the size of exhast really would depend on the number of horsepower that you are making and the type of bends created.if you are running less than 500 whp then a 2.5 inch exh. is plenty sufficient if it is mandrel bent.If you half to run crush bent piping,then run a 3 inch to compensate for the size difference along the crush bends.2.5 inch makes it easier to fit on the bottom of your car also.You really wont see the full potential of a 3 inch exhuast untill you reach upwards of 700 whp.