The common term used to describe simple bolt on engine mods such as a high flow air filter, Hi flow mufflers, performance headpipe and ECM tuning to match. Typical power gains with Stage I mods are 10-14 HP.
A high flow AC is the very first thing required to start the process of waking up your stock sleeping Harley. For years, all non CVO Touring models came equipped with a very restrictive AC design that was targeted at sound reduction more than performance. Simply unbolting the stock AC and throwing it on the ground is good for 5 HP. What you bolt back on may cost you half of that gain. The garden variety stage I AC from S+S, Ness and Harley that accept the stock cover will all cost you about 1-2 HP of that gained 5HP as you bolt them back on netting you a final improvement of about 3-4 HP. The forward facing air filters with the large 90 degree elbows are the exception to this. The HD Heavy breather, hands down the best performing air filter. These types with their large plennums will actually show a gain in TQ with no loss in HP when bolted on an open throttle body in back to back dyno testing. Just an FYI.
The number one most purchased product for every HD motorcycle. Aftermarket muffler designs in this industry are all over the moon. We have rounds, ovals, skinny, fat, you name it. We even have drag pipes with fishtails, I call em Fish sticks. 90% of these are crap on the dyno because there was never any design effort to improve performance. The only objective was noise, and to make more of it. Why even call them mufflers? How about amplifiers, or a little truth, Torque killers. Mufflers that perform the best are hard to find, because in this industry of louder is better, they are the quieter ones. Air speed, not back pressure is the key. A large bodied muffler that has a continuous full length core between 1.75 and 2.25 will give the best results by maintain decent air speed. Bigger than 2.25 and the bottom end power drops dramatically. The larger the muffler body, the smaller the core can be and still make good power. The larger volume in the muffler body gives the pressure pulse a place to go with out having to actually exit the muffler. Check out the muffler on any stock 1000 cc Crotch Rocket. Huge muffler, pea shooter exit, ludicrous HP, quiet and stupid fast. Yes, we can still learn from things we hate that pass us at twice our speed.
Headpipes and cats!
If your Touring Harley is 2010 and newer it came with a restrictive cat headpipe. How restrictive? The first 96" 09s that I dynoed made about 65 HP off the showroom floor. Hi flow AC, mufflers and tuning bumped them up to 75HP. Removing the cat after that only netted about another 2 HP. Safe to say the stock cat headpipe can support about 70 HP. Conduct the same test on a 110" CVO and the gain or loss is about 5 HP. Now compare the stock cat headpipe to a properly designed tuned X Pipe or 2 into 1 and your looking at about 8-10 HP between stock. As power levels rise, so does the power loss from the cat and the potential power gain from getting rid of it.
Fuel injection. ECM remapping and tuning. Closed loop?
More air = more power, right? Not with out maintaining the proper air fuel ratio. The stock ECM will adjust and even learn to compensate for minor air flow improvents made within its "closed loop Range". Operating in "Closed loop" is when the ECM is receiving and using live data from the intake and exhaust side (02 sensors) of the engine to calculate and maintain precise target AFR. Closed loop operation is primarily active during steady highway cruising. Grab a handful of throttle to pass and you are no longer in closed loop! You are now running on fixed fuel calculations that better be matched to the aftermarket parts you added or your engine will run extremely lean, down on power and begging for fuel.
Stage I summary.
So, if all you did was add some hi flow mufflers behind a restrictive stock cat headpipe to get some rumble, your going to be fine with out remapping the ECM. Add a hi flow AC and remove the cat, you have crossed way over the line and are running seriously lean at heavy throttle. Not recommended unless your willing to ride like Granny at 1/4 throttle and in closed loop everywhere. Not the way I roll.