I already answered this, but I'll answer it again:

There are two ways of doing this, one within the model itself:http://www.simcraft.nl/afterburner.html, and one with effects in the sim: Just look at any currently available aircraft with afterburners to see how it's done (look in the "aircraft.cfg" file).

I think it's alot easier to go with the fx files, although this does require a gauge. Just search around, you'll find alot of good tutorials.

Ric B.