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Jabiru 3300 CHT Sensor Shields
I recently completed AVDALSR085-1 on my Jabiru 3300, the factory-approved relocation of the CHT sensors from the ring terminals on the plugs to direct connection to the cylinder head. This worked well and as expected, my overall CHT's were indicating a little higher with the more direct attachment to the head.
However, any CHT sensor that is either on a spark plug ring terminal or screwed directly to the head still receives air blast cooling from the air passing along the head and through the cylinder fins. CAMit, the Australian firm that actually manufactures the Jabiru engine to Jabiru's specifications, has come out with a CHT sensor relocation kit that includes small shields to insulate and divert cooling air from the sensors to further improve the accuracy and consistency of the CHT readings. The shields are available separately from the kit.
Today, I installed the shields. You can see the Fiberfrax ceramic fiber insulation attached inside the shield:
Here is what the sensor looks like before the shield is installed:
Installation is very simple. The "fingers" of the shield are pushed down between the cylinder fins and a piece of safety wire secures it to the sensor:
This arrangement still allows for torquing the cylinder head bolt immediately above without removing the shield and also the plugs can be installed and removed without disturbing it, either. The insulation should give a more-accurate CHT and the shield should provide some added protection to the sensor.
Tomorrow, if the weather improves, I'll do a test flight and see how the readings compare to when the sensor was uncovered.
John
N750A
(no affiliation with CAMit)
Replies to This Discussion
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Permalink Reply by John Austin on
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Test flight this a.m. revealed an almost 20°F increase in indicated CHT's in the front two cylinders, but the other four showed only very slight increases. This probably makes sense as the front cylinders get much more air velocity and therefore more cooling of the surface-mounted sensor. This was very reassuring as it makes my CHT spread even tighter and the hottest cylinder (#3) was only indicating about 5-8°F hotter than previously with a max of 311°F in cruise, well below the 356°continuous limit.
These shields give a more accurate picture of what's going on cooling-wise. I'd strongly recommend them if one was operating anywhere near the temp limits and wanted a better idea of just how close they were!
John
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Permalink Reply by Mack P. Kreizenbeck on
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John,
Thanks for the pix and flight test report.
Mack
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Update:
As you probably know, CAMit closed its doors and these shields are no longer available. However, it appears it would be fairly simple to fabricate something similar from some aluminum and bond some Fiberfrax insulation inside. I've had one or two of the shield's insulation deteriorate a bit and flake off. I found laying around the shop a scrap piece of what appears to be the identical insulation with a self-adhesive backing, so it was simple to just cut a small piece off and stick it in place. I don't think the adhesive is very critical since the insulation will be held in-place once the shield is safety wired.
John
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Permalink Reply by Alan Cameron on
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Hi John,
Just saw your post regarding air deflectors for the CHT sensors.
Did you consider using the red high temp silicon to surround the lead junction thereby preventing cooling air from coming into contact?
Alan Cameron
Australia
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The shields are a neat, clean solution and were originally supplied with the fiberfrax insulation installed. The insulation covers the terminal, the screw, and the junction all the way to the lead, so I saw no need to further try to insulate them. I think your idea is a good one and if someone was fabricating some shields, high temp red epoxy would certainly seal them well! However, the silicon would be a bit more of a mess if later one had to get to the screw to remove the sensor for replacement, etc.
I did have had at least one lead fail from flexure where the terminal transitions to the lead, so I strengthened or reinforced this area with a drop of JB Weld epoxy (you can see the epoxy in the middle picture above). This has worked well as I have never had another problem with the leads since then.
John
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