The numbers and characters on the plugs, are the product number, wire thickness, and thermal range of the plug’s body. Example: The Novarossi C6TG (F) or (C)
Product: C (Conical)
Wire Thickness: 6
Plug Type: T (Turbo)
Plated: G (Gold)
Thermal Range: F (Fredda = Cold) C (Calda = Hot)

The plugs with cold thermal range F (Fredda), have a body with thinner walls and shorter body, so they dissipate heat better and faster to the engine head. The plugs with hot thermal range C (Calda), have a body with thicker walls and longer body, so they dissipate heat better and faster to the engine head. The number of threads is the same on both types of plugs.

How to select the proper plug:
·When the ambient temperature is high, we have to use a plug with thicker wire.
·With higher compression, we have to use a plug with thicker wire.
·Humidity determines if we use a Cold (F) thermal range, or Hot (C) thermal range.
·With high humidity percentage we should use a Hot (C) thermal range plug.
·If we have high temperature and high humidity, we should use a plug with thick wire and a Hot(C) thermal range.

The best instrument to determine which Turbo Plug to use, is those table digital weather stations, that have Temperature, Humidity, and some also Barometric Pressure on them. That is the key to success or failure tuning engines with Turbo Glow Plugs, because they are so sensitive to weather variations that is unbelievable.

Following is a selection chart for Novarossi Turbo Glow Plugs

Type Thermal Range Nitro% Air Temp.C° Humidity%
C5TGC Hot/hot wire 10-20 0-10 70-100
C6TGC Hot/average wire 10-20 10-16 70-100
C7TGC Hot/cold wire 20-30 16-25 70-100
C8TGC Hot/ultra cold wire 30-up 25-up 70-100
C5TGF Cold/hot wire 10-20 0-10 40-70
C6TGF Cold/average wire 10-20 10-15 40-70
C7TGF Cold/cold wire 20-30 16-25 40-70
C8TGF Cold/ultra cold wire30-up 25-up 40-70

For Altitude you have to adjust your compresion ratio (higher) reducing shims on chamber, and/or use higher nitro % fuel. Depends on how high you are. Plugs will be the same as per chart.

This info is courtesy of AFM.

Also, the heat range of the plug alters the timing. A hotter plug will ignite earlier in the cycle than a colder plug. This effects power delivery. All things being equal, a hotter plug will usually make more bottom end grunt than a cold plug, and vice versa.