Motors,
Great question! And when I went poking around, I (re)learned a few things. Thanks!
Apparently A3's are called No. 6's nowadays. No idea why they needed to be renamed--they had a great reputation, so it wasn't that. I think that they just wanted to reserve all the lettered plug designations for the car guys. Now, all of the so-called "air plugs" have number-only designations.
The available OS plugs show up on this chart, arranged by heat:
https://www.osengines.com/glowplugs/index.htmlI have been running P6 turbos ("hot") in engines that take turbo plugs (OS 18TZ, etc.), and A3s in engines that don't (OS 15 CVA, etc.). I don't know why P6's don't show up on the chart, unless they have renamed those as well. Perhaps they are RP6's in order to remind us that it is illegal to run these in anything but
road vehicles? That does not make sense based on the packaging of the P6's that I have in hand, which says "Hot."
The chart implies that the P3 "Ultrahot" is the same heat as the No. 6---but I wouldn't count on it. Sales brochures mostly fall short of technical accuracy. You could pull out the ancient Wild Bill magazine article where he tested and ranked the available plugs by heat range and repeat his glow plug test methods on the No. 6 vs. P3 and see what you get. I would add a test given today's technology: connect to a calibrated voltage source and measure with an infrared thermometer. Might take a imaging sensor to be able to resolve just the plug element though.
I have only run OS-made turbo plugs, so don't have a correlation to other brands. Sorry.
McSlow