unless your upwards of... i think 500 horses ... i can't remember there is a recommendation chart somewhere... unless your upwards of 500 3 inch is over kill... i have 2.5 inch on my little sixxer and for the little cfm it has its way over kill...
side note... if you do a single turbo 3 inch is average size
+1 back pressure is a myth..... I learned this on here .... go to the sticky on the dyno info and you will see some numbers.....from what I have heard here you do NOT need back pressure like a lot of people seem to think
3" isn't going to hurt you but dollar per horsepower is probably not to your advantage with a mild set up. The only thing you need to worry about when it comes to back pressure is eliminating it. Contrary to popular belief you do not need back pressure. If you did these exhaust companies wouldn't be spending thousands of dollars every year designing better exhaust systems that eliminate more and more back pressure. Back pressure is an obstruction.
"BACKPRESSURE = TORQUE?
An old hot-rodder's tall tale: Engines need some backpressure to work properly and make torque. That is not true. What engines need is low backpressure, but high exhaust stream velocity. A fast-moving but free-flowing gas column in the exhaust helps create a rarefaction or a negative pressure wave behind the exhaust valve as it opens. This vacuum helps scavenge the cylinder of exhaust gas faster and more thoroughly with less pumping losses. An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of this scavenging effect, which has the greatest impact on low-end torque.
Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters. The only two exceptions to this are turbocharged engines and engines optimized for large amounts of nitrous oxide. Both of these devices vastly increase the exhaust gas volume and simply need larger pipes to get rid of it all."