Im my Fly RC reviews, I use a direct contact thermocouple (with the RCATS data unit) tomeasure both the engine head temperature, and exhaust temperature at the stinger.

In my experience testing almost 16 engines so far, I would say that the majority of glow engines running 5% to 15% nitro fuel, on a synth/castor blend fuel run somewhere between 250 and 325 degrees F with a lot of them liking to run just around the 300 F mark. I know Dave Gierke likes to see the temps a little higher, around the 335F mark and in some of his reviews will modify the engine (wrap with foil or change the plug, etc) to see the temps get to at least this mark. I basically test "as is" and record them. Of course these temps are measured at full throttle with the engine broken in and set at just under peak, on an "average" day (not in the middle of winter, nor on a 95 F day).

With a GOOD brand of infrared meter, you can certainly get very close. My Greelee meter often reads within 20 F of the thermocouple (on the low side). But you have to hold themeter almost touching the glow plug. You can't point it from a foot away and expect to get a good reading on such a small object, due to the way these things operate.

Hope thats of some help/interest.

Andrew