Replies to This Discussion

Permalink Reply by John Austin on October 4, 2020 at 7:34am

As far as using the ring thermocouple under the spark plug, typically the CHT is falsely low, not high.  This is due to the fact the ram air flowing over the exposed plug cools the thermocouple several degrees.  If, in fact, your temps appear to too high, make sure the ring thermocouple is correctly installed - when installing, you have to take the spark plug sealing gasket/washer off the plug, put the ring thermocouple on first, and then replace the gasket/washer so that it is between the cylinder head and the thermocouple.  It is a common error to simply put the ring thermocouple on with the sealing gasket/washer remaining above it on the plug.  The ring thermocouple provides a poor seal and allows the plug to leak hot gases during combustion and falsely elevate the cylinder head temperature.

I did the Jabiru Engineering Bulletin AVDALSR085-1 several years ago and it works great!  I simply cut the rings off my sensors and carefully drilled a hole in the lug of the remaining ring terminal.  I would definitely not use self tapping screws!  I think you would be creating stress points that would be prone to cracking!  I tapped the hole and applied some heat sink compound to the screw and installed the old sensor:

This works very well, but if you want to get even closer to the actual CHT, you must block the ram duct's cooling air from cooling the sensor.  An Australian company, CAMit, made a Jabiru engine clone and several accessories.  I got these sensor shields from CAMit:

They have Fiberfrax insulation on the underside and are safety wired to the sensors like this:

Unfortunately, these are no longer available as CAMit is no longer in business, but it would be simple to fabricate some from aluminum and put a small piece of Fiberfrax or similar fiberglass-ceramic insulation underneath.

All of this gets you very close to the actual CHT.  A probe "down the hole" is likely more accurate, but the installation would be more difficult.  I'm very happy with this mod and the big plus is you don't have to disturb the sensors when changing plugs.

John

N750A

• ▶ Reply

Permalink Reply by Stephen R. Smith on October 4, 2020 at 10:39am

The "under the spark  plug" location is bad for a number of reasons. The reading in that location is too LOW, not too high.

I am one of the folks who moved to thermocouples "down the hole". But before I did that to all 6 cylinders, I flew for many hours with three sensors on the center cylinders only; one under each spark plug and one "down the hole". What I learned is that the sensor down the hole registers higher temperatures and shows increasing temperature much sooner than the sensors under the spark plugs. I really like that because I can react more quickly to a rising temp problem.

With with sensors "down the hole" I use the following temp limits: 386 - yellow - five minutes max; 410 - red - fix it NOW - nose down, reduce power - richen the mixture (HACMAN on Bing).

The sensors are held in place by a combination of red RTV in the hole and a wire tie that fastens the wire to the head as a strain relief.

The absolute way you know your heads are getting too hot is if the head bolts move when you attempt to re-torque them. On my CAMit engine I don't even bother to check them anymore. They have never moved. My CAMit engine has been by far the best "Jabiru" engine I have owned. :)

Steve

• ▶ Reply