If you're looking for efficiency check out the new brushless technology for powering your props. They are basically mini 3-phase motors that offer tons of torque for their weight and size. In combination with Li-Ion batteries, their unbeatable! Cheap too!

Here's just an example, Lots to choose from....
http://cgi.ebay.com/1700kv-Helicopter-Outrunner-Brushless-RC-Motor-30A-ESC_W0QQitemZ120290659490QQihZ002QQcategoryZ84444QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

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Current Stable:
01 ML55 AMG
92 500E (a few mods)
87 300E (lots of mods)
00 Chevy 3500HD Diesel Box Truck
68 18' Donzi Marine
06 GT i-Drive7 1.0 Mountain Bike (with GPS!)

PREVIOUSLY OWNED:83 300SD, 87 420SEL, 88 420SEL, 90 420SEL, 86 560SEL, 86 190E 2.3-16V AMG, 94 E320