Bravo CHTs

[Awful_Charlie]

Folks

Just trying to get an opinion on your Bravo CHT spread, and wondering if I can improve mine at all.  I'm EDM800 equipped, and as to be expected, #3 is always the coolest due to sharing the thermocouple well (why doesn't someone make a single screw in with two probes inside it?), but #1 is always my hottest - by about 30degF or so. (standar order from hot to cold goes 1, 5, 4, 6, 2, 3)

Reading the Conti thread and about the pixie holes made me wonder if I should consider something similar, but thought I'd ask around to see if my spread is normal in a Bravo, or if it is something else that needs attention.

FWIW, I don't normally have a CHT problem other than the spread, after establishing the climb at about 120KIAS and 20-21GPH, I can close the cowl flaps and maintain below 400, unless it is very warm outside.

Ben

[Cruiser]

You need to know if the CHT spread is from external cooling or internal heating.

Check you "GAMI test" spread for balanced fuel flow. The closer together (in fuel flow) your cylinders peak (EGT) the better. When you set the mixture for your normal operation, some cylinders will be closer to 50°F rich of peak EGT than others and these will tend to run hotter then the others etc.

External cooling (baffles) draw off the generated heat and flow across/around the engine will cause variations.

You need to know what to change.

[Skywarrior]

Of course, check your baffling. I had not checked mine since I bought the plane, and my BFR instructor showed me that it had gotten bent out of position. Temps, overall, are a bit better now, but I don't recall what the spread looks like.

[Awful_Charlie]

Thanks chaps.  The baffling is good, with new rubber on the front section to the lower cowl, and the original (but in good condition) to the top cowl.  I've checked for gaps and leaks by leaving a light underneath and buttoning it up.

My GAMI spread is far from ideal, but #1 is one of the early cylinders to peak, most frequently #5 is the last to rise and fall, but it depends on the MP and altitude.  I've got about 70 data points so far, and when I get to 100 I'll try to do some correlation and speak with GAMI.  The spread is on avereage about 1GPH with a minium of 0.3 (34"/2400 at FL170) and a maximum of 2GPH (30"/2200 at 4000')

[johnggreen]

Awful,

My cht's, egt's, and peak cht's are simply "weird".  I put in GAMI's right after purchasing in 2005 and from that point, the engine runs great but the numbers don't make sense.

Don't recall the numbers but after changing out a couple of injectors, my cylinders all peak at "about" the same temp.  GAMI says the numbers are good.

From there it gets weird.  At cruise power, there is a 150 degree spread in cht from #3 to next lower and 200 degrees to the coldest which I think is #6.  Never varies.

My EDM shows all CHT's to be around 340 to 360 with #3 the highest, but my OEM cht which is also on number three shows 40 degrees higher.  Sitting in the hangar at OAT, the EDM and OEM guage agree.

Yea, I've had it checked out by several mechanics including a MSC (that I would have the least faith in) and everybody shrugs their shoulders.

I had a valve guide problem on #3 and replaced that cylinder last year and nothing changed.

Go figure.

If there is some genius out there who can analyze why the EDM and OEM guage don't agree at operating temps and why I would have such a spread from #3 CHT to everything else and still have the cylinders peak at almost the same temp, I'm all ears.

It's Sunday so there may be some good replies, but I've got to go load out some hay that's going to Texas so I won't be back on line until tonight.  Just so you won't think I'm not paying attention.

jgreen

 

 

[Awful_Charlie]

Thanks John - seems this is not going to be an easy one to fix!

My EGTs are all fairly consistent, with #5 a bit hotter, which is to be expected, as this one is at peak or just LOP, whereas the others are well LOP by the time #5 has dropped.  Like you, on the ground at ambient temps, everything agrees nicely within a couple of degrees.  Sometimes I wonder if we'd be better off without all this fancy instrumentation, as it seems to add to the worries as much as it suppresses them

My CHT averages for this year for flights over an hour:

[carqwik]

My CHT's run with about a 70 degree spread between hottest and coldest.  Hottest is always #6 and coldest is #3 or 4 (don't remember exactly which one it is).  Go figure.  Key is just keeping the hottest one below 400 degrees which requires either more fuel and/or trailing cowl flaps in the summer (southwest desert).