So with the underdrive pulleys you get one idler pulley to swap out. But which one do you change? There's three idlers and they only send you one new one in the kit.

 

Swap out the tensioner pulley... or do what I did and order 2 more idlers and swap all three...

 

Are pulley's really worth it, I have gotten mixed feedback on the subject? Kinda like going with a stage 2 CAI, the MAF on the 05's is huge.

 

Umm..I can understand changing the crank, waterpump and alternator pullies...but what performance gains do ya get from changing the tensioner pulley?

 

According to MotoBlue, Saleen, and the other companies that make the UD pulleys, it's supposed to free up 'robbed' horsepower from running the electronics. Since I put mine in along with a CAI, throttle body, and tune, it's impossible to tell if there's any direct HP coming from the pulleys. It's a sound theory though, so I think they provide some benefit. Still, the most they give is like 10 HP.

Oh, and about the tensioner pulley specifically. The new idler pulley Motoblue supplies is supposed to produce less friction on the belt, allowing it to travel faster, which is where the extra HP comes from. It's the same with any other idler pulley on the belt system though(there's 2 more).