Max EGT

[jatkins]

I cannot seem to find an absolute number for a max EGT. I realize that the max , will vary day to day , with altitude and temperature , as well with power settings. The reason I am looking for this value ,is I am installing a 6 cyl EGT guage , and I need to program it to the max value for the engine.I found the max CHT 525, but I cannot sem to locate the Max EGT ? Any one out there have a number / source ??

[jrenwick]

There isn't any max EGT value; EGTs are not absolute values in the first place, because the temperature varies over the length of the exhaust path, so what you see depends a lot on where you inserted the probe. Assuming the probes are all at the same distance from the flange, then what's important is the temperatures of all the exhausts relative to each other. With fuel injection, they should be pretty close if the jets are balanced well. With a carburetor, expect to see variations of 100 degrees or more between cylinders. It's just in the nature of the beast, and you can't do much about it.

The electronic EGT instruments have high and low alarms. If you have the EI instrument, the manual has a recommendation for setting the maximum EGT limits, and I think it's some number of degrees above the highest reading you can get for each cylinder (I don't have the manual handy, or I'd look it up). I don't know what JPI recommends.

Best Regards,

John

[jatkins]

I have installed the probes very close to the exact same location from the flange in all the exhaust stub pipes. I guess the value which I will
( should ) never exceed, is what I am looking for??

I do follow that what the gauge shows will depend on the temp, at the exact location of the probe, and the length of each stub pipe varies slightly.I am thinking that after flying it some I could reset the value up or down as required ?

[jrenwick]

What you set the max/min EGT values to is a matter of experimentation with your particular aircraft and installation. They may be different for each cylinder. The manufacturer's installation manual or user's guide should have instructions on how to do it.

The idea should be to set them such that the alarm will be activated whenever one of them goes outside of the "normal" range by some factor, and what's "normal" is really unique to each cylinder on your particular engine. There is no general guideline.

Best Regards,

John

[blueldr]

You're going to have to determine the highest or peak EGT for each unit and then decide if you're going to operate either rich or lean of that peak.
Most naturally aspirated engines cannot be over leaned at or above about six thousand ft.MSL. If you're flying below that altitude, be cautious leaning when at or over about 70% of max power.
On an O-300 engine the fuel mixture distribution is really too erratic to gain much from accurate instrumentation.

[jrenwick]

What BL said! You can use the EGT to lean the engine, but I long ago gave up doing that. I lean for peak RPM, sort of like what the owner's manual says to do, and then just monitor the EGT for something unusual going on. If you get a bad plug, the EGT (with a mag check) can tell you pretty quickly which one to look at. It can help you debug other problems as well, but as has been pointed out here in the past, for leaning, it's overkill for the O300. I like it mostly because I have additional sensors on it for oil temperature and oil pressure, and I can set limits that will light an idiot light. Because as I once heard Dick Rutan say, the gauge you're not looking at isn't working!

[Haydon]

Howdy All,

After reading the info on this thread, I think I have a pretty good understanding of what has been stated. My 170 has a single probe EGT and no CHT. I have an 0300A with a D accessory case. I am not shy about leaning...even on the ground...and for run up and take off....field elevation is 2000'...On the ground and for take off I lean by the tach, in the air/cruise I have been leaning by the EGT. In cruise flight I lean to about 1325 degrees indicated on the EGT and then enrich the mixture to about 1250 degrees....I burn about 8 gallons an hour over all and cruise at 2450 rpm. My oil temps are good, well below red line and in the lower portion of the green arc.....Plug fouling is a thing of the past... I use avgas only.... Are these temps that I see displayed, are they in the "ball park"????? Should I just disregard the EGT and do all leaning by the tach? Any comment or suggestion would be appreciated...

Thanx,
Richard...

[jrenwick]

1250-1325 is lower than what I see on my EGT, but remember that the only valid comparison is between cylinders on the same engine. The temps you see depend very much on how far away the probe is from the flange, and there's no standard for that.

Another way to say it is, the readings are all relative to where you see the peak EGT value. One temperature reading, by itself, doesn't really mean anything.

At higher power settings, the danger of leaning to too high an EGT is that the exhaust valves don't get enough cooling, and they can burn.

If you're running at 75% power or less, you can lean to the absolute peak EGT without hurting the engine. And with our O300s, that's almost always the case. The Cessna recommended method of leaning until roughness and then enriching until smoothness returns is basically leaning to peak EGT on the first cylinder that peaks. All the others are running rich of peak. Your single EGT gauge may or may not be on the leanest cylinder, so unless you know it is, maybe leaning the recommended way will give you better performance. The problem is that with our carburetors, the fuel is distributed rather unevenly, and they'll all generally be running at different mixtures.

[blueldr]

Haydon,

The standard tach in this airplane is usually very difficult to read fine enough for leaning purposes. In my experience, the best method is as the factory recommends. Lean to roughness and then enrichen just to smoothness. If you are cruising at 2450 RPM and somewhere arounf six thousand feet MSL, your fuel burn is a little high by my experience. On a relatively long leg, I would expect the burn to run between 6.8 and 7.3 GPH.
Between my home base in California and Idaho with a cruise climb from 1600 MSL to 9500 MSL and a 7000 ft descent over about twenty minutes (About 350 ft/min.) I will usually burn about 6.7 GPH for the 3.9 hr. leg.
I'm running a 50 inch pitch and cruise at about 2550to 2600 RPM. My tach runs slow so I set the power with my electronic tach checker.

[Haydon]

Thank You John and "Blu-elder" I have a lot of respect for my"elders and my betters." From now on I will lean by engine sound. Guess I will have to "duck tape" over the EGT gauge.... I would be inclined to remove it....but, the hole in the exhaust would probably whistle...warning George that I was about to pass him.......

Richard......

[GAHorn]

Ha!
Well,.. the instructions I have regarding the "factory" techniques for leaning do not mention "listening" or leaning for "roughness".
The instructions I have call for leaning very slowly to max RPM, then continuing to lean until the first sign of loss of RPM, then enrichening back to the point of max RPM. (My own technique is to push the mixture very slightly further rich from that point.)
There is a term called "Best Power" that is generally accepted as somewhat rich of peak EGT. While that may not give the absolute greatest range, it will give the best speed/shortest flight time all other things being equal. It is a bit richer (according to the texts I've read, anywhere from 50-100 degrees richer than peak EGT) than I personally enjoy. But it is where I prefer to be as opposed to the absolute leanest possible (within the capabilities of still having a smooth engine.)
I don't subscribe (or perhaps I just don't fully understand) to all the logic of Mr. Deakin...(aviation author) ... but he makes a good argument that an engine can be in a range of increasing RPM while leaning... yet have one or more cylinders that have become lean beyond peak. I can see the validity of that. (If 6 cylinders have 2 very rich, 2 almost at peak, and 2 right at peak EGT... then further leaning will take the 2 at peak beyond peak and down the lean-of-peak side of operations. Yet the 2 almost at peak cylinders have improved their horsepower output as have the 2 very rich cylinders. In other words, of the 6 available cyls, 4 have increased power output as they approached closer to peak EGT, while only 2 have begun the downhill slide on the other side of peak EGT. Therefore the engine may actually exhibit a slight increase in RPM despite the fact that two cylinders are lean of peak (LOP), 2 cyls are at peak, and 2 are rich of peak (ROP).
Also, it's been postulated that an engine running at peak, or even slightly rich of peak, can suffer damage as conditions change and one or more cylinders have cause to lean still further. (A change of density altitude, for example.) In the above example of the 6 cylinders.... any further leaning OR richening, regardless of cause....might place 2 cylinders at risk of excessive EGT. If the power setting of the engine is above 65%, it's possible to damage those cylinders that become leaner...because they are running hotter. Capt. Deakin makes a persuasive case for running lean of peak because further leaning will only result in a cooler EGT. Of course that can only be accomplished in properly instrumented, fuel injected engines. (And it ignores the possibility that an engine might have an equal chance of an inflight change that makes it run richer....and therefore hotter.)
The problem is, with our engines, we're back to measuring things with a micrometer when we use digital EGT gauges. We then mark it with a grease pencil when we are running a variable-length induction engine like this one. (Or when we're "listening" for roughness.) And we're then cutting it with a hatchet with this carburetor and mixture control. (Does it sound as if I have very low regard for digital gauges in these fairly crude engine installations we have?)
I am quite happy with using the tachometer, and my fuel burns average 7.8 gph and speeds run right at 104 kts/120 mph like the factory claims.
What is an EGT gauge going to tell me that I don't already know?

[jrenwick]

Thank You John and "Blu-elder" I have a lot of respect for my"elders and my betters." From now on I will lean by engine sound. Guess I will have to "duck tape" over the EGT gauge.... I would be inclined to remove it....but, the hole in the exhaust would probably whistle...warning George that I was about to pass him.......

Richard......

Oh, I certainly wouldn't get rid of it. As I said before, it's very nice to have a picture of what's going on with the engine, and I really like the alarms, especially on oil pressure and temperature if you can add those sensors. A six-way EGT is also really nice for debugging problems, like finding a bad spark plug.

Best Regards,

John

[GAHorn]

Yes, you are right, John. My question was a rhetorical one ("What is an EGT gauge going to tell me that I don't already know.") that is not applicable to a multi-cylinder, multi-function gauge.
You are certainly correct, that a good quality, multi-cylinder, multi-function gauge can be very useful in trouble-shooting and in early detection of changes that are occuring in an engine's cylinders.
And my opinion of digital gauges is only that...a personal opinion heavily influenced by personal preference. (I see gee-whiz stuff all day, every day at work and the simple-life is a return to serenity/relaxing pleasure to my personal flying.)
Being an originality nut is also to be taken into consideration by anyone reading my posts. I love original airplanes. (Except, of course, my digital radios, tail-pull handles, tow-adaptors, hatch-latches, 172 door locks, wheels and brakes, strobes.......)

[cessna170bdriver]

... The problem is, with our engines, we're back to measuring things with a micrometer when we use digital EGT gauges. We then mark it with a grease pencil when we are running a variable-length induction engine like this one. (Or when we're "listening" for roughness.) And we're then cutting it with a hatchet with this carburetor and mixture control. (Does it sound as if I have very low regard for digital gauges in these fairly crude engine installations we have?)...

...(I see gee-whiz stuff all day, every day at work and the simple-life is a return to serenity/relaxing pleasure to my personal flying.)
Being an originality nut is also to be taken into consideration by anyone reading my posts. I love original airplanes. (Except, of course, my digital radios, tail-pull handles, tow-adaptors, hatch-latches, 172 door locks, wheels and brakes, strobes.......)

George is correct in that our engines probably don't need super-accurate digital instrumentation, but then again we probably don't really NEED a gyro panel or GPS, or the airplane itself for that matter; they’re all just toys to make us happy. To each his own. Amen brother on the flying as a means of escape! (In more ways than one ) But we all have different thresholds on what "floats our boat" (or airplane, as it were). If we were all the same it would be pretty boring wouldn't it?

I put an Electronics International UBG-16 6-cylinder EGT/CHT in my airplane as part of the engine overhaul project last year. (I’m a bit of a gadget geek, so sue me! ) I can’t say that I’m operating any better or more economically, but I do enjoy the insight I get into the operation of the engine. In about 70 hours of operation, I have gotten a pretty good idea of what is “normal” for each cylinder for whatever the situation and conditions happen to be. If someday I have an induction or exhaust leak, or a plug start to foul, hopefully I’ll have some insight on where to start looking for the problem.

Probably the best upgrade, if you can call it that, I did during the overhaul project was to replace the tachometer (my preference is still mechanical analog for this instrument, go figure ). My old tach was so shaky that it was difficult to discern a peak, and I usually just resorted to leaning until I got an audible drop in RPM or slight roughness, then richen till that indication went away. For rough leaning, the new tach responds much more quickly than the EGT. With the new rock steady tach, I can fairly quickly lean to a definite RPM peak, at which point I can switch to the EGT for further leaning, but I have to take it slower due to the response time of the thermocouples. As I recall, my peak EGT of around 1450-1460 occurs on #5 somewhere around 50-75 RPM lean of peak RPM. I can lean a bit past this point, but not much, as there are several other cylinders peaking at this time (although at lower indicated temperatures), and I can feel some roughness begin to set in and the RPM begins to drop more rapidly. From peak EGT, I usually just richen about 25-30 degrees (just because) and I’m where I want to be.

George and I have discussed this here before, but IMHO, the hatchet analogy is right up there with the "fire hydrant valve on a water fountain" analogy, i.e. it’s just an exaggeration to make a point. With my vernier mixture control and the digital instrumentation, I can set EGT for any one cylinder within 5 degrees of where I want it, and once set, it rarely varies by more than a degree or two. That doesn’t necessarily accomplish anything especially useful, as there are other cylinders that are 150 degrees different, and there’s nothing that can be done for one that doesn’t affect all in one way or another; I just think it’s interesting. So there.

Miles

[johneeb]

What Miles said plus. My multi pickup (JPI) engine monitor has paid for itself for sure 3 times, one of which is pitured below.