OK well I really didn't believe that in transplanting my 2.5L engine for my 3.6L X51 motor that it takes 15 more hp to get the power from the crank to the wheels.
I think I found the answer. It turns out that the 15% loss has to do with the rate of acceleration of the large drums on an inertial dyno. On a load control dyno, you assume a constant amount of parasitic drive train loss.
A good explanation can be found here for those that are interested.
http://home.earthlink.net/~spchurch/churchautomotivetesting/id12.html
Todd