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Permalink Reply by John Austin on May 12, 2019 at 1:17pm

I've always had mixed feelings about this topic!  Jabiru is adamant about staying within their specified EGT limits, but Mike Busch (the "Savvy Aviator" maintenance guru and AOPA contributor) has a totally different take on this issue.  "IF" I understand his analysis correctly, absolute EGT measurements are not "real" and are meaningless -  relative EGT measurements can be useful diagnostically.  The critical absolute temperatures are CHT's, which more accurately reflect the stress to which the cylinders and valves are being subjected.

Mike has written a number of articles and given a number of seminars on EGT's vs CHT's.  Just Google "Mike Busch EGT CHT" and you'll see what I mean!  He has a YouTube video seminar on this subject 1h 43min long!  But, one of the most concise summaries is here. 

The latest 3300A maintenance manual (JEM0002-7 / June 2016) lists 1364F as max cruise EGT and max cruise CHT at 356F.  In my STOL 750, my typical CHT's are 250-275F in winter and 275-300F in summer (measured with probe attached to hole between the plugs and the probe is insulated from cooling blast air, which generally gives a closer-to-true CHT vs the original probe ring under the plug).  I pretty much operate in upper 1200's/lower 1300'sF EGT's on take-off and climb and 1350-1400F in cruise, all the time keeping an eye on CHT's to ensure they are well below the published CHT limits.  I feel very comfortable operating within these parameters and so far have 580+ relatively trouble-free hours.  So, I guess I'm a little closer to a "Busch Believer" than a "Jabiru Believer" on adherence to EGT's.

One significant factor in my experience is that I switched from the Bing to an AeroInjector throttle-body carb.  This gave me a mixture control which gives me the ability to very precisely control the mixture and hence the EGT's and CHT's. 

I think your EGT's are fine if your CHT's stay within limits!

John

N750A

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Permalink Reply by Steven Mogg on June 29, 2019 at 8:29pm

Jeff,

yes I agree with Johns comments. Absolute EGT is not useful. It’s all about change in EGT. Would be easier if they all started at the same value. If you have mixture control you can get some useful info fromEGT. Also doing a cruise power mag check will give you EGT change. Rises slightly unless you have an issue. 

I have ave included a cruise picture of my MGL. You can see #2 EGT is low. Always is. That cylinder is also the richest but not because the EGT is low in cruise. It has more to do with the sensor location in the pipe. I have moved it twice now and varied the temp bu 100C moving it 3mm in or out. Just haven’t got it in a better position yet. Very sensitive to position of the sensors. They are all same distance down the pipe from the value. Started all the same distance in but that didn’t work out for #2. Note the CHT spread. That is a good indicator also. They move slower thanEGT but I find them a more reliable indicator of what is going on. #2 is low there because it is rich. Lean the mixture and it will rise. Unfortunately #5 & #6 are lean of peak so the CHT on them will fall. Can’t win them all.. 

i am am using a Rotec TBI

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