Why is the S15 6-speed not desirable?
the S15 6-speed is 6 gears crammed into the same transmission housing as the S15 5-speed. this obviousley means that the gears have to be thinner in order to fit that extra one in there. since the gears are thinner, they tend to break alot easier. but, as J Spec said, sometimes the tranny can handle power and be fine. there ARE ways to strengthen it as well, but generally the best choice for strength is to use the 5-speed.