TTTHOMAS3

Why would it be nesessary to get the ASP or SFI or MACH underdrive pulleys, and is it worth it?

2_wacko

iTrader: (2)

Get the ASP and yes it is worth it.

LS1LT1

iTrader: (16)

It's not necessary at all, you can do without them if you so choose.
But there are gains to be had by installing an ASP or March underdrive pulley so yes it's worth it IMO.
And SFI is just a certification/racing safety thing that gets stamped on the pullies, not an actual type of pulley.