whats the point of increasing the pipe size when all of the std pipe work after the UBP is still 2.25"? the one I used was 2.25", not 2.5" like process west... its not about volume, its about restrictions in diametre, and there are none in the standard pipe, cut one up and measure for yourself. it may Look bad from the outside, but its not a problem... internally its a smooth shape transition if you already have a cooler and are looking to get a tune, your better off throwing that cash at a high flow cat ( now that will help you make more power off less boost, compared to an UBP which is a marginal gain at best)