2 1/2 inch exhaust on a 318...

On the street you are looking at great variations in RPM range. In racing you are focusing on a realitively small RPM range. Also, for instance, in racing if the car is a little soft in leaving you might add a little longer collector. ie:you tune the exhaust...Not practical for the street. As in most things you do for the street, you have to make compromises.
Mac