Lowering rotational mass do you not lose some torque tho?
No, torque is a factor developed by the engine, less resistance = more torque.
When dyno tuning certain track/race cars we often remove the alternator belt to see what the difference is as they donr actually need them for short bursts of power such as drag strips. A gain of much more than 3-4hp is rarely seen, this is removing not just a percentage in drag from the pulley but the actual entire drag of the alternator.
On aircooled cars such as beetles where the belt runs a large fan, the belt being removed adds 30+ hp in some cases, on an engine that makes 120hp! (car we tested this on:
http://www.dyno-tuning.co.uk/gallery/vwBeetle.htm )
personally I cant see these lightweight pulleys adding any real power or bhp, or mpg. If I was building a high spec £15k+ engine for competitive race use then I may consider testing them, but for a road car then I find them pointless.
If I was a retail operation looking to make high profit margins from shiny bits of metal, selling such parts to car enthusiasts would be very very easy. Unfortunately we get a large amount of customers visit us for 'dyno test -fit part - dyno test' type work and often my sceptisism was well placed
