How the narrow band O2 sensor reacts to the fuel mixture has always been something of a mystery to most riders. It can be a little hard to visualize what is going on when some simply tells you that the output changes based on the fuel mixture changing, which is really the amount of oxygen in the exhaust changing. So we thought we would make all of this a little more "visual" and show some very accurate graphs showing what is really happening  at the O2 sensors.

What the table shows is that the Narrow Band O2 sensor does is react to a fairly wide range of Air Fuel Ratio's. What is harder to see from this table is see that the sensor does not react in a linear manner, which can best be described as in it not always highly predictable. The other note is that narrow band O2 sensors are sensitive to the operating temperature of the exhaust system. We have specific note here that this response curve is for an exhaust temperature of 1,200F, which is going to be fairly typical of your Harley operating in an urban environment when traffic is moving fairly well.

From this graph, you can actually see that the hotter the engine gets, the wider the response curve of the narrow band O2 sensor. This little quirk in the operational ability of the NBO2 sensor can be used to your advantage if you are trying to manipulate O2 sensor voltages directly to the ECM. There will be more on this later on for you DIY tuners.

We also made note of voltages over 1.000. Many people will tell you that a narrow band O2 sensor is only a 0-1V device. All you have to do is connect a voltmeter to your HD's O2 sensor during startup and watch the voltage go to 1100-1200 mV to realize that 0-1V is not correct. This extra 200 mVolt may not be important for normal operation, but if you decide to do some modifications to the centering voltage or bias voltages that your ECM wants to operate at in closed loop to manipulate your fuel mixture, this little piece of information becomes very important.

So how exactly does this information help you with your EFI system?

With this information in hand, it is easy to start entering new closed loop bias values into your software tuning programs. So riders with SE Race Tuner, SE Super Tuner or TTS software know that 750 closed loop bias values are going to provide closed loop fuel ratio's of at least 14.2:1. A value of 800 would provide an AFR of about 13.8:1.

You can take this one step further and start manipulating the O2 sensor voltage to the ECM. Since it is well known that the ECM generally wants a Bias Voltage of 500 mV, why not use a voltage divider to drop the O2 sensor voltage to the ECM from one voltage to the other. While the manipulation of these voltages does require some pretty fine calculations and a high degree of accuracy for the desired results, it is quite possible to manipulate the input and output voltages to achieve some dramatic results.