The OEM Harley-Davidson closed loop fuel mixture is 14.6:1. On Twin Cam 96 engines, the closed loop range generally covers under 4000 RPMs and under 40% throttle. This is going to be 95+% of the time an average rider spends on his bike. Even the most aggressive rider would find it hard to spend more than 5% of this time in open loop.

For those of you concerned about open loop fuel mixtures, on TC engines these are set from 13.8 to 12.5:1 depending upon RPM and engine load. So these mixtures are quite adequate for performance work. Riders that are worried that exhaust and air cleaners are going to lean these mixutres out need not worry. Changes in exhaust/air cleaners are only going to change these AFR's by .3-.4 point, yielding 14.2 to 12.9 AFR in a worst case situation. These are still not fuel mixtures that are going to hurt an engine under wide open throttle situations.

The average HD rider is probably considering upgrading his exhaust system to get a better sound from the engine and maybe wants to install a performance air cleaner. Many riders do notice the heat from the exhaust/engine from the Twin Cams large engine, which can be uncomfortable. When they go to the Dealer about getting these exhaust/air cleaner upgrades done and ask about the heat, the typical response is that they need to "remap" their ECM with SERT and need to put the bike on the Dyno. This will end up costing the rider anywhere from $800-$1000. Even if the rider goes to an independent shop, they are still likely to get a similar answer to 'remap' and $400-$1000 cost using PowerCommander or TTS. Still very expensive when all you did was install an exhaust system.

While riders can rely on the fact that the OEM closed loop H-D Delphi ECM will maintain the fuel mixture at the 14.6:1 AFR it came from the factory without any ECM remapping, there can be advantages to seeing how an engine might react to various fuel mixtures. It has already been shown that a voltage divider can be used to manipulate a narrow band O2 sensor. Specifically, this effects the ECM the same way that altering the closed loop bias values do in the software tuners like SERT/SEST/TTS. The Table below shows the relationship between fuel ratios, bias values and voltage divider percentages for narrow band O2 sensors correct to 1200 degrees EGT.

You can use the table to match a voltage drop back to the standard bias. Under normal operating conditions, the ECM looks for a .500 Volt signal from the O2 sensor which indicates the expected lambda value of 1.00 or in the case of the Harley-Davidson Delphi ECM's, a .99 lambda. If you can somehow manipulate the incoming voltages or bias values, you can effectively change the closed loop programming of the ECM very similar to the same way SERT does. By matching a voltage drop and fuel ratio, you have created a quick way to test AFR changes on your ECM.

General electrical characteristics of narrow band O2 sensors presented earlier provides a detailed description on how these devices react to changing fuel mixtures and the the expected voltage outputs. A simple voltage divider circuit can be used to bring an output voltage down to the target bias voltage. By manipulating the target bias voltage you might be able to provide a more favorable operating result for your bike. The table above provides some details on what can be reasonably expected from a modern narrow band O2 sensor and precision voltage divider.