I had my engine dyno'ed with the side pipes attached. My side pipes were specially crafted by Stainless Specialties to add just a little extra baffling but still provide a satisfying exhaust note and rumble. But, those side pipes rob you of a good 75 horsepower and I wanted a true number from the engine. You can play lots of games with a dyno, but putting side pipes on the engine when you test it is the big crippler. At my local club though we have a set of shorties you can borrow and bolt on if you're doing something serious. You have to wear hearing protection though when you do.
