Sorry if this question seems a little retarded but when i bought my GTA a few months ago the guy included what he said was hypertech underdrive pulleys in the trunk.MY question is this:when i compare the crank pulleys, the one on the car is about twice as big and the alternator pulley on the car is smaller so which is the underdrive pulley?I have never seen any before so i am little confused.Thanks,chris.

 

Under drive is just that... something is "under driven".

True underdrive would be something that is less that 1:1 drive ratio. Like .9:1, or .8:1.

Since the drive ratio of a stock crank/alt pulley is overdriven at ~2:1, then anything lower is what they call "underdrive". Even if it's only lowered to 1.8:1.

Anyway...

Think of it like a 10 speed bike. When the chain is on the smaller sprocket up by the pedals, and on the largest sprocket in the rear by the wheel, it's very easy to pedal, but you won't go very fast. That's 1st gear. That's ~ a 1:1 drive ratio.

When you change to 10th gear, the chain is on the larger sprocket on the front, and the smallest on the rear. It's very difficult to pedal, but once you get going, the bike can go faster IF you're strong enough to pedal it faster than if you were in 6th or 7th gear. That's because it's an "overdrive". Although it allows the bike to go faster, it increases the strain on your legs.

That's the same concept with the "underdrive" pulleys. They bring the drive ratio closer to 1:1 to make it easier for the engine to spin the alt. This supposedly frees up HP. How much? It's hard to say without a dyno, but it's certainly not the "up to 25 HP" like they advertise.

So... to sum it all up;
The smaller crank pulley, and the larger alt pulley would be the "underdrive" pulleys.

 

For some reason i was thinking it was the other way.Aj thanks for the great answer.Yeah and i dont even plan to use these i just wanted to make sure that they werent already on there.