Dual EGTs in a Single Engine?

write-stuff

write-stuff

En-Route
Joined
Jul 25, 2006
Messages
4,604
Location
Atlanta
Display Name
Display name:
write-stuff
I'm putting a new 0-470-R into my 182 and need to replace the EGT. The stock EGT just checks the exhaust manifold on the left side.

There are EGT gauges designed for twins that have two meters. Any reason I cannot expect proper function if I use one of these and put one of the probes on one exhaust and the other one on the other exhaust?

Now that I've asked the question. I have tried it but I'm getting hugely different temps on the two sides. This make me wonder if there is some fundamental flaw in using a gauge designed for a twin on a single engine.
 
Yes, I got the probes part. I'm not looking to monitor each cylinder. The question is can I use a gauge like this one (designed to monitor twins) on a single. I ask because I'm seeing MUCH hotter temps on the right bank than on the left.

upload_2019-8-25_7-19-32.png
 
write-stuff said:
I'm putting a new 0-470-R into my 182 and need to replace the EGT. The stock EGT just checks the exhaust manifold on the left side.

There are EGT gauges designed for twins that have two meters. Any reason I cannot expect proper function if I use one of these and put one of the probes on one exhaust and the other one on the other exhaust?

Now that I've asked the question. I have tried it but I'm getting hugely different temps on the two sides. This make me wonder if there is some fundamental flaw in using a gauge designed for a twin on a single engine.
Click to expand...

I wouldn’t think so. Try switching them. Not the probes, the connection at the gauge. First do just one. Left probe to left needle then try it to the right needle. Note any difference. Repeat from the Right probe. That will tell you if there is a problem with the needles. Then hook both up, left to left and right to right. Then left to right and right to left. That should give you a starting picture of what’s happening. You could then switch the probes from side to side on the engine and repeat the whole process. You should then be able to tell if one side of the engine is running hotter than the other or the gauge itself is unbalanced.
 
Last edited:
Yes, I understand about the switcher/gadget. I'm just trying to debug my existing situation with the 2-bank EGT gauge. Either
(1) the gauge is inaccurate,
(2) the probe(s) is/are inaccurate,
(3) the gauge is not meant to be used on singles,
(4) or I have some horrendous problem with exhaust temps in the engine, right bank.

Note on (4). In the standard (single-bank) EGT config, I would never have even known about the right bank temps.
 
write-stuff said:
Yes, I understand about the switcher/gadget. I'm just trying to debug my existing situation with the 2-bank EGT gauge. Either
(1) the gauge is inaccurate,
(2) the probe(s) is/are inaccurate,
(3) the gauge is not meant to be used on singles,
(4) or I have some horrendous problem with exhaust temps in the engine, right bank.

Note on (4). In the standard (single-bank) EGT config, I would never have even known about the right bank temps.
Click to expand...

What I described in post #4 should answer those questions except 3. I can’t think of anything other than length of the wire from probe to gauge. The gauge can’t tell what kind of plane it’s in. Length of wire from probe to gauge might have an effect on accuracy, it’s going to be longer on a twin. But that isn’t going to account for difference from side to side. If you have a huge discrepancy and the engine seems to be running just fine, I think that one of the gauges is off. It is two gauges you have, they just put them both in one unit.

EDIT: Have you checked the plugs? Are they all pretty much looking the same color?
 
Last edited:
Most of the variation in EGT is simply probe placement. Absolute EGT is pretty meaningless. Even a slight difference in location in the exhaust stack can make a pretty substantial absolute value difference. EGT tuning is always done with respet to the peak value. On a non-injected engine, it's going to be a fairly rough number anyhow.
 
flyingron said:
Most of the variation in EGT is simply probe placement. Absolute EGT is pretty meaningless. Even a slight difference in location in the exhaust stack can make a pretty substantial absolute value difference. EGT tuning is always done with respet to the peak value. On a non-injected engine, it's going to be a fairly rough number anyhow.
Click to expand...

Yeah. Do you think the airflow around the exhaust would make much difference?
 
Doubtful. I don't think much cooling is going to propagate into the gases inside of the exhaust stack with the usual placement.
You want to judge cooling, put CHT probes in. It's required if you have cowl flaps usually. There it can be very handy to see what the individual cylinders are seeing.
 
The main thing that is worrying me is that it's pegging the temperatures on the gauge. During a normal mag check (with normal results), it's pegging the right side within a second or two. I don't know if the gauge is simply wrong or if I'm cooking the engine with pegged EG temperatures. CHT, oil temp, and oil pressure are all normal.
 
I get the feeling you have the probes very far down the exhaust, but I have no solid reason to think that other than my interpretation of what you're saying. The further down the exhaust you put the probe, the more chance the gasses could be flowing unequally and not hitting the probe consistently. My probes are only a few inches from the cylinder.
 
I cannot see a reason why it would not work. Are the probes in the same location in distance from the cylinder? You did not cut and splice the wires correct? Have you tried swapping the probes to see if you have a bad one?
 
Thanks, guys. Yes, they are far down the pipe, right where the three pipes converge. They are the same distance, each side, but temperatures differ a lot between sides. Not something I would expect.

We're going to get back at it this evening swapping leads and probes, and even testing with another gauge. Monumentally slow since we have to cowl/uncowl the thing each time. Don't want to run it uncowled for too long.
 
I hooked up a digital EGT to the two probes and got what seemed to be "normal" readings (1200-1500F).

Looking back at the two-bank Alcor gauge, I noticed the specs say:
Resistance: Scale 1600F at 7.44 Ohm

The original single-bank Alcor gauge in the airplane specs out:
Resistance: Scale 1600F at 3.63 Ohm

I find this odd since both use the same K-type probes. But could the impedence differences between the two gauges account for the high temps shown by the two-bank gauge?

?
 
Thanks, Doug. That would certainly explain the pegged temps on the new gauge.
 
The problem is that old style gauges aren't "smart" and rely on the resistance of the cable as part of their calibration, this is why you can't shorten(or lengthen) the cable without recalibrating the instrument, and why I have like 5 feet of bundled up thermocouple wire under my cowling for my factory EGT.

Newer digital meters just measure the voltage with a high impedance amp, so they don't care about resistance variations.
 
Problem answered. The gauges meant for twins have longer cables and are calibrated for a higher impedance. Lesson learned.
 
Hi, Doug. I could probably recalibrate the gauge but at this point I'm just flat worn out with it and have settled for the traditional Cessna single-bank set up. I will say I have learned a lot in the process and that's worth something.
 
I'm sure you considered this, but really get an engine monitor. You'll be able to take much better care of that new Conti.
 
Hi, George. Yep, I was just considering a cheap upgrade for more accurate EGT temps. I agree that an engine monitor would be the better option.
 
You must log in or register to reply here.
Back
Top