Originally Posted by speedbump
I have never heard anyone say serpentine belts are less efficient, in fact, just the opposite although the difference either way is probably negligible.
Go back and re-read my post. TRUE serpentine belt drives, like the factory installs, do, in fact, have less drag and use less HP. The factory serpentine drives typically use a reverse rotation water pump and LOTS of belt engagement on the pulleys to get the same drive force with less tension in the belt.
Aftermarket conversions for engines like the Pontiac that don't have reverse rotation pumps available rely instead on three or four idler pulleys to force the belt against the water pump pulley (and the other accessory pulleys). The sole purpose of these idlers is to increase the force on the belt, making up for the fact that there is only a limited amount of engagement available between the belt and the water pump pulley. This extra force must be reacted somehow, and that reaction takes the form of added friction and thus more HP required to turn the accessory drive. Is it a large number? No, of course not, but the whole point is that you are spending somewhere well north of $1,000 for a pseudo-serpentine drive that provides exactly NO performance improvement but increases complexity (due to the additional idlers).
Again, factory-style serpentine is fine. Aftermarket "pseudo-serpentine" is a waste of money.