A vehicle’s oxygen sensor is a device responsible for measuring the oxygen content of the exhaust gas and supplying that information to the Engine Control Module (ECM). The ECM uses the information from the oxygen sensor to maintain a proper air/fuel ratio. “Narrow band” oxygen sensors typically operate between 0 and 1.0 volts and can only gauge air/fuel ratios in the range of 14.7:1. “Wide band” sensors usually operate in the 1 to 5 volt range and can accurately measure A/F ratios across a much wider range.

With a narrow band oxygen sensor, a rich mixture will trigger a high voltage, generally between 0.8 and 0.9 volts. The higher voltage signals the ECM to reduce the amount of fuel delivered to the engine. When the mixture is too lean, the sensor’s voltage drops to around 0.3 volts, signaling the ECM to add fuel to the mixture. The narrow band oxygen sensor thus acts like a switch, only indicating whether the A/F mixture is too lean or rich. It does not indicate how lean or rich, however, and so the ECM is constantly adjusting the mixture to try to maintain a 14.7:1 ratio, which is ideal for producing the lowest emissions.

By contrast, a wide band sensor supplies a gradually changing signal that indicates the exact A/F ratio in real time. While narrow band sensors are only accurate around the 14.7:1 range, wide band sensors can measure A/F ratios as rich as 7:1 or as lean as 20:1, making them an extremely useful tool for tuning a high-performance engine.

Automobile manufacturers aim for the best combination of fuel economy, emissions, and power. This is why they maintain a stoichiometric A/F ratio of 14.7:1 at idle speed and part-throttle engine speed. On the other hand, high-performance enthusiasts have a simpler agenda: Power.

A low-boost supercharged engine uses almost the same A/F ratio that any high-performance hot rod would use, somewhere around 12:1, while higher boost engines might run around 11.5:1 under heavy load. The use of an aftermarket cam affects the A/F ratio as well, and a high-boost engine with a racing cam the ideal A/F mixture could be much leaner. Because there are so many variables affecting the ideal A/F ratio under so many different operating conditions, performance enthusiasts typically spend a great deal of time at the dyno using an exhaust gas analyzer to tune the A/F ratio just right.

But the dyno can only provide limited, circumstantial information. You might gain 15 horses by tuning at the dyno, but that doesn’t necessarily equate to a properly tuned engine. There are many factors aside from A/F ratios that affect performance, including fuel pressure, boost pressure, and exhaust gas temperature, to name just a few. And the best monitoring is done under actual operating conditions, not just on the dynamometer.

This is where the real beauty of a wide-band sensor comes in. Not only do wide band sensors respond quicker and give more accurate data over a wider range of air/fuel ratios, they can also incorporate additional channels. For example, the Bosch LSU 4.2 is a 5-wire wide band O2 sensor that is packaged with Innovate's LM-2 Digital Air/Fuel Ratio Meter. The set-up can be taken out on the track and can record multiple full-throttle passes at a rate of 12 samples per second. While the engine is cooling down, you can download the data into your PC and analyze it using the included software.

When used with an optional auxiliary box, the extra channels allow you to view the Air/Fuel ratio in concert with other critical engine parameters such as throttle position, fuel pressure, engine RPM, injector pulse width, and so on. This makes for a fairly comprehensive data logging system that can rival the capability of a dyno with the advantage of logging actual track data.

Systems range in price from less than a hundred dollars for basic A/F meters to several hundred for a rather complete digital tuning system, but that’s a drop in the bucket compared to several days at the dyno.

By Vanessa Wilson