Sooty residue look from cowling exhaust...how do I do it?

If you're skilled with an airbrush, that's the most permanent method. I wouldn't use graphite, that looks too metallic, but chalk pastels are great for this. They feather nicely and if poorly applied, you can just wash them off.

The color of exhaust stains on a radial engine will vary from engine to engine because of the way a particular carburator is set up. An all black stain indicates the engine is running very rich, while an almost white stain indicates it is running lean. I usually apply a base stain of black using thinned flat black applied with an airbrush. I then apply a very light grey pastel chalk to the center area of the of the stain which fades into the black leaving a gray stain outlined in black.

Tamiya Smoke has worked for me in the past-- but now I use Doc O'Briens Weathering Powders with awesome results....see here:

...and to the forum.

Great idea, I'll give it a try and see how it works. Thanks heaps.

how about ground-up chalk pastels? you can even just file away some pencil lead/graphite and use that. just brush that stuff on, presto!