Your transmission tuning is a big deal. If you are running a soft, or weak center spring, your transmission is changing too fast. This could be the difference of pulling wheelstands off the line, or your bike being a total dog until 15mph. I've seen it all too often. The red is a 35% stiffer spring, and is best for higher rpm bores. . Keep in mind the stiffer the spring, the more you need a zx pulley. The oem S pulley can break due to the higher tension of the stiff springs.