I currently have underdrive pulleys on my SC GT. I know I need to swap them out but I was curious whether it would work to just swap my crank pulley from the UD to the factory one and keep running the UD alternator and waterpump pulleys. Or do I need to swap all of them out to get the SC back up to speed?
I was also looking at the two crank pulleys, and I might be wrong in this but my factory crank pulley is bigger than my UD crank. Wouldn't the smaller pulley spin the belt faster than a larger one?
Just wanted to get some opinions before I start pulling things apart.

 

You want to do them all. Think of a motorcycle. If you change the rear sprocket to a bigger one it gives you a lot more torque. Now if you change the front one bigger it gives you more speed. When changing pullies you want to make sure they are all matched up closely. You can overspin the alternator and end up buying a new one every month. Lose boost on the sc. Not spin enough coolant thru the water pump and run hot ect. Not to mention you have trial and error with belts

 

Change all the pullies back to stock. And bigger crank pulley means Belt spins faster.