In a very interesting article (
https://gami.com/articles/egt_myths.pdf), Mike Busch points out among other things that the absolute values of EGTs are not really meaningful, that "differences between EGT values are both normal and benign," and that in carbureted engines it's normal to see spreads of 150 degrees or more between different cylinders' EGTs.
I've got a carbureted O-320. It's got a #4 EGT that routinely goes 120 degrees or so above the rest on on takeoff (hitting the 1510 degree mark, or thereabouts). There doesn't seem to be any appreciable effect on CHT. Here's a typical data run:
https://apps.savvyaviation.com/flights/3859992/e2a4b957-b49c-46b8-b4e5-2221a200e218
I read Busch to be saying that I have no reason to be concerned about this. And I certainly wouldn't have been concerned at all in my old Warrior, since my only engine gauges were RPM, oil temp, and oil pressure. Blissful ignorance vs. analysis paralysis.
However, any EGT above 1500 sets off the flashing red light on my EIS 4000, which a previous owner set to alarm at anything over that level. Before I turn that alarm off: is an EGT alarm actually capable of alerting me to anything useful? If so, what should the alarm limit(s) be?
As MEL said EGT is not an absolute but a reference. More important is what is the RELATIVE EGT to each JUG. Also important is what is NORMAL for your plane. If you have one get cold or super hot relevant to the others, it means something.
I love your "analysis paralysis". The other current thread no doubt you saw. He is plotting it out and scrutinizing the delta of #1 Cyl at low power (at very low MAP). It could be a fuel injector issue at low power but not an important one. All is good at higher power. He has checked for leaks gaped plugs... Engine is running smooth. We tend not to LEAN in the pattern even if at low power for the GO AROUND. However there is nothing wrong with leaning in the pattern, the engine would be happy. Personally I don't enrich until late in the pattern. Once on the ground, taxi speed/clear of runway, clean up and lean the heck out it. I don't look at the engine monitor. No need to even look at EGT. It does not mater if the engine is almost at idle.
This is a pet subject. I grew up with NO EGT and if one of the planes I rented had ONE analog EGT.... it was WOW.... Later in life I flew hours off a plane going through a STC Engine mod. It was fully instrumented before it was wide spread in GA, but digital engine monitors were starting to be available and were very expensive. No glass mind you but just an engine monitor alone. I think the experimental EIS less than half the cost of these fancy certified Engine monitors 2 decades ago (and still today).
So what can you do with it?
- Watch it.
- OP, FP, CHT are more absolutes and most important
- Make mental notes what is normal.
- Look for DELTA (difference)what is normal.
- Use it to to LEAN... **
** Carb LOP Ops - Are more difficult but can be done. Running LOP with Carb is hit and miss (Pun intended).
To get smooth LOP you need to get all jugs near same mixture. If one Jug leans out much more than the other three that might be still ROP or Peak, it will start to misfire well before the other Jugs can get to LOP. With FI the EGT does help operators trouble shoot issues with injectors and happy juice delivery system.
OLD blind days: with ZERO EGT to look at, you lean to roughness and shoved the mixture in a bit 1/X inches.... per the AFM/POH. Pretty crude compared to scanning atomic microscope we have attached to our engines but NOT IGNORANT... It worked. However never lean above 75% power. If you are at low power 65% or 55% lean as much as you can until engine stumbles (at least one Jug is LOP) and enrich to smooth. Even if some Jugs are LOP, Peak, slight ROP you will do no harm. We now know the old 50F-100F rich or peak was not ideal. There are many great explanations on mixture techniques to be found.
LOP: With Carb you might not get PURE LOP but a hybrid LOP with one or two Jugs being at Peak or just ROP. However SMOOTH is the key. [
What I just wrote might be controversial so I have my asbestos flame suit on. I can defend it however please operate your Lycoming powered sky scooter at your own risk and fell free to do what ever you want, up to you.]
Trouble Shooting: If the engine loses power and start shaking and one EGT gets very low.... well you know what Cyl is the problem. Can you do anything about it? It could be a fouled plug. If it gets way hotter than other Cyl . It could be an air leak or pre-ignition.... (different but related and precursor to detonation).
Dreaded detonation. It can happen so fast you will not see it on EGT. So engine monitor will not likely save your bacon. If you have some precognition due to ignition system failure or carbon deposit, you might get some clue. If you like Savvy aviation he has a great webinar on this. BTW a Carb O-3230 (assume 160HP) is a fairly high compression engine and you should always run 100LL. As long as you leave it RICH above 75% power you are good.
BTW LOOK AT YOUR PLUGS... are they toasty brown? If they are bare white you are lean at full rich. This is common with RV's with high flow air box and headers.... no muffler. You need a larger main jet. On my old RV-4 I reamed the jet ever so slightly (in tiny steps of a few thousands at a time) to dial in the jet. This is old hat to OG hot rod guys be it car or motorcycle.... I have a box of jets for my scooter (a real scooter not airplane). Mechanical FI goes back to around 1930. However GA planes and cars did not start going FI until 1960's technology. However cars still mostly came with Carbs (as did GA planes) into the 1980's. Nothing really changed until now with electronic FI. However
I LOVE ME SOME CARBS (the fuel delivery system and bread /pasta).
