[cobred]

You should also check what mufflers are inside your sidepipes. When I first built my car I bought the sidepipes from ERA and didnt ask anything about mufflers. The ones they gave me had 2" mufflers inside the pipes. A few months later I bought a used set with 2 1/4" mufflers from ERA then a year later I sent the first set to the exhaust shop ERA uses and had 3" mufflers installed.
Each change made a difference but I made other changes at the same time so its hard to say how much of an increase but what goes in must come out.