One thing I do want to explain before I get too far into this is backpressure. I hear "You need backpressure to make torque." all the time. And it even comes from "tuners" as well as customers. This is flat out not true. Before I arrived into the company of true tuners I fell into the same trap myself. The reality is that backpressure is the enemy. You want to keep it as low as possible. What you do want is to keep velocity up. However doing the things that keep velocity high involves slightly more backpressure under some conditions. You want to keep the gasses moving as quickly as possible to make both good torque and top end power. The perfect exhaust system would keep the gasses moving as fast as they did coming out of the cylinder and have zero backpressure. However this is impossible to achieve in the real world.

Pipe diameter does have an effect on flow rates as well, but again it is not the major factor in most cases. 2.5" may flow enough for 300-350 h.p. without being a restriction. 3" is usually capable of flowing 500-600 h.p. before becoming a restriction. This is assuming that you have designed the rest of the system up to par. There are also full 3.5" systems and those that start out at 4" and taper down. Unless you are making over 500-600 h.p. anything over 3" is a case of diminishing returns and in most cases has no advantage. There is more to gain going from 2.5" up to 3" than there is going from 3" to 3.5". A 3" system will not loose torque compared to a 2.5" system if designed properly. In fact if designed properly 3" may be capable of making better low end torque than 2.5". Again, since the way to make the most torque with a turbo exhaust is to get the turbo to spool-up as quickly as possible, it should be the main goal of the entire exhaust system and good flow after the turbo is one way to achieve it. We use 3" as we want our system to flow enough to be capable of coping with a customer's changing goals. Properly designed we can offer it to the big power crowd while still appeasing the low end torque club.

The only reason to reduce the size towards the end of the pipe is for packaging, cost, and noise reasons. Tapering the diameter does not make more power, torque, or bring on boost faster. However having smaller pipe towards the end has less effect that having smaller piping at the beginning. In other words a system that has 3" pipe for the majority, and necks down to 2.5" at the end will flow enough for more power than a complete 2.5" system. The further downstream you neck down the exhaust the better……..if you decide to neck it down.