Alright, let me get this straight. If you change your exhaust you run leaner. How? It doesn't matter if you run mufflers or straight pipes. If the engine can't take in more air then it can't run leaner. It can't take in more air if you don't change the air filter. It has nothing to do with how much back pressure you get with mufflers versus just baffles. It all has to do with no more air in means no more air out, which means no more fuel needed. Please prove me wrong. I need documented, tested, proven proof to change my thinking. Not just what some one thinks or what the dealer told you.

 

Your stock air filter flows the same amount of air as any aftermarket filter...or no air filter...as long as you're not asking for more than it's flow capacity. In other words...for normal riding, your air filter is not restricting your engine's ability to breathe. Ultimately this makes your theory nonsense.

 

Like I said, show me the proof, as in flow numbers. I want to see more air flow on a chart. I mean airflow thru the engine. Not just at the exhaust tip because that measurement is useless, because of the lack of baffling when changed to and aftermarket exhaust. Not just what you believe and I will gladly change my way of thinking. So what you are saying is that backpressure from the exhaust controls air fuel ratio.

 

My two cents. I put on the Rush 1.75 slipons last week on my 09 FXDL. No air cleaner, no downloads. No popping, bike runs great. I did the whole thing on my 06 Ultra with a PC3 and the biggest difference was very poor fuel mileage and a lot of money. So my take on it is don't go to big on the mufflers and don't do anything else and save a bunch of money.

 

i removed my reply... when I re-read, it sounded like i was bein a smart *** and I wasn't.
sorry if it offended anyone.

 

I think you are asking for the book. I would not do the subject justice. The easiest way to learn, perhaps, is to plug the terms into the search engine or google or WIKI and read learn the results. There are a number of sites as well as the FAQs over on the tuning forum ( https://www.hdforums.com/forum/elect...-injection-55/ ) Perhaps check out nightrider.com or http://tuneyourharley.com/biketech/ . It could take days if not weeks for you to gain some understanding, I hope this helps you get started.

 

Bought straight shots 4 my fxd & I was told I would'nt need to retune only if I did a intake & pipes, right or wrong ?

 

Yes it does help a lot thanks I heard that some mufflers do not require remapping...like the Rush slip ons with 1.75" baffles....do you know if that is true?

 

I read the hdforum link you posted. Jesus EFI is just a scary thing lol...I love Harley because the bikes look like they never changed that much...except for the EFI I don't like too much technology....did they put EFI to make the bikes more reliable?

 

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.