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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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Permalink Reply by John Burns on
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For the longest time on my Gen 4 I had one cylinder that would alarm EGT on climb out. Lower the nose and all was well. I used all of the normal tricks, clock the carb, flow divider from Nick, changing carb jets. Each change helped extend the time before the alarm, but it still had the issue. Suddenly last summer after about 100 hours on the engine the problem went away. My fuel economy has also been improving as I have reached 150 hours. I have no real explanation except to think that the engine took some time to settle in. Oil analysis does not show any abnormal wear metal and everything else is good. Just my recent Gen 4 experience. I look forward to hearing more about your results.
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My Gen 2 used to have one or two cylinders that on full throttle climb out would have very high EGT's but no problem with CHT's. Not so with the Gen 4 - with the AeroInjector, EGT's are very low and like most engines, I'll get slow, steady rise in the CHT's and oil temp but not so much I have to abort the climb.
At the present moment, I still have the carb mounted as depicted above. The last time I rotated the straightener (probably the "12 o'clock" vane is now over at about "11 o'clock) the EGT's on cylinders 1-4 all went up and are higher relative to #5 & #6, but the CHT's are still well within normal limits. #4 was still marginally higher at cruise settings, but not much. Out of curiosity, I tried retracting the #4 probe and put enough thin stainless washers under the probe head to pull it back about 3 mm. This did reduce the EGT, but now it is only marginally less than the other 3 high EGT's. If there are no exhaust leakage issues, I might go for another 3 mm, but really since the first 4 cylinders are very close, I don't see much point in it. Now, it would be nice bring up #5 & #6 to have a nice balance, but I suspect what really would have to be done is move the probes closer to the exhaust valve - but who knows and how much? Not that I would want be drilling multiple holes in my exhaust system! LOL!
So, I'm going to leave well enough alone for now as #4 is not so out whack compared to the majority and what's really important is that my CHT's are well balanced and doing great! (#1 & #2 CHT's are a little higher, but well within normal limits. I'm not happy with the placement of the rubber baffle material I have between the cowl openings and ram air ducts and will be reworking it after I get back from a trip. There's presently a bulge on both sides in the upper part of the rubber on both sides and I think it may be doing weird things to the airflow, so hopefully I can rework it, straighten the area, and then see the effect on the #1 & #2 CHT's. Again, this is not a serious problem but I do think I can achieve a better fit between cowl and ram air duct.
John
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