2.5" is way to big for a inline 4. I had a V6 camaro with 200 hp and for that much power 2.5" is more than what that engine needs.I had a 2 1/4" intermediate pipe stock on the camaro. Went up to a 2.5" for better performance. You will lose a lot of low end torque and barely any gains with such a big pipe. Take it from magnaflow and borla they know there stuff because they are the biggest muffler makers in the US. Here go to dynomax's website and read all about the pipe sizes and why it can hurt your performance.
DynoMax Performance Selection Guidelines
Getting the most out of your exhaust system modifications is easy. Here are some common-sense ways to maximize your horsepower. . . with DynoMax!
Restrictive Exhaust
A restrictive exhaust creates backpressure in the exhaust system that hurts your vehicles performance in two ways:
The engine has to work harder to force exhaust gases out of the cylinders.
Engines cannot scavenge burned exhaust gases out of the cylinder efficiently. This leaves exhaust gases in the cylinder to dilute incoming air/fuel mixtures and rob your engine of horsepower.
Pipe Diameter
A common misconception is that the larger the diameter, the better the system. But bigger isnt always better. Systems that are too large in diameter can actually hurt performance.
As a general rule, switching to a performance system that is 1/4 to 1/2 inch larger than stock will provide you with the best horsepower increases. To determine which pipe diameters will be best for your system, decide what RPM range your engine will operate at, most of the time. Smaller diameter pipes will produce low- to mid-RPM torque. Larger diameters produce mid- to high-RPM torque.
Horsepower Depends on Flow not Pipe Diameter Alone
Bigger isnt always better. Systems that are too large in diameter can actually hurt performance. Improved flow is what you need.
Having large diameter pipes isnt whats cool. Going fast and making horsepower is.
http://www.dynomax.com/techsupport.stm#TechTips
