EGT Spreads and CHTs

[JimMardis]

Good Morning, #2 runs about 30 to 35 hotter on the EGTs while number #3 runs about 30 to 35 cooler than the rest. Any thoughts??? I did have #3 spark plugs cleaned as it would drop down with a little occasional roughness. But I’m thinking I want to replace both plugs. What about #2 running hotter. If I try to lean to find peak, #2 will get to 1700 and I don’t like that so I have to yet to find peak. Always running on rich side. Any thoughts??? Is the injector not working efficiently???

 

 

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[dkkim73]

Jim, 

There is always some distribution of both absolute EGT peak values and locations of peaks. 

As I understand, this is due to injector variations, cylinder and port/flow variations, and other factors. 

From what I've heard, people use who EGT to lean will use the first or last to peak, depending on what direction you're adjusting the mixture. Ie, to avoid some cylinders being in the danger zone. Eg running LOP make sure you're past the last to peak by 50F etc. 

Some have said absolute EGT values don't matter, may depend on probe placement etc. 

POH says to use TIT as a guide though I've seen good arguments for individual EGT based adjustment as above. TIT looking values would still apply in those cases. 

OTOH have you noticed a recent *change* in EGT values? That could indicate something eg if one went suddenly high you could have a non firing plug. 

HTH,

David

The above reads like a list, so I think I should follow tradition and add "PP thoughts only, not a wizened engine guru".

 

[Danb]

You’re most likely going to have a EGT spread similar to what you mentioned. I doubt you have any issues

D

[Fly Boomer]

On 6/23/2024 at 8:25 AM, JimMardis said:

If I try to lean to find peak, #2 will get to 1700 and I don’t like that

I agree.  Sounds too high to me.  This may be why some are in favor of the BMP (Big Mixture Pull).  When you are certain all cylinders are on the lean side, you can enrichen slowly, watch EGTs climb, and try to determine peak coming from the lean side.

[JimMardis]

Will you please describe how to do the big pull. I’ve read about it but not sure on how to do it. The Salesman that sold me the plane showed me how cool the temps were at level off and stressed going real slow with the temperature changes as there was going to be 150 to 200 degree changes and you want to do that slowly. So could you help me out with this big pull.


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[Fly Boomer]

1 hour ago, JimMardis said:

Will you please describe how to do the big pull.

First, here is a series of three articles by George Braly (one of the GAMI guys) that is something like a summary of the leaning taught in the APS class.

https://gami.com/articles/bttfpart1.php

https://gami.com/articles/bttfpart2.php

https://gami.com/articles/bttfpart3.php

Then read this topic on BeechTalk which includes a couple of posts by John Deakin that, in a few words, describes it way better than I could.  I believe it may have been John that came up with the BMP phrase.  There are quite a few topics on BT regarding the BMP, so search there.

https://www.beechtalk.com/forums/viewtopic.php?t=82606

The point of the BMP is to quickly get each cylinder on the lean side of its peak, without dillydallying at high EGTs which will quickly lead to high CHTs.  Generally, John advocated powering back to a modest level, completing the BMP and then, if the engine is happy, putting back some of the manifold pressure (while watching EGTs) to get some power back as long as temperatures don't get too high.  If you have never done it, it might be good to fly with one of the several people around here who own or are familiar with the M20TN and the process.  If your engine doesn't have GAMI injectors, or doesn't have a near perfect ignition system, and you pull it back to the lean side quickly, you might get a really rough engine, or you might put the fire out.  Find out what the previous owner did.