The International Cessna 170 Association

I want to install a single probe EGT on my 170B with the C145.
My question is has anyone done it?
What cyl did you install it on and how does it work?

Dick
1954 C170B
N2704C

I had a single cht on my O-300 on the back right cylinder. I always assumed that was the hottest and thats why it was there. I have heard other opinions on the hottest cylinder, but am not knowledgeable enough to confirm.

The difficulty in single-probe systems is the question: What do you wish to measure.. the LEANEST point of the LEANEST cylinder..?? or the richest point of the Richest cylinder?

The O-300 has a pretty crude induction carbureted system and which cylinder is the leanest or richest can only be reliably determined with a 6-probe system.

If it were me… I’d select cyl no 2 for the EGT probe because that cyl is the one the OEM thought to be the hottest, is already instrumented with a CHT, and therefore at least ONE cylinder would be completely instrumented.

For what it's worth, Cessna published AK172-211E to install an EGT on the Lycoming-powered C-172. As they couldn't decide which cylinder riser to mount the probe to, they installed it downstream the muffler.

I don’t have an answer for you but have noticed some single-EGT installation instructions recommend installing the probe at the cylinder that is leanest during climb. My personal experience with a 6-cylinder monitor in my 170B (with C145) is that the #2 is the leanest cylinder (by far) at full power and is also the hottest cylinder. During cruise EGT evens out pretty close to the rest of the cylinders, but it is still the leanest (and still the hottest). Cylinder #1 is one of the coolest. Go figure.

I’d also be leery of installing it below the muffler. While the O300 is unlikely to be capable of running too lean above 65% power (which is the commonly-held criteria to avoid burned valves)…. I can imagine a situation where the leanest cylinder will be run too lean while the other two on that same muffler could mask that situation.

My thinking on that is in-line with the age-old concept of running a few degrees rich of lean on the leanest cylinder (which is where “best-power” occurs.) The carbureted induction system of this engine is simply too crude to be able to run lean-of-peak as one popular writer suggests. (That recommendation might work on a well-instrumented fuel-injected engine…but not a carbureted one, IMO.)

(I operated a very well instrumented Navajo Chieftain (turbo-charged fuel injected TIO-540-350 Lycomings) for about 800 hours and tried the Lean-of-Peak operations and gave up when it resulted in poor engine maintenance problems and borderline high CHTs. I returned to 25-degrees rich of peak and had no further problems and good performance. The actual fuel consumption was insignificant on those flights which were most-common (ATL-CLL) and (CLL-ORD)

BAck to our Continentals: It’s a personal choice and certainly reasonable one to install an EGT system, but I believe I’ve flown several thousand hours on these Continental with fixed-pitch props and the simple method of setting cruise power and slowly leaning to Peak RPM… continuing to lean to the First Indication of RPM-loss….then enriching back to Peak RPM…. is about as good and accurate as it gets …without any EGT system at all. However, even if a good EGT system is present…using the EGT system to lean the engine ends up being the exact same results.

A single cyl EGT system also is of little help with regard to enroute engine diagnosis. If an engine develops a malfunctioning cylinder in-flight a six-cyl system would likely pinpoint the defective cylinder…but it’d still be a question-mark if it actually was the only cyl with a probe installed in it because a change of that cyl indication would continue to be a puzzle and tell you nothing more than you already knew…. a rough engine. That’s the way I think of it anyway.

Hope this helps.

My experience with 819 hours with a 6-cyl CHT/EGT system is also that #2 is the hottest on both CHT and EGT, but not by a whole lot. I’ve used the system to lean a few times but I always end in the same place as I do when lean for a RPM drop then richen to get it back. I usually end up around 1450 EGT (probes 2” below exhaust ports) and 350 CHT(bayonet probes). Surprisingly, I can get a smooth decrease in RPM after leaning past peak but I don’t run it that way.

My recommendation is that you don’t waste your money. I installed an nice EI single cylinder combination EGT/CHT probe and gauge a number of years ago thinking I would use it for lean of peak operation, engine diagnostics, etc. Because of the aforementioned sloppy mixture distribution system, it’s just not very useful. You can lean it just as accurately by ear or the tach. YMMV.

I have one installed and never really use it. It's not really worth it unless you're gonna put probes in all 6.

My single EGT is not very useful. It validated my instructor’s leaning technique. Lean to max RPM and push the knob in about a half a knuckle and you’ll be about 100 degrees rich of peak. The engine is so simple and reliable, I’m afraid of messing that up by being able to micromanage every CHT and EGT.