VW CHT gauge/probe questions

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cluttonfred

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This topic came up on the margins of another thread about instrument panels, so I wanted to ask the questions here in the VW section.

  1. For those with experience flying behind air-cooled VW conversions, do you use a cylinder head temperature (CHT) gauge?
  2. If so, do you use one, two or four CHT probes, installed where? Under spark plug(s), threaded screw(s) in the head, head mounting stud(s)?
  3. And lastly, what CHT range do you consider normal for your installation?
By way of example, the Aeroveee folks lists these CHT numbers for their normally-aspirated conversions: cruise 350-375 degrees F; climb (5 min) 420 degrees F; max 450 degrees F. I asked them about it and these numbers are for probes mounted under the spark plugs with other probe locations yielding results differing by 50-75 degrees F (presumably cooler).
 

Pops

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I have used a CHT probe under the spark plug but its down in the spark plug well and the probe has a short life from removing and reinstalling the plug, so I put the probe under the bolt head mounting stud closed to the plug on #3. Also used a black marker and measured and put a mark on the other cylinders at the same location. Did WOT test and used a point and shoot temp gauge to make sure the others were running almost the same. On the 1835 cc VW engine the CHT's are 265-270 degs at cruise of 2650-2700 rpm and can climb as long as you want on a 100 OAT day and have never seen over 290-295 degs. Oil temps are always 180-185 at cruise and never over 195 degs on the extended climb on the 100 OAT days. Hot oil pressure 42 lbs at cruise with a hot idle ( 850-900 rpm ) pressure of 20 lbs.
 
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Marc Bourget

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Pops did a smart thing and established a black field on the surface at the spot you want to take a temp with an optical thermometer. They are calibrated for a "black body emmission"

Keep one thing in mind on CHT temps. The classical NACA report fixed something like 34 Thermocouples on the back side of the cylinder (downstream). IIRC, the temps varied from 180-200 all the way up to 435 deg F.

The place chosen for the location of the thermocouple was the most consistent over the full operating range, not the lowest.

"You makes your choice and pays your price"

Onward and upward
 

cluttonfred

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To be perfectly frank, one of the reasons behind this question is not wanting to deal with the bother or expense of an analog gauge + selector switch or digital analyzer plus four thermocouples in simple project like a VP-2. Plenty of VWs have flown (and most Beetles have driven) without CHT gauges at all, though I certainly understand how, especially in an aircraft application, you might want one. So, if I am going to use an simple automotive CHT gauge and one probe, I was wondering where it should go. This little article from the old GPASC newsletter also go me thinking as it says that the VW factory location for the CHT probe is under a screw in a threaded hole in the head itself. http://www.greatplainsas.com/bf20032.html
 
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Dana

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I have used a CHT probe under the spark plug but its down in the spark plug well and the probe has a short life from removing and reinstalling the plug, so I put the probe under the bolt head mounting stud...
Same here, for the same reason. On my Mosler (only two cylinders) I put it on the top front stud (closest to the exhaust) on the RH cylinder. I don't remember the numbers, though; the gauge was flaky and I always paid more attention to the oil temps, which would be touching but never exceeded the continuous limit (220 per the manual) in a full power climb on a 95 degree day, I forget where it was in cruise but it was less than that.

Dana
 
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