[schwarze-man]

Hi Chris,
Yes, the exhaust-gas plume is visible on large piston aero-engines at night. Certainly, stubs are notorious for giving off a lot of light, just look at night-flying Hurricanes with those large flat plate shades to stop blinding the pilot.
The light is certainly reduced by flame dampers, although many are not very good. I think the Bf 110 night fighters with the very large flame dampers are a good illustration of an effective system. The exhaust gas needs time, distance and dilution to cool and loose the glow. Here, the glowing gas is contained in tubes that prevent it being seen to the sides and forward. Cooling air is allowed in at the front and again, about half way down the damper pipe. This is probably one of the effective dampers but, the pipe is still long despite the dilution and baffling inside.
Another factor is that the flames do vary considerably with power changes.
Overall, flame-dampers did vary in effectiveness. I am not certain that Me 410 night fighters or intruders were all fitted with flame dampers, and their large 44litre DB 603 engines would certainly give a large exhaust flame, so possibly the Me 410 was weak in this respect.

Cheers

SM