In investigating getting my speedometer repaired I have done a little investigation of cylinder head temperature gauges. I was disturbed to discover that one must use the wire that comes with the sensor. I had hoped to use a wire I already have running from the engine to behind the dash.
I read on aircooled.net that the sensor is calibrated to give you a correct reading at a particular ambient temperature, say 70 degrees. At 100 degrees the gauge would read 30 degrees lower than head temp. At 40 degrees the gauge will read 30 degrees high.
I found a discussion of sensor location on VW heads on an airplane site:
Cylinder Head Temperature Thermocoupler (Sender) Location
Although it seems obvious to install the cylinder head temperature sender underneath a spark plug as the sender is 14mm, this is really not representative of the average temperature of the cylinder head. This is the hottest location however! In a test of 10 locations on a VW cylinder head, that included under the spark plugs and under each one of the cylinder head stud washers, the hottest temperature was under the spark plug. In fact, reading the temperature under the plug was a little better than 150 degrees F hotter than the coolest place on the head, which was under the two center cylinder head stud washers inside the valve cover.
We find that many builders that complain of high cylinder head temperatures really don't have a high cylinder head temperature average. Recently a builder called trying to cool down a 450 degrees F head. The engine runs fine he said. "Good oil temp, good oil pressure and the EGT was 1250". But he just could not get the head to run any cooler. He was measuring the temperature under the spark plug. After a brief chat he moved the probe around, measuring temperatures on the head. He installed it finally under a cylinder head temperature studs washer that was on top of the head behind the intake manifold where he found his average temperature. His gauge now reads about 325 degrees F - his average.
If you've ever looked at the cylinder head temperature probe on a VW Beetle equipped with one, the probe is installed in a cylinder head boss that is just above the rockerarm cavity, not under a spark plug (see photo). The cylinder head temperature runs about 100 degrees F cooler at this location than under a cylinder head stud washer and about 200 degrees F cooler than under a spark plug. The hole in the white circle is for a fuel injection enrichment temperature probe.
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