Originally Posted by agraham2252

I was looking at the specs yesterday for the 550i and the 550GT. For the 550i they say:

4.4-liter BMW TwinPower Turbo V-8, 32-valve 400-hp engine. Combines two turbochargers with variable valve control (Double-VANOS and Valvetronic) and high-precision direct injection

For the 550GT it says:

4.4-liter, 32-valve 445-hp V-8 engine with TwinPower Turbo technology, piezo direct fuel injection, 4 overhead camshafts, and Double-VANOS steplessly variable valve timing

I believe the key is the twinpower vs the two turbochargers. That should be easy enough to spot. However, I am still waiting on my car that was caught up in the Sandy delays.

There is a nice explanation on why the single turbo twin power is more efficient at:

http://www.popsci.com/content/two-one-turbocharger