they got it wrong.. (although i'm from utem myself
)
car alternator doesnt operate on 2:1 pulley ratio...its more than that.. probably atleast 3:1 or 4:1 (see the alternator pulley size vs crank pulley size to get the idea)
2:1 is crankshaft to camshaft ratio....
yes u can change the alternator pulley into smaller one (or bigger crankpulley) to increase the rpm ratio between those 2...
problem is on the market, most is people selling under-drive pulley (which lower the ratio and effective rpm of alternator) for race purpose and use it on the street (wrong concept)....
the reason of under-drive pulley invented for racing purpose, race engine never saw rpm lower than 4.5k rpm...it will spend most of the time at high rpm range..so to reduce alternator speed damage (as well as power steering pump)...race car build install the under-drive pulley....
installing under-drive pulley will make alternator (and power steering pump and ac compressor) working under its designated speed make it worst in slow moving traffic or on idle during traffic jam...
what people on the street is over-drive pulley...
in this jz bimmer case, is ideal solution (if not changing bigger amperage alternator)....but since sourcing the over-drive pulley which definately need to be custom made is freakin expensive, then bigger alternator is the simplest route to follow... because bigger alternator is more common mod especially in car audio shop...