Factory pro has rule of thumb stuff to get you in the area, perhaps right on. I like what sean barney said about the tuba and I hear him a little but i think factory tunes for emissions and quietness and all around rideability for your average Joe and everything is a compromise. I kinda remember air intake restricts before exhaust, but opening up your exhaust will make it run leaner (because more air). And more air is more horsepower. And so after intake, jetting, and exhaust, the next improvement (still a compromise, you might loose some idle or low end torque), is a cam change.. Then you could start flowing your head and changing valves but that's out there with higher compression pistons, bigger bore, stroke... Cheapest hp gains are bigger displacement bike to start with.
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