Just for my own sanity..... when we switch to an aftermarket pulley kit like what banzai offers, shouldn't the alternator spin faster?

For something to be under driven, doesnt that mean it spins slower than normal? The main pulley is larger than the alternator pulley and it's also the driving pulley, so shouldn't that mean the alternator is actually being overdriven/spinning faster?

Just want to get my terminology right, since I'm reading some stuff that's conflicting with my understanding of what's going on.

 

If it's underdriven, the crank pulley is smaller so it moves LESS belt per rotation. Usually a kit will include a slightly smaller alternator pulley so it rotates quicker to offset the difference. Hopefully with that it will still charge at idle.

In theory you should gain some power because all the accessories are spinning slower, requiring less HP to turn them.

Vince

 

That's why there's a difference between "lightweight" pulleys and "underdrive" pulleys. Just lightweight and your accessories still spin at factory speeds, underdrive and they spin slower. Under and over drive deal with the ratios between the pulleys, like with gearsets. Smaller alternator pulley will do more rotations for every one of the crank pulley. Changing the diameter of either one one way or another changes the ratio - like the CVT.
Then there's the other factors. Increase the electrical load and the alternator has to work more: remove water pump and go EWP, alternator needs to work a lil more, upgrade the alternator, etc....

Prob preaching to the choir here but...i'm drinking...

 

Our kit has a smaller diameter main, that underdrives the accessories

 

Yeah I wouldn't run one personally on a road car, the FD struggles enough power wise as it is without under driving the auxilaries. I've got Banzai pulleys with an oem e-shaft pulley.

 

I'm running an underdrive pulley on the eccentric shaft and a stock alternator & PS pulley. I didn't feel any difference while driving but did notice that the car cranked a little faster/easier on startup. As far as battery/PS feel, everything's been working 100% and feels normal. I've had the pulley on for two years.

 

The advantage of underdrive pulleys is mainly seen in race cars that are constantly operating in higher rpm ranges. It is less about freeing up power than it is to prevent overspeeding the alternator and water pump. Overspeeding can see premature wear of the alternator and, in particular, cavitation in the coolant, which can be very bad for cooling performance.

The OEM pulley gearing is designed for a good balance across the full rpm range and is best if that matches your driving habits.