A few questions

[The-sky-captain]

I've been messing around with running LOP and like the results as far as GPH (8.5 at 7-8k) and CHT temps ( -30 degrees) go. My question is regarding my EGT's. While running 100 degrees ROP the highest and lowest are on average only 10 degrees apart. LOP they are 80-90 degrees apart and overall about 75-100 degrees hotter with the hottest being in the mid to high 1400's. Is this consistent with running LOP vs ROP? Engine runs smooth, although it did feel a little weird pull back so much the first few times.

[carusoam]

We're you guys really up at 1am? Or possibly a time bug?

-a-

[DrBill]

Carusoam, were you up at 221 AM ? This posted at 1036PM EDT

BILL

[The-sky-captain]

Hey Jim,

I haven't paid much attention to the speed loss but I'd say it's around 5-7 KTAS. With a descent tailwind on a recent flight I was up in the low 20's nmpg but am averaging about what you are. I'm with you on my EGT temps I was just curious if other pilots are seeing the same info on their engine monitors. I make a lot of 150 NM trips to Little Rock for business and running LOP is shaving about $40 per round trip. I'll take that any day of the week over the extra few KTS.

Carusoam- it's a time bug...

[The-sky-captain]

Carusoam, I edited my time zone in my profile and it looks like the correct times are posting now... Enough of that, anybody else have any input on the OP??

[danb35]

If you were running 100 ROP and change to running 20 LOP, your EGTs will be roughly 80 degrees higher than they were before. It's obvious when you think about it, but it sometimes takes a little while to "click" in your mind. I don't have an explanation for the increased spread in EGTs either, but as long as they peak close together (in terms of fuel flow) and the engine runs smoothly, it doesn't really matter. I honestly couldn't tell you if mine do the same thing, as that's simply not something I pay attention to in flight.

[jetdriven]

Do the GAMI spred test. Absolute EGT values on a naturally-aspirated engine are pretty meaningless. The number of degrees from peak is what you are looking for.

[fantom]

Absolute EGT values on a naturally-aspirated engine are pretty meaningless.

I have a tough time buying that. Excess heat is excess heat, not a good thing, either inside or outside the engine block. Mufflers are expensive to rebuild, and can cause havoc in the air.

No reason not to enjoy LOP, but maybe something to watch as we go way LOP to "save" on fuel.

[garytex]

I think 80df diff is something worth puzzling over. Not necessarily a big deal, but sometimes little "does not follow" indicators can be your engine trying to talk to you. Maybe it is saying, "my EGT probes are not equidistant from the valve, and now that the burns are shorter, it makes a difference in the readings" Maybe it is saying "I have a slight intake leak at the #1 tube flange" or, I have high impedance on a spark plug or I have a partially blocked injector" or any number of things. It is in my experience always worth trying to understand when an engine speaks. Frequently it is a "Oh thats what it was", after the fact understanding, and sometimes it's nothing, but 80df dif. would cause me to cock my ears.

Is the dif between 3 cyls all pretty much the same and one out in left field? That would be more pressing, and easier to diagnose.

There is a point around 1600df where the stainless steel in our exhausts looses around 90% of it's tensile strength. In the situation where poorly distributed large fuel globule charges from badly jetted and venturied carburetors do not completely burn in the cylinder, EGT'S can get up in the mid to high 1500s as the fuel burns in the muffler. I owned one such, fouled up by the Marvel Schnabler multi piece venturi AD, and was welding cracks every annual. I did the carb at the same time as an engine overhaul, and didn't recognize the effect of the new venturi and jet for a couple years. A new multi hole jet and a properly configured venturi brought EGTs down 100 df and gave a 50 -75 bump in static rpm. High absolute temp in EGT's can be another communication, which may or may not have information content that is important. Part of the fun of airplanes is the "inquiring minds want to know" part. You don't want to think about work or whatever all the time anyhow, do you?

Gary

[jetdriven]

The GAMI folks concluded that EGTs on a naturally-aspirated engine cannot exceed a safe threshold for 321 stainless exhaust. Also, if it was something to consider, why is there no EGT limit in the POH?

TIT limits of 1650F must be observed for turbocharged aircraft or things can start wearing out fast. IIRC some models of the M20M were certified for a 1750 TIT limit and that would cause the exhaust to wear out in 1000 hours. 1650 is safe. We peak at 1510 at sea level. Higher altitude is less.

[fantom]

I know, I know, but I'm still skeptical. The GAMI folks are sellers and the old POH leaves out more than it includes. High probability it's fine, but......

[aviatoreb]

The GAMI folks concluded that EGTs on a naturally-aspirated engine cannot exceed a safe threshold for 321 stainless exhaust. Also, if it was something to consider, why is there no EGT limit in the POH?

TIT limits of 1650F must be observed for turbocharged aircraft or things can start wearing out fast. IIRC some models of the M20M were certified for a 1750 TIT limit and that would cause the exhaust to wear out in 1000 hours. 1650 is safe. We peak at 1510 at sea level. Higher altitude is less.

Hi Byron, Where can I read this conclusion by Gami folks? What is the safe zone for 321? What is the magic EGT that turbo operators should be careful to avoid?

[KSMooniac]

Here is a snip from the MMPDS publication for the 321 steel tensile strength temperature reduction factor.

EDIT: I can't attach the pic.... dangit. Upload failed, exceeded disk quota, etc.

The ultimate tensile strength reduction factor at 1500 is 0.26 (meaning almost 75% of strength is lost) and at 1400 it is 0.36.

[bd32322]

I have also observed the same thing as the OP, higher spread because of cyl 2 having a much lower EGT than its brothers. None of the EGTs are past mid 1400 for the other cylinders.

My guess is cylinder 2 is producing much less power than its brethren because of an overly lean mixture maybe? I'll pay attention to how many degrees less than peak it is next time i fly. Normally i lean with the big pull method and dont pay attention to how many degrees the cylinders are running below Peak, as long as the engine runs smooth and i get my target fuel flow of 8.4 gph

Ps - my spread was also 80-90 F

[jetdriven]

Ok, so a 321 exhaust has 25% of its tensile strength at 1500 degrees. I submit it is strong enough at that temp to still do the job. The EGT may be 1500 degrees at 2" from the cylinder head but the pipe is not that hot, and certainly the muffler is not that hot. The combustion temp flame front is 3200f and the aluminum piston melts at 1220f. Its the boundary layer that protects it. Same with the exhaust. Pilots started operating LOP about ten years ago, and more every year. Yet no cascading avalanche of exhaust failures.

You see much higher exhaust temperatures in turbocharged aircraft and no rash there either.

Another point is economics. Say you limit your EGT to 1400, which is 11 GPH at low altitude in a M20J. Say the LOP pilot is running 9 GPH. (our block average over 250 hours) Ok, so the LOP pilot has to totally replace his exhaust every thousand hours for 3 grand. He saved 18 grand on fuel and spends 3 grand on a new Powerflow exhaust. Every thousand hours. The old adage that fuel is cheaper than parts no longer applies, folks.

[marks]

Bodie, I have GAMIjectors on my J model and my GAMI spread is about .3 gallons and I have to say that I do almost exactly what you do, often flying at 8.2 gph and I experience the same temperature spread and higher EGT temps that you do. I've only been flying LOP with GAMI for the past six months or so and I've also been a little nervous since I started, but I'd watch the CHT drop slowly and the engine ran almost as smooth as when I ran it ROP, so I've begun to figure that everything's OK.

[wishboneash]

I have a data point for my J model - At 8.6 gph, the hottest was Cyl#4 EGT at 1495, the coolest was Cyl#2 at 1430. Diff=65 deg. The CHTs were between 325 and 350 (the coolest and hottest cyl same as the EGTs). Cyl 1 & 3 were close to each other for both the CHT and EGT. I have stock injectors and the FF spread is about 0.3 gph. Running a bit richer (9.4gph) brings the EGTs closer together and the EGT difference drops to 13 deg.