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Tips for Installing EGT Probes?
I'm about ready to install the EGT probes in my Jab 3300. I know that they are supposed to be 100mm from the exhaust outlet (I assume you measure from the flange securing the exhaust?) and I know that probe placement is critical to getting accurate information!
Soooo, since the exhaust pipes have several different bends or angles of bend in them, how do you get a consistent placement? It appears to me that if you measure down the inside radius of a exhaust pipe bend, measure down the side of a bend, or measure down the outside radius of a bend, you will get three different distances from the exhaust flange!
Would the best approach be to measure 100mm down the "side" of the exhaust pipe (90 degrees from either the inside or outside radius of the bend), mark a circumference there, then drill anywhere on that circumference that seems best for probe clearance and access?
I would especially appreciate someone's advice who has "been there/done that"!
Thanks,
John
Replies to This Discussion
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Permalink Reply by Phill Barnes on
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Hi John
I recently have been in contact with the Jabiru factory technicians and they advised me that the probe placement should now be 120mm from the top of the flange on the No. 6 exhaust pipe. I installed mine at the back side of the pipe and facing towards the cowl a little for easy maintenance. If you would like a pic I can take on for you.
Phill
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Permalink Reply by Craig Payne on
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Actually I wouldn't sweat the distance too much. I have seen a number of reports from the real world where moving the probes didn't change the readings. In a closed tube how much can a hot gas cool in a few centimeters? For example Marvin Williams posted this on the MGL list in October:
"I would NOT fret too much about the location of the EGT probes. We changed ours from 100 mm to 120 mm and couldn't see any difference at all."
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Permalink Reply by Bob Pustell on
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The advice I have heard most ofton is that regardless of what distance you use, you need to be sure that all of your probes are that same distance from the cylinder (assuming you are doing a multiple EGT installation). If the probes are at different distances (especially if there is a significant difference in the distances) you will get EGT spreads that "lie" to you because there is some cooling in the pipe.
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Permalink Reply by John Austin on
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Bob,
I agree, the absolute distance is not critical, but it should be the same for all cylinders. After all, my 6-probe Grand Rapids EIS recommends "2 to 8 inches" - that's quite a range! However, from what I've read, the longer the distance from the exhaust flange, the less wear on the probes, BUT, the longer the distance also can create a slower reaction to temp changes.
The HomeBuiltHelp DVD shows someone installing probes - they don't comment in the video, but it appears they have marked a fixed distance on a piece of tubing or something to use to measure from the exhaust flange. So, I guess the proper procedure would be to measure on the same orientation on each pipe, perhaps at 90 degrees to the inside or outside of the bends, mark a circumference, then pick your spot to drill based on best access/clearance, etc.
John
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Well ... I took a long, deep breath, closed my eyes, and installed the EGT probes today! (Actually, I did it with my eyes open! LOL!)
I checked distances and 120mm definitely seemed to work better - it put the probe's clamp right below a spring hanger on a couple of cylinders - 100mm would have probably put the spring hanger in the way of the clamp.
About the only problematic spot was #4 cylinder - the clamp cleared the spring hanger, but with the #6 exhaust header in front of #4's and the intake runner behind, it was kind of tight to find a spot where the worm gear on the clamp wouldn't interfere with the intake runner. I did some careful "spotting" of the location and finally found a spot that worked - I removed the #6 header while I installed it to give me more working access - made it a lot easier.
John
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Permalink Reply by Chris Sinfield on
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John
got a photo of how you did it?
Chris
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Chris, I don't have a photo of the process, but I do of the end result! I cut a piece of malleable wire 120mm long, put it up against the exhaust flange and oriented it along the pipe 90 degrees lateral to the bend of each header, marked the spot and made a circumferential line there. I then picked a spot for best probe orientation and good access to the clamp's screw. Here's some pics to show the probe orientation and the tight spot for #4:
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