edited May 27, 2017 by carusoam

[MV Aviation]

In my case it doesn't seem to be very dependent on altitude and temps. No matter if I'm cruising at 5k or 10k, temps are usually between 270°F and 280°F, persistently. I haven't measured the GAMI-spread yet, since the FF transducer is not yet installed. However, if I use the "lean find mode" of the EDM, EGT deltas from peak are typically 20-30°F apart. Meaning the first cylinder to peak shows e.g. 45°F LOP while the last cylinder to peak shows 15-20°F LOP, if that makes sense.

Thanks.

[carusoam]

The FF transducer makes things easy to understand.

Best regards,

-a-

[jaylw314]

Does the JPI-830 use a piggyback sensor or a spark plug gasket sensor?  The spark plug gasket sensor can be 60 degF high or low depending the plane.  The piggyback sensor is supposed to be more accurate, but I don't know how much more accurate.

If you want to confirm the accuracy of your CHT probes, run your engine to working temps, then immediately after shutdown use an infrared thermometer to check each cylinder temp and compare to the 830's temps.

[Guest]

If you power up the EDM without the engine running the EGT and CHT values should all be very close, within a degree or two.

In flight there is nothing wrong with the values you are seeing, Lycoming says below 150 degrees F is bad.  If your baffle sealing is  in good condition your temps will be cooler because your cooling is efficient.

Clarence

[Bob_Belville]

"My Aviation", those CHT levels are quite low for an E, particularly with the original cowl/baffle. I'm would not think they are a problem but they are a little puzzling. Is there any possibility there's a conversion from Celsius to Fahrenheit error?  Your 280F CHTs are equal to ~138C.

I note you do not have Fuel Flow readings yet, that will be helpful. At altitudes above 7 or 8000 you should be able to lean to under 9 gph. (That's USGallons which are 3.785 liters) Have you checked your average fuel per hour? Running really rich will cool the cylinders though it would seem not as low as you're seeing.

Your "lean find" description does make sense and seems reasonable but you won't know much until you can get your gami spread values with the FF transducer. FWIW our IO360A1A engines in the E model usually have pretty tight spreads, well under 0.5 gph, and GAMI (not to be confused) Injectors are not needed to run LOP. Have you continued to slowly lean past that 25LOP on the richest cyl until the engine runs rough?  

Do you have pics of your cowl, baffles? I suspect some of us would like to copy them!

 

[MV Aviation]

22 minutes ago, Bob_Belville said:

Do you have pics of your cowl, baffles? I suspect some of us would like to copy them!

 

Interesting that you mention this. Now if I think about it, my engine is completely encapsulated in a metal box and not "only" sealed with baffles. See attached Foto (it's an old one, before the EDM installation). It didn't occur to me that others might have another type of "under the hood" airflow management. However, this might explain the temperatures. By the way, the EDM is set to °F, so no conversion error here.

5 hours ago, jaylw314 said:

Does the JPI-830 use a piggyback sensor or a spark plug gasket sensor?  The spark plug gasket sensor can be 60 degF high or low depending the plane.  The piggyback sensor is supposed to be more accurate, but I don't know how much more accurate.

If you want to confirm the accuracy of your CHT probes, run your engine to working temps, then immediately after shutdown use an infrared thermometer to check each cylinder temp and compare to the 830's temps.

I do have 3x the screw-in bayonet type probes and 1x the gasket ring probe, NOT for the spark plug, but for the factory CHT. All four CHTs read roughly the same temps at all time within +/-5°F, I'd say.

The FF transducer will be installed after my current holiday trip. We didn't wanna mess with the fuel system shortly before departure. I'll measure the gami spread after installation of course. Before the EDM installation I flew ROP only. Now, for the last 10h I flew mostly LOP in cruise. For now I'm using the lean find mode and lean until the richest cylinder shows about 20°F LOP. The other cylinders then typically show up to 45°F LOP, as I mentioned before. With this setting I lose about about 8KIAS, safe 20%FF and am not "too far away" from engine roughness. I suspect that swopping around the injectors after a qualified gami-analysis might enhance the performance a bit.

Thanks for your input, guys!

Best, Marco

[Tommy]

My J does the same. And my CHT probe is installed at the top plug. I had it at the bottom plug before but it consistently reads 40 higher and gives me the warning on take off when factory says it's around 360. Once I switched it reads 40 below factory! 

Tom

[jlunseth]

What's wrong with cool termperatures?

Somewhere in the front of your POH there may be a table listing some operating and redline temps.  There is in mine.  The normal operating range for CHT's is 250-460 dF.  The 460 we know would be unhealthy.  There are metallurgical changes that take place above 380.  I have asked the same question about low CHT's several times on this forum, and have also done a little of my own technical research and talked to my piston engine engineer friends.  I get immediate answers on hot temps, but just head scratching on low temps, no one can think of a reason why it would be bad for the cylinder to run at, say, 240, or 220.  There are some reasons unrelated to the cylinders themselves.  Oil temp must stay above the minimum specified in the POH, in mine it is 100.  There is an extra reason for it in my 231 engine, oil flow through the turbo depends on warm oil.  CHT is indirectly related to OT because, obviously, OT goes up or down if the engine is run hotter or cooler.  But there does not seem to be an issue with cool CHT's.  Look for the NOR numbers in your POH and if you are within that, don't fret.

[takair]

My E has the same doghouse configuration and very similar temperatures.  While I live in a cooler climate now, we lived in Georgia, USA for years, and had cool temperatures there as well.  On a rare occasion, on a hot day, and in a steep climb, one CHT may break 330F.  I can't think of a time where I got all CHTs above 300 no matter where I lean.  I suspect you have 20 degree mag timing which is a likely contributor. I suspect those timed at 25 degrees see more temps in the 300s.

[MV Aviation]

1 hour ago, jlunseth said:

What's wrong with cool termperatures?

Somewhere in the front of your POH there may be a table listing some operating and redline temps.  There is in mine.  The normal operating range for CHT's is 250-460 dF.  The 460 we know would be unhealthy.  There are metallurgical changes that take place above 380.  I have asked the same question about low CHT's several times on this forum, and have also done a little of my own technical research and talked to my piston engine engineer friends.  I get immediate answers on hot temps, but just head scratching on low temps, no one can think of a reason why it would be bad for the cylinder to run at, say, 240, or 220.  There are some reasons unrelated to the cylinders themselves.  Oil temp must stay above the minimum specified in the POH, in mine it is 100.  There is an extra reason for it in my 231 engine, oil flow through the turbo depends on warm oil.  CHT is indirectly related to OT because, obviously, OT goes up or down if the engine is run hotter or cooler.  But there does not seem to be an issue with cool CHT's.  Look for the NOR numbers in your POH and if you are within that, don't fret.

I'm not concerned about cool CHTs in general. I'm concerned that the indicators may show considerably lower temperatures than actual, which might make me believe that the cylinders are running cool, although they aren't. As I described, I didn't trust the factory analog CHT from the beginning with its cool reading. But now that the EDM confirms the temps., they cannot all be false. And I was curious where this comes from.

[Shadrach]

Given those power settings, I'd say you're with in 15-20 of what I see in my F. It's a challenge to keep 1 and 4 above 300 when LOP.