As delivered, nearly all production motorcycles are tuned to run as lean as possible to meet emissions standards. They often run miserably until fully warmed up as a result, misfiring under any kind of load. To meet noise standards most manufactures will introduce a lean spot in the power curve at whatever rpm the noise test is accomplished at. By accessing the ecm through something like a Screaming Eagle Supertuner or TTS Mastertune you can change the look up tables that determine how much fuel is delivered at a given rpm and throttle opening, air temperature, coolant temperature and manifold absolute pressure. You can also usually go in and change the spark advance on it's own look up table. What usually happens with a stock bike is whoever is tuning the bike will richen the mixture closer to 13.0 to 1 for best power (at the expense of emissions, stock air-fuel ratios, AFRs, are usually around 14.2 to one except for full throttle and high rpm) and using a dyno you can see what the air fuel ratio (AFR) and power are doing at every rpm to tune out rich or lean spots that lead to dips in the horsepower and torque curves. The engine and exhaust have enough airlow in stock form to burn more fuel and make more power, but you have to program the ecm to deliver that fuel.
Since the SE pipe has to be street legal, it won't change stock airflow. To do so would put Harley Davidson in violation of a stack of laws in the US. If you install an exhaust that changes airflow, you really have to remap the ecm or the bike will run very poorly. This type of fuel injection does not measure airflow into the cylinder head directly. It reads rpm and throttle position to determine the basic length of time the injector remains open (called pulse width), and trims the pulse width longer or shorter depending on coolant temperature, intake air temperature and manifold absolute pressure. There will also be cold start maps to keep the bike rich for a short period of time after a cold start. If you change the airflow through the engine through a different exhaust, intake tuning or whatever, you have to remap the ecm to deliver the right amount of fuel for the new airflow. The ecm has no means to figure this out on it's own.
Don't kid yourself into thinking a different pipe on it's own will make a big difference in how much power the bike makes. Most stock exhausts are very good in terms of airflow, they just don't make a lot of noise.
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Last edited by Desert Tortoise; 05-28-2014 at 11:27 AM.