from what i can see from a little rescearch is that HRC stacks are longer, but factory pro stacks are shorter. still checking on erion's used parts list. and if longer is better so you say(i only say that cause i have noanswer at this point)then you should be able to run a set of 900rr or 954rr in the f4i and it would be suffice[sm=dontgetit.gif]

 

gots sum mo info
http://www.superbikeforums.co.uk/ind...7486.msg591273

 

longer is almost always better, hence the term 'stack' in velocity stack. Its supposed to increase the velocity of the incoming air by funneling it through a longer tube, probably not much if any difference. all kinds of engines have aftermarket stacks available for them, although its a associated with TBI or a carb 99% of the time

 

well after my rescearch i think im going stay with the stock ones not much of a performance gain with a little sacrifice of my topend