The question was “is there a hotter glow plug than the Merlin?” I was looking for a yes or no answer and a brand name if the answer was yes. It appears that no one knows the answer.

Downunder is correct about keeping the engine cool. I have found that a head temperature of about 225 to 250 F) on the ground and 250 to 275 in the air works best for maximum in flight power. I used an infrared meter. By running rich on the ground there is enough fuel for the engine during high G maneuvers such as a high speed pylon turn so the engine will NOT overheat.

Here is how I determine that one glow plug is hotter that another. I used an OS-F plug as my base line. 15% nitro fuel, 10X4 APC prop, engine either a TT PRO 40 or a GMS 40. The RPM should be around 16,500 peek when you pinch the fuel line. Set the needle valve to achieve 15.500 RPM or 1000 RPM rich from peek.

Now let the engine cool and change the glow plug, make NO other changes i.e. do not touch the needle valve. Start the engine and let it worm up then go to WOT. If the RPM is greater than 15,500 and somewhat rich, the plug under test is hotter than the OS-F; if the RPM is less than 15,500, and very rich, than the plug under test is colder than the OS-F.

Remember this test must be performed with the same fuel, prop and the same engine, and on the same day within 15 to 20 minutes apart.

This works for me.

Bob