Question:
Why only a 2oz header fuel tank used for RC nitro Heli's.
I am guessing it has to do with:
#1. The location under the Cowling limited space and
#2. To promote cool down periods for the Engine to extend
Life Run Time, (TBO).
I am looking at installing a
S380 Ducted Fan Fuel Tank 11 oz as a exterior Saddle tank
to one of My Scale MD-500E Nitro Heli's. It is curved to
fit to the exterior fuselage and low profile.
Has anyone tried this before?
Currently I get approx 13-14 minutes of air time using
stock fuel tank with the 2oz header tank and like many
wish to stay Airborne longer.

Header tanks aren't used for longer run times. They are to provide an air free feed to the motor. Most rc heli header tank users quit flying when the main tank is empty. If you use a 11oz header tank, you'll be defeating the purpose of that tank by running it with air in it (after the main tank empties.) There's nothing wrong with that, you'll just be running a 2 tank setup. Not a header tank setup.

year ago I run 2oz header tank just for that reason, the fuel tanks on my old nitro birds were 24oz and was easy to get any air bubble on the line, header work great.