I purchased this kit to go along with the other components of the Stock to Brock's package. I haven't run the bike on the dyno with and without these stacks, so cannot say for certain how much better they allow the bike to run. I trust Brock's judgment when it comes to tuning a motorcycle like this and just went ahead and installed them at the same time as the rest of the mods. I can say that the stacks are of great quality, right on par with the stock tubes. The kit only includes 2 stacks, as 2 of the OEM units are retained, though in different locations. I had a lot of trouble fitting them into the airbox, as the rubber is very stiff. After struggling excessively to install them, I was told by another rider to heat them with an air dryer until they are pliable enough to go through the holes easier. DOH! Anyway, hope this helps someone else out there who has the same troubles I did. I'm glad I went ahead and installed these while I had everything apart. Brock makes neat stuff.
This part is just one component developed to increase the performance of the Z900RS. How much it contributes to the overall performance when used with rest of the "S2B Performance Package” is hard to quantify. I'm sure Mr. Brock can tell you exactly how much it contributes to the overall performance. Myself, I wanted to maximize the performance of the motor and initially purchased and installed the full Street Meg exhaust before Brocks released the “S2B Performance Package”. Then as I went to purchase the ECU Flash with Fuel tune I discovered the velocity stacks and decided to give them a try. So, with just the exhaust I found the midrange and top end rpm performance to be phenomenal. I then received the ECU and the velocity stacks and installed them. The performance gains from these components combined is nothing short of amazing. The dyno graph on Brocks site is impressive but to feel the performance gains is something entirely different. The bike’s motor performs well but is docile, with the “S2B Performance Package” it is a completely different motor, might I even say aggressive. Aggressive in a good way, the drivability is still good though the throttle response is instant and requires re-calibrating your right hand to adapt to the new instant acceleration. So how to describe the increase in performance? The low end is as strong if not stronger than stock, throttle response is instantaneous, the midrange is quite a bit stronger and now the motor just revs freer all the way to red line. The top end pull is so strong and fast you must pay attention to the rpms or you will hit the rev limiter before you know it. Absolutely incredible Mr. Brock, now the motor’s performance matches the looks of this incredible motorcycle.