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Retract EGT Probe?
I've got a few hours on my new Gen 4 and my #4 cylinder EGT is consistently high relative to the other 5 cylinders. Ironically, the #4 cylinder is also one of my coolest cylinders, running about 250-275F. (I've checked to be sure #4 is not lean-of-peak, which would cause it to run cool. I've also checked for induction leaks and swapped probes to verify the probe is not faulty. I borescoped the cylinder and the valves look normal and the plugs' colors are identical to adjacent cylinders.)
At this point, I'm thinking that my EGT is falsely elevated due to the probe's position on the exhaust manifold. This position was pre-drilled by Jabiru - they pop a stainless rivet in the hole and then you drill it out if you're installing an EGT probe.
I saw "somewhere" on the 'net that someone calibrated an EGT to make it closer to the others by putting a washer under the probe's head to back it out of the manifold a few mm. As we know, the exhaust gas temp varies greatly both in distance from the exhaust valve and where the probe lies within the diameter of the exhaust manifold.
Why would I want to do this? Mike Busch, the "Savvy Aviator" guru, has for years preached that absolute EGT values are meaningless because of the multiple variables involved and the CHT's are more of a valid indicator of cylinder/valve stress. However, EGT changes relative to other cylinders' EGT's can be used for early detection of problems and or diagnosis of problems. I simply would like to lower the absolute (meaningless?) EGT value so that it is more in-scale with the others and not so distracting.
I envision putting a small stainless washer under the probe's head to see what will happen - can't see any harm unless it causes a leak, which will be easy to detect. Anyone ever heard of this mod or knows how effective it is in changing the indicated EGT?
John
N750A
Replies to This Discussion
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Permalink Reply by John Austin on
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Update: I emailed GRT Avionics (the probe vendor) and tech support quickly replied. They said that it was perfectly acceptable to put one or more washers under the probe's head to back it out of the manifold a bit and observe the effect on indicated temperature. I'll probably give that a try the next time I have the lower cowl off.
John
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Permalink Reply by Jimmy Young on
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I’ve got a similar situation on my C-90 #1 cylinder. It is the coolest cylinder in standard cruise mode, but always has EGT temps around 100° to 150° higher than the other 3. I understand the logic Mike Busch has stated regarding the ease of having temp variations due to probe positions, and have never concerned myself with it. Having said that, I’d like to know how this washer experiment works out, it would make it easier to spot any trends developing with engine health.
Congrats on getting a Gen 4 engine, I was not aware you had done that. I plan to fly up to the Sept. Zenith gathering this year, hope to see you there.
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Hi Jimmy,
I've decided to try one more experiment before adjusting the probe. I have a "flow straightener" that Pete Krotje (at the former Jabiru USA) gave me to straighten the airflow for the original Bing carb. It has 4 vanes internally 90 degrees apart to stop any swirling air that might cause uneven mixture distribution. I've just mounted it between my AeroInjector and my filter and will see what the effect is. I've convinced the #4 mixture is OK, but who knows, if this affects the distribution it might even things out better.
You can't see much of the straightener as most of it is inside the blue silicon hose.and the air filter flange. I left the blue hose long so that it butts up against the AeroInjector and provides a bit of a brace to the cantilevered assembly. The straightener itself looks like this internally:
See you at the September Fly-In!
John
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