I have a 2010 FXDC and have had 2 different sets of pipes on the bike and of the last set I had 2 different baffle sizes. The first set of which I can't remember the name was 3". When I came off the throttle it would backfire some. Plugs looked very good though. With the 2" and 1.5" Rush pipes no backfiring and plugs are the same as the 3" I think engine runs a little cooler with the Rush pipes. I was told that 2" or less baffle and you don't need a tuner. I have bore scoped the bike several times and all looks good inside.