do you need to buy the whole pulley kit or is the just the crank pulley by it self good enough?

 

it depends on what you want. the crank is where most of the hp lies. the other two are mostly for looks. it has been said that they will and another hp or 2. i say if you have the money then get the set. but in the end it all depends on you. if you get the underdrive you will need new belt which will be another $40 so keep that in mind also