Fatboy

possible options

Ness Big Sucker Air filter (best price I found was here at direct parts about $85ish), Power Commander (again, best price I found was at Direct parts, about $350ish), then some slip on tail pipes..... cost varies with what you get. You need something to adjust the fuel injection with less resistance from air filter and pipes (that would be the power commander)...if not your engine will get HOT (like burn up hot). This has other options, but this is one way of doing it..... you can also put in over $600 for a HD Race Tuner, or you can go with a couple other options that do not have as much "tunability". You can also increase the cost of the pipes by going true duals or 2 into 1.... reason they cost more is that they go from the engine to the end...slip ons are just that....they are about the last 2-3 feet of pipe. The junk that HD puts on to call air filter is a joke, so a K&N type filter by either screeming eagle or ness will help. Good luck and have fun!!!

oh ya, cheapest option

and for that REALLY cost effective option....take a long steel rod or broom handle, stick it in the stock pipes, and HIT IT REALLY HARD.....to punch a hole through the baffles.....have heard of this, tho do not know the exact results in tone, loud, or performance.