CHT/EGT selector switch?

[Birdseyeview]

I've got the Rotax 912 supplied CHT senders already on the engine and haven't purchased any EGT probes yet. I've also got a Westach quad gauge which only monitors one CHT and one EGT at a time. Any recommendations on which cylinder I should monitor, or should I get a selector switch and monitor them all? Spruce has these switches and I could get one for all the CHT senders and another one for all the EGT senders. Also, what location in an exhaust pipe is best on the 912 for mounting an EGT probe?

[avidflyer]

I have one of those 4 position switches from ACS that you mention. You can actually use the same switch for CHT and EGT. As the switch comes, it has wires on only one side, but there are connections on the opposite side of the switch that you can connect to. That is what I did with my Avid Flyer with the 4 cylinder Jabiru engine in it. In # 1 position, I'm reading #1 cylinder EGT and CHT, in position #2, #2 cylinder EGT and CHT, ect, ect. I usually just leave it on #3 cylinder as that one runs the hottest. Once in a while I will switch through and look at what the others are doing. JImChuk


PS I just went to Westach's site, and looked at that switch there. Looks like you can get the switch wired for the other 4 termanials from them (model 254-4FD) or just wire the cheaper model 254-4F yourself westach.com/products/ACCESSORIES/SWITCHES/SWITCHES%20FOR%20THERMOCOUPLES/index.php

[Birdseyeview]

Thanks for the info avid flyer. Just curious, do you have your selector switch mounted on the instrument panel near your associated temp gauge or somewhere else a little more out of the way since you don't use it often?

[avidflyer]

If I can post it, here is a pic of my panel. I use an MGL-E1 engine monitor for CHT and EGT and the 4 way switch is right below it. After much back and forth, I think I figured out how to reduce the size of the photos. The panel is not finished of course in this picture, but it does show the switch and such. JImChuk

[Birdseyeview]

I'm assuming that your MGL engine monitor only shows one cylinder at a time, and that's where the need for the rotary switch arrises. Now that you've had the ability to monitor all the cylinders for a while would you do it differently if you were doing it again? For instance and for simplicities sake, would you only monitor the temps on one cylinder and ignore the rest and eliminate the switch and a few temp probes? What's your take on the reliability of the switch? I saw someones comments in the Spruce catalog review that he had to replace his switch every 3 years. In your situation and set up do you worry more about CHT or EGT? I know this isn't exactly a fair comparison to my Rotax 912 but I'm trying to get at the underlying logic for what is important to monitor and what my options to display it are, while staying within the confines of my existing quad gauge. I also have an underlying goal of keeping my panel and wiring as simple as possible but a rotary switch would certainly be included if its function fulfills a key need.

[avidflyer]

I think it's nice to be able to see all 4 temps when you want to. I'm rebuilding a Kitfox that will have a 912 in it, and will maybe go the same route. I've never had trouble with the switch, actually I used to have 2 of them, one for CHT, and one for EGT. When I rebuilt the plane and panel, I learned I could use just one. The Jabiru may be a bit more finicky then the 912, so maybe monitoring all 4 cylinders is better with it than the 912. My monitor could see 2 EGTs and 2 CHTs at once, but I only use one side. Looks like all you would need is 4 EGT probes plus the switch, and this should work fine on your quad gauge. I would probably go with it. JImChuk

[Birdseyeview]

OK, thanks so much for your insight and advise. For your 912 install have you settled on which EGT probe to use? The clamp on type or would you weld a boss on each pipe for the NPT type attachment? Seems like the clamp on type is the easier install but the other may have some advantage.

[avidflyer]

My exhaust pipes already have the holes drilled in them, so clamp on is probably the way I will go. JImChuk

[Birdseyeview]

where are the holes drilled exactly and is it the same distance from the flange on each pipe? I assume you are going with a certain p/n clamp. Do you know which mfr and p/n?

[jiott]

Rotax 912 Installation manual specifies exactly where to mount the EGT probe in the exhaust header. I believe it is about 4" from the flange, but don't take my word, look it up. The recommend the two rear headers.

[Birdseyeview]

Jim, Thanks for pointing me in the right direction. I found the location of the EGT probe on page 10.1 of the Rotax 912S manual and the distance from the flange is stated as 2.76". It doesn't seem to say anything there about which cylinders to monitor. On page 21.1 of the installation manual it does mention that CHT should be monitored at cylinders 2 and 3 (one in front and one in back), in which both ports have a temp sensor installed. However, there's a CHT sensor port location on every cylinder head so this gets back to the basic question on what everyone is doing with CHT and EGT monitoring - Is the most common practice and/or recommendation to monitor CHT and EGT on all cylinders or just specific ones (#2 and #3)?

[Rodney]

The EGT probe is a type K thermocouple

Vktech 2M EGT Thermocouple K Type Temperature Probe Sensors Exhaust Screw Threads New
by Vktech

These are available on Amazon for about $18.00. I welded a 1/8" boss to the exhaust pipe and installed two on my engine. Plenty of cable to reach inside to the back of the instrument panel.

Regards
Rodney

[jiott]

Larry, CHT in cylinders 2&3 where the stock sensors are installed is where most of us leave them. EGT in the two rear exhaust headers is also standard for most folks. In my opinion, this is minimum but adequate. I do know of several that have installed EGT sensors in all 4 locations. I think this is more common for those people who have adjustable props and ignition/fuel systems where leaning and MP must be managed.

[Birdseyeview]

Jim, Thank for the clarification. It is very helpful for me to know what works for others before I get into it.

[jrevens]

Larry, CHT in cylinders 2&3 where the stock sensors are installed is where most of us leave them. EGT in the two rear exhaust headers is also standard for most folks. In my opinion, this is minimum but adequate. I do know of several that have installed EGT sensors in all 4 locations. I think this is more common for those people who have adjustable props and ignition/fuel systems where leaning and MP must be managed.

I know this is going to raise a few eyebrows, but I have a couple of thoughts about this. First of all, the use of EGT measurement is a relatively recent development in the history of aircraft engine instrumentation (I’m showing my age here). Remember the history of Alcor? There’ll probably be disagreement with this, but I think that EGT is probably one of the least useful of engine monitoring instruments for a 4 cycle Rotax engine that has no manual mixture control. In case someone thinks I’m against technology somehow, I designed and built a simple comparator circuit that I used with an automotive oxygen sensor to compliment my EGT gauge on the Lycoming in my first homebuilt. That was well over 20 years ago before anyone I knew had done it. Then, consider that for over 30 years most Rotax 912 engines have somehow run just fine with only one CHT, or water temp probe. There are exceptions to every rule, and I’m not against having access to all the engine monitoring data you can, but saying that having a “minimum” of 2 & 2 is a necessity strikes me as possibly overkill. We live in an age where displayed data is over-abundant. I’m curious about how useful (and used) EGT is for the vast majority of 912 owners. I’m measuring EGT on one of my cylinders of my 912, but frankly I’ve thought about other things I could’ve done in place of that on my “old-fashioned” panel that would be more useful. For me it’s kind of like the uselessness of having an ammeter in an airplane, when a voltmeter is much more useful and can tell you everything you need to know in flight. Again, as an old guy, there is just way too much to look at on a modern glass panel for me. That’s my rant for the week.

[jiott]

I agree with you John, the standard carbureted 912 has nothing you can do in flight to adjust EGT, so it is kind of useless. However, I like to look at it once in a while to make me feel good. As I see it, its only value in flight might be to give you some early warning if that cylinder is getting colder due to fouled plugs, ignition problem, fuel supply problem, etc.

[Birdseyeview]

John, Thanks for the historical perspective. This is a good reminder to me of what is necessary vs what is nice. In my attempt to keep things as simple as possible in the cockpit it is often easy to get swept up in the idea that more data is always better. Although I do appreciate hearing what others are doing with their instrumentation, I'm finding it way more enlightening on why they are monitoring certain parameters and the associated logic behind their decisions. This logic is ultimately what drives the implementation of how to gather data and what to do with it once you have it. From what I've been gathering so far in my investigation EGT seems to be a lagging indicator of what's going on inside a 912 with a fixed pitch prop and may be most useful typically in troubleshooting engine issues after they occur. CHT seems to be more important. That being said, I haven't heard a compelling reason yet to monitor either CHT or EGT in every cylinder so I'm leaning toward the minimalist approach. From another historical perspective: my 912 is a 2001 vintage which apparently had coolant temp sensing that was treated as separate from the CHT, which was sensing the head casting temp. In some later engine versions it sounds like a head change was made so that CHT was sensed in the coolant passage. From stories I've heard it sounds like CHT and coolant temp are usually very close either way so I'm wondering if I could treat them as equivalent and only monitor CHT and not have the separate coolant temp sensor that my build manual calls for.