I love it. Spoken like a true HDF veteran.

 

This is a common misconception. Because they are narrow-band sensors they have a very limited range of air-to-fuel ratios over which they can report (essentially ±3% from center). This does not mean the ECU is itself limited to that range. In fact, the ECU can control fuel nearly 10 times that range to get it into the window which the O2 sensors can use. What's even neater about the system is that it "remembers" the offset it needed to do that, and uses that same offset in the operating areas of the engine where the sensors cannot be used (because the design air-to-fuel ratio is out of range).

This is autotuning. The only downside to the system is that it's absolutely imperative that the ECU calibration be correct insofar as whatever the needed fuel calculation offset was in the closed-loop area is actually the same in the open-loop area.

Said another way, and in a manner which actually pertains to the thread-starter, the fueling calculations are based initially on tables of values which indicate the amount of airflow through the engine at various engine speeds and throttle positions. If whatever it was that caused the calculated fuel value to be out of range of the O2 sensors is the same everywhere (maybe a change in altitude or fuel additives, for example) and, here's the important part, those airflow tables are accurately populated, the correction factor will be "correct" everywhere in the operational range of the engine.

If the reason for the calculation errors was that the engine has been modified, unless those modifications resulted in the same offset (percentage-wise) in every area from the programmed tables, the "assumption" the ECU makes in the areas where the O2 sensors are not used will not be correct anymore. It might be close enough, but that's an assumption the operator is making. Monitoring the exhaust gases for content would provide the most accurate answer, however, failing that ability, carefully and accurately reading the plugs would suffice.

Changing out just mufflers on the stock pipes is generally considered to be safe to do, though I've not seen actual (measured) data to support the claim. It would certainly be dependant on the type of muffler at any rate. I know H-D says no remapping is necessary for their slip-on mufflers, and it's got to be safe to assume they know what they're talking about.

Changing over to a different exhaust system type is another matter, however. Although the ECU will no doubt accommodate the changes while in closed-loop operation (though the success/ease of doing so will have to be not as great or simple as it could be), all bets are off whenever the throttle gets opened past 40% or so. I would not do it on my own engine and would strongly discourage anyone else from doing the same.

If you're going to change out the exhaust system entirely, it really becomes necessary to reprogram the ECU's fueling calculation factors in some way.

When intake changes are considered, H-D says their assembly requires ECU recalibration. I don't believe it's because the extra airflow exceeds the range of what the ECU can account for [i.e. it would not just become "too lean"], but rather because it changes the shape of the airflow tables. When I say "shape", I mean the contours of a graphical representation of the table. If the table were merely raised or lowered with no shape change it would be no problem (and this touches back on what I said earlier about the mufflers vs. headers).

I forget just now what the original question was and it's not present on this web page. I'll just assume it was "do I have to make fueling changes for just a true dual exhaust?" My answer is "yes." And if you're going to do

 

With what you have now, you don't need to do anything else. But... with the next upgrades (breather, etc) depending on your budget, I'd go with either the PCIII (and block off the HD 02 sensors) or the Thundermax Auto Tune. The HD SERT is junk with the limited tuning of the narrow band HD 02 sensors.