I'm thinking about putting in a new gauge in my homebuilt Supercub (PA18-150), and wondered which would be more useful: a CHT gauge or an EGT gauge? I've got a recently rebuilt engine, and lean whenever I'm in cruise by pulling back on the mixture until the engine stumbles, then add some until it smoothes out. I generally get around 7.5 gph with that method and a little bit of grey/white on the exhaust. Any thoughts?

Thanks!

ASW.

I'm gonna be first and then everyone else can first correct me and then answer your question. This will help take the heat off you.
The reason we have EGT gauges is to limit the CHT temperature. Too high cylinder temperatures is where the damage is. Last May I flew with HC out of Sidney to Steamboat Springs. On climb out at that airport 6900' and 60 degrees five miles from the airport my engine began to heat. Cylinder temps hit 430. My EGT was not out of line but I still began richening the mixture to cool. I richened and the engine began to run rough. I pushed the nose down and pulled the throttle back some, readjusted mixture and the engine began to cool. Later at home I found the reason for this heating was the baffling at the rear of the engine was just too long a reach to the cowling and had pushed itself over back toward the firewall and let the air pressure escape out the rear. EGT gauge would not have saved me on this one. Abnormal engine heating but not able to be adjusted out with mixture. No amount of richening would cool it.

Do both.......http://www.aircraftspruce.com/catalog/i ... combo2.php

I've got this unit in my plane: http://www.buy-ei.com/Pages/EC/EC_Overview.html

Definitely both. CHT and EGT both tell you distinct and equally valuable information about combustion and efficiency of. There are lots of cool combo engine analyzer options for Exp.

I can highly recommend MGL Avionics gauges. I have their 2 1/4" 4 probe TC-2 gauge and works excellent. I use it for EGT. Easy to read in daylight. I am going to order a second one for CHT. I only have an analog single CHT gauge right now. If you have space for 3 1/8" gauge, you can get their 12 probe model and have all in one. www.mglavionics.com

Bugs66 wrote:I can highly recommend MGL Avionics gauges.

I don't often plug my own business, but I am a MGL dealer. PM me for more info and a fairly decent BCP discount.

If I had to pick one I would go for CHT, make sure you have a reading on EACH cylinder. A CHT will show a potently dangerous condition (valve train failure, rings, and shock cooling) better the a EGT. If it were me I would go for both, some nice analog style units from spruce (multiple readings on the same gauge), or if your jumping on the glass panel digital wagon can't go wrong with a JPI

At what temp on CHT is shock cooling a concern?
Gary

shortfielder wrote:At what temp on CHT is shock cooling a concern?
Gary

Shock cooling isn't a certain temp, shock cooling is a result of going from warm temp to a cool temp rapidly (I.e. Power off rapid decent) and cooling the engine quickly, same theory of a very hot glass being submerged in cold water and cracking.

I believe the JPI CHT units will flash if you start cooling too quickly.

Lots of info here.
http://www.buy-ei.com/Information/Pilots%20Manual.pdf

Thanks for posting that Terry! You beat me to it:)

Without my sales hat on, just trying to make mention of what I have seen.

Burnt valves always have been and remain one of the largest repair issues in an aircraft engine. Some are due to valve guide issues, but most are due to overheated valves.

1. Exhaust gas temperature is the major contributor to heating a valve.

2. The valve seat to the cylinder head is 80% of the cooling of the valve.

Therefore, operating with high CHT’s and EGT’s can overheat an exhaust valve and cause valve damage, as many pilots have experienced.

From purely a diagnostic stand point:
Changes in engine health show up as trends in EGT first.

My humble conclusion: Engine health relies on a good and accurate understanding of how a pilot's actions affect the engine. Both EGTs and CHTs have a significant affect on the exhaust valve temperature. Both should be monitored if at all possible (all EGT's and CHT's on the motor). Doing so will allow you to keep your temps down to "acceptable" levels decreasing probability of developing issues and at the same time adding a significant amount of safety because engine issues won't "sneak" up on you.

Best regards:)

Matt
.

Terry
Yes thanks for posting that link. I read it and tried to absorb as much as my thick head could. I've got to get some things cleared up and determine how to calibrate the Alcor EGT/CHT gauge I've got (can't afford a 6 place monitor, can't afford another overhaul) . I know where peak is on the EGT side but it doesn't show the temps only graduations. On the CHT side it shows 25 deg grads up to it looks like 500 deg F. I haven't seen the CHT over 400 deg, it stays about 325-335 in cruse and 375-380 in climb (according to the grads).

Last motor I had had 4 burnt exhaust valves at annual running 50 deg ROP (as I've been told to run them) on a 1750 hr tt O-470 (I put the last 300 hrs on it) one of those cylinders only had about 400 hrs on it (#2). There was so much reading I'll have to go over the data again and weed out the LOP IO stuff. I can't afford to burn up another motor.

ASW, this is a really GOOD, but tricky question. As my old Control Systems prof used to say, "You can't manage what you don't measure". Ummm, did I ever mention that Control Systems put meager bread on my table? This topic always engenders heated discussions on the local aerodrome "liars bench", perhaps topped only by observations on the circuit etiquette and landing skills on view to all

Some good observations previously posted. Perhaps the best discussion of these topics for users is the "Sky Ranch Engineering Manual", John Schwaner; sorry don't have the Sky Ranch website handy. This discusses the complex relation between EGT, CHT, and TONS of other related topics. I HIGHLY reccomend this to all serious piston aeroplane powerplant operators.

And TCM and Lycoming have their own reference parameters, see, e.g., their websites.

By way of example, on the topic of "Shock Cooling", the Lycoming guideline is <50F CHT reduction per minute. The serious Glider Tug and Skydive operators have figured out descent profiles with powerplant/airspeed configurations to conform to this, and some routinely go over TBO, without cylinder rework.

On the tug I regularly operate, I'm running Matt's nice digital CHT and EGT's, but only one point each. Whilst ALL cylinders is most desireable, it's easy to get fixated on watching those numbers/bars dance, and forget to look out the windscreen and FLY! And on the cubscout 'cub, I have neither, but both are on my shortlist for improvements. That said, the "click-stop" mixture control (PMA from Spruce) eliminated mixture creep, and significantly improved fuel burn, and plug fouling. If I had to pick only one, it would be CHT, installed in the factory CHT well [unLESS you're cursed with one of those early Continental cylinders which use the Spark Plug ring washer ] .

Since you're running an XP S'Cub, with a Lycoming (the last we chatted), that would be MY starting point. But be VERY aware that that 4-cylinder Lycomings have notoriously uneven mixture/CHT distribution. FOBJOB on another popular wesite where you sometimes lurk has discussed his trials and tribulations in this regard.

There, MY opinion and worth what you paid for it.

Thanks. cubscout

Wow, lots of great info - thanks! I'm leaning toward the EI dual gauge. It sounds like I really should have both readings to protect my engine. It's the right size (2 1/2") for the hole in my panel that's currently filled with a very useless vertical card compass.

Since I'm planning on going to Sun-n-Fun, I'll be sure and check out other gauges, but I like the EI.

Thanks again! ASW.

This was one of those questions I had, but once I bought my Stinson, it already had two gauges, CHT and EGT already in it. This thread got me thinking to go check to see them though as the last plane I rented didn't have numbers on the gauge, just hash marks. Turns out both my CHT and EGT had temp numbers on them too. I never really flown a plane regularly that had CHT, so I'll be reading the article linked early, need to do some learning.

It's all confusing to me......to much info for my head. I get what they are measuring but what are the perameters? I have never seen CHT's over 330 or 340 at most, but see EGT's in cruise peak at mid to high 1400's. Seems to high to me?
For example...today cruising at 1000ft, mid 30*, carb heat on to maintain about 40*, peak EGT was in the 1470's but full rich was only 1330. Only 140* difference? CHT on hottest CYL was around 315.
EGT's seem to hot?
I was tought to lean by backing out the mixture until RPM's drop, push it back in a little and then pull carb heat to make sure it is not running to lean. Seems a lot simpler and less time consuming to me.
And to top it off EGT's change in different conditions?

Terry
Seems to me, if your are going to be flyin w/carb heat on, you should lean the mixture again once you have that on, otherwise you will be runnin pritty rich. Look at your exhaust stack. probly pritty black?????

If you are flyin with the heat on all the time, you want to be careful about runnin around on the ground with the heat on, and suckin in dirty air.

Gary

Gary,
I do just as you suggest and carb heat off on short final.
I think my EGT's are to hot...maybe not?
The numbers I posted was at 22 square.
What are you guys seeing?
O-470L