Originally Posted by ColdCase

There a number of books out there written on the subject. Perhaps you may find a few at your local library. There are more graphs and studies than you can shake a stick at. It is just not that simple.

The nutshell is that the engine is basically an air pump and the factory tunes the entire system to maximize its efficiency (so called volumetric efficiency), everything from setting up intake harmonics to push fuel and air into the engine just when the valves open to setting up harmonics in the exhaust to pull (scavenge) the exhaust mix out just when the exhaust valves open. The better the efficiency the more power/torque/fuel economy. The computer uses sophisticated algorithms and several sensors to determine the optimum time to inject fuel and spark for the volumetric efficiency characteristics of the entire engine system. But then life is a compromise, there is EPA and noise rules to contend with which places some constraints.... and even the bean counters get involved.

Regardless, any time you change one component or another you mess with the tune, the harmonics, the efficiency of the pump. It just turns out that opening up the Harley exhaust typically makes these engines run a little leaner (unless you run a tuned exhaust). Changing the air filter (they all offer resistance to air flow in one form or another) typically has more of an upsetting effect on the Harley, tending to even more lean conditions. The factory provides a map to bring the engine back into EPA compliance given the factory stage mods, but it does have the EPA constraints and that tune will not get the most out of the components.

The ECU can adjust closed loop operating parameters to the point of running OK and avoiding damage, but the VE of the engine is now different, so the math is all off. A tuning tool or pro determines the new VE, determines the right parameters, and applies the right math to bring the engine back to the optimum for the components.

Sorry so long winded, but its just not that simple a subject. It is a many faceted subject, facets with complex physical relationships that you need to be willing to read a lot to understand completely.

Throwing on a slip on or changing the air filter hurts the engine performance unless you also re-tune the fuel and ignition system for the change in VE. It turns out that exhaust pipes have just a small effect on the tune, most don't bother with a tune and live with the torque reduction. Replacing the stock air filter with a free flow air filter affects the VE a little more than an exhaust, replacing the entire filter assembly has much more effect. The sound of an open exhaust is one thing, but why hurt performance by putting a filter on without a re-tune, is it looks? I dunno.

BTW we have EFI because its the cheapest way for Harley to build a bike that meets EPA regulations.

Oh, and one more thing, every bike off the assembly line has a different VE, not one is the same. The factory MAP is designed to make sure the worst case bike meets EPA compliance with some margin. That is usually why one bike feels great while another just feels like it is off power.

Originally Posted by FireRyder09

09 Street Bob is about to get her Vance and Hines Short shots pretty soon and either a SE stage1 or Arlen Ness...haven't decided yet. So Im doing my best to stay away from letting HD have my entire bank account and all I was going to do was let them remap it with the new accessories and call it a day. The PCV will have to wait some time. Then I go to the local shop near the house and the guys say that they can do the pipes and put a K&N Filter (skip out on the SE stage 1) on and be done with it. No re mapping just letting it run around for about a hundred miles or so to see how it adapts to it. This a route that anyone has done before or I am just headed down a road of future engine damage? Gracias

Originally Posted by BugDr

Thanks ColdCase, that is the best explanation that I have read. I have been hanging back and reading about everyone's exhaust, air flow, and tuning modifications and trying to decide where to start with my 2009 Street Bob. I now have a basis to try to understand which way to start. I still have a lot more to learn, but at least now I have a better understanding of the balance of the system.

Originally Posted by ghostd00d

I just got done talking to a master mechanic for HD about the issue. He works on the side for the Indy that does the work on my bike. If you are not messing with the air flow and just the slip on mufflers no remap is necassary. Once you mess with the air flow the bike will run lean and hotter. Some bikes don't mind it...others will complain loudly. You can go to the stealer and pay 150 or more for the SE downlowd....or just buy a PCIII for 225 from fuel moto, tell them what your mods are and they will ship you a custom map. Either way, once you change from the stock air to a AN Big Sucker or SE you will need to modify the ECM in one way or another.