Bob,

I imagine (but it is your jusdgement) that you can increase pretty much the precision of your
measurements in the following manner:
1. Get a stroboscop from the car guys which is used to see the ignition instance, or use a photo-flash.
2. Drill and place a nipple beside the glow plug and use that pressure to trigger the stroboscope.
3. Then instead of propeller place a heavy wheel like those used for the boar engines. Behind or in front of it
place a disk on which outer surface you mark every 1mm axial line for about one half inch distance.
So the idea is pretty classic one: - when the pressure in the head chamber triggers the flash then you will see
which mark on the disk is seen due to the stroboscopic effect. With different plugs you should be able to see
different mark on the disk and from there you will find which plug is hotter = it should flash the earlier
mark on the disk, isn't it.

Check also my post earlier today. Using that old DDR-idea you can install second plug on the head of your
engine, drill it trough and fill it with transparent material, like melted quartz. Then you will see what is going
on inside of the head-chamber of your engine... It is amazing thing ...

Of course the professional laboratories for engine research have much more precise instruments.

For example, why not visiting the nearest University, I believe that is University of Dallas or Texas A&M
university and ask if they have some lab where the undergrad students will enjoy having a course project
and measure some bunch of glow plugs for you. For sure in those Universities there are many who are
at least interested also in RC flying.

Cheers,
Nick