I just installed a de-catted stock head pipe today with stock mufflers. See my post in intake and exhaust section with videos before and after.
It's my understanding that as long as you don't mess with the intake/air filter, a tuner shouldn't be needed. The air/fuel ratio coming into the combustion chamber hasn't changed, no matter how much you flow air out the back(within reason) ..... it still has to come in from the front.... and the air/fuel ratio is mixed per HD specs up front before going into the engine.
With no baffles in the mufflers, the exhaust flow might increase marginally, but the incoming charge of air is still mixed to HD specs (kinda lean) ....regardless of flow going out (within reason). As long as it doesn't overcome the specs HD has programed in for the intake charge, no tuner is needed.
It's my understanding, simple exhaust mods won't exceed these stock specs, without combined intake changes.
If you have extensive engine mods (cams, intake, turbo, supercharger, head mods, larger valves, etc) where you might exceed the preset intake charge specs, a tuner would be needed.
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Now, the factory a/f ratio is kinda lean, they say, so a tuner might be a good idea to richen up the a/f ratio and help cool the engine and prohibit bluing.... at a slight mpg cost.
With completely open baffles in the mufflers, you present a different problem.
All street engines (unless full-on drag racing where wide open throttle is needed all the time), require some backpressure. It improves low end torque for rideability . The huge decrease in backpressure causes the exhaust gases to not escape as designed, and you will experience some popping on deceleration. I can't explain it perfectly because I don't understand it thoroughly ..... but in a nutshell, raw fuel is sucked out of the intake charge during the exhaust cycle and it burns in the pipe or head due to the hot valves or other hot surfaces it encounters on deceleration.
I'll have to google that........
As stated above, some backpressure will be beneficial . Google it.
EDIT: Just copied and pasted from another forum about popping on decel. Some highlighted items to better explain.
...........This is exactly correct! If you think about the mechanics of what is going on upon decel, it makes perfect sense:
You start out with an engine that is turning high RPM with a carb that is open, flowing freely. Next, you close that slide. This has the effect of creating a tremendous vaccuum in the intake manifold (CHOKE!). This vaccuum does curious things:
1. It pulls air and fuel from wherever it can--including the idle and low speed jetting in the carb.
2. To counteract the vaccuum, air will take the path of least resistance--including sucking exhaust gases back into the combustion chamber during valve overlap (the brief period of time that the exhaust and intake valves are open at the same time.
So what does this mean? Well, now you have a disproportionate rich mixture heading for the cylinder AND what little oxygen is drawn in is further diluted by gases that are coming back into the chamber from the exhaust valve. The result is poor, incomplete combustion! Depending on the temperature of the exhaust pipe and gases, reignition is possible once this mixture is introduced into a more oxygen rich environment. This produces the "pop."
Why does a lean running engine have a tendency to excessively pop? The exhaust gas temperature of a lean engine is substantially higher than one that is running on the rich side ( an EGT probe will show this). Therefore, the high EGT serves to heat the pipe more and the gases themselves become a much better source of re-ignition. By richening things up, you are cooling down the exhaust temp and therefore reducing potential re-ignition sources.
Any 4 stroke engine--even those that are flawlessly tuned will pop on decel. It is harmless, but can be annoying--granted. However, I would never alter the jetting of an otherwise performing motorcycle to get rid of it. Go by the seat of your pants and the color of your spark plug rather than the popping as an indication of the jetting of your scoot!
