ORIGINAL: Juice
I went to the LHS to pick up a colder glow plug. I explained to him why I need it. He ended up talking me out of it, explaining why it wouldn't work for what I want to do. He gave a couple reason's:
1) 4-stroke engines (like the one in question, RCV 90SP) needs HOT glow plugs to keep the filament lit up for all four strokes of the piston. A cooler plug would lose it's glow between ignitions.
2) Cooler glow plugs are used in 2-stroke engines for high performance applications, like racing. It wouldn't work to put in a cooler glow plug and just lazily fly around.
I'm going to try lower nitro after my current jug of fuel is done.
For now, I'm trying to get my head around this peak RPM thing. How can I find it? My propeller is custom made and I do actually have the ability to adjust the pitch (on the ground only). I have some RPM numbers at various pitch settings. Can anything be determined from these numbers?...
18x13, 3840 RPM
18x11, 4400 RPM
18x10, 4450 RPM
16x13, 4760 RPM
16x12, 4960 RPM
16x11, 5200 RPM
The typical RPM for this engine is between 5000 and 5400 RPM. This number is about half of a typical engine because it has built-in 2:1 gear reduction.
Juice
Four-stroke glow engines need a glow plug with enough mass to stay lit between power strokes. This does not take the heat range into consideration.
The heat range of the glow plug is a completely different matter than the persistance (how long in time-units the glow plug element will stay sufficiently hot to fire the next fuel/air charge).
Had I known that the engine in question was an RCV four-stroke, I would probably have answered your question in a different manner, though I'm not backing away from what I stated previously. It is still true, in general.
I'm not at all current on RCV engines, however I seem to recollect that one of the customer complaints concerning them was the exceedingly long break-in period required before the engine was usable. Once broken-in, they appear to operate quite well, so it goes.
Since this engine does use a rotary intake/exhaust valve (rotating drum, IIRC), it is possible to damage the engine without using the specific glow plug(s) that are recommended by the manufacturer (over reach, damaging the drum valve). I would contact the manufacturer or distributor about your concerns, since the RCV engines are different enough from other engines that they have their own idiosyncracies and tricks. Good luck.
Ed Cregger