Performance & Mileage impacts of the screaming eagle exhausts

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.

I haven't used auto tune so I cannot comment knowledgeably on it. Later V-Rod ecms combined with O2 sensors permit a degree of "learning", but I think there is only ten percent authority, meaning the learning feature can only over ride the settings on the look up tables by ten percent. There is a good explanation of this towards the back of the Tuning Guide on the TTS Mastertune website. Interestingly, while 2006 and 2007 V-Rods didn't come with O2 sensors, their ecms have the software installed to permit closed loop operation and learning with O2 sensors. The TTS Mastertune Tuning Guide shows you how to wire a pair of O2 sensors into your 2006/07 V-Rod if you can either buy a later model stock exhaust or after market exhaust with O2 bungs or have a shop weld them into your existing exhaust headers.

What I don't like are cutting and splicing someone elses box into the stock wiring harness. From experience, any time you splice something into the stock harness you open a can of worms. You also have to find a place to mount the aftermarket tuners control module and associated wiring. That isn't always easy to do. I prefer something like TTS Mastertune or the SE Supertuner because it remaps the stock ecm. No cutting and splicing into the harness, no gyrations trying to find a place to put the aftermarket tuners module.

Yusef said:

I never really understood how exhausts would significantly improve the performance of a bike. I could see it happen from taking off weight, but other than that it seems to be more about the exhaust note and the look of the bike. Can a new exhaust change the performance that much. Maybe a little, but something that is noticeable?

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Eh, depends on how well or poorly the stock exhaust was designed how much of a change in airflow you can achieve. Also consider the effects of pressure waves moving up and down the exhaust system, with negative pressure helping to scavenge the cylinder when the exhaust valve is open. If you replace the stock staggered dual exhaust that comes with most Harleys with a well designed two-into-one you can almost always gain horsepower, but you have to remap the look up tables in the ecm software to give you the right amount of fuel for the increased airflow through the engine. Those staggered duals trade away a bunch of airflow to get a look and they have less than ideal pressure wave characteristics. On a V-Rod, the stock exhaust already has good flow and pressure wave characteristics. Most aftermarket exhausts don't add much more power. Several actually reduce power. They were designed for appearance and sound, and the chimps who make them don't know up from down about making a performance exhaust system. They just chase style. Whatever floats your boat I guess, but darned if I would throw my hard earned at something that reduces function or performance. To me that is stupidity.