It used to be that tuning a hot street engine or race motor was one of those black arts that only those carburetor and ignition wizards seem to understand. When (and why) an engine needed more or less fuel was a gray area that only those guys who seemed to be plugged into the engine could figure out. Well gang, air-fuel ratio tuning just got a whole bunch easier.
There's nothing new about plugging in a sensor into the exhaust and reading the free oxygen level and establishing an air-fuel ratio based on the sensor's voltage readout. The OEM's have been doing that since the early '80s. But these sensors were designed to be accurate only around 14.7:1 air fuel ratio. Attempting to build a "gauge" that could accurately deliver air-fuel ratio around 12.5 to 13:1 was challenging and usually inaccurate. Several companies tried and the result was a slew of inexpensive lean-rich indicators that were only really accurate around 14.7:1. The rest of the time, the best these gauges could do was tell you that you were "richer" than 14.7 at wide-open throttle (WOT), which wasn't very useful. These gauges were all built around what were called narrow-band exhaust gas oxygen (EGO) sensors.
A few years later, companies like Bosch and NTK began building wide-band oxygen sensors that could accurately deliver dependable air-fuel ratio readings in useful ranges for the performance tuner. The problem was these sensors were verrry expensive. But with advanced technology and time, these prices have come down, making a handheld device for reading and data-logging air-fuel ratio finally affordable. Enter Innovate Engineering with an air-fuel ratio meter for a mere $350.
Innovate Engineering has created a slick, digital handheld air-fuel ratio meter that can also data-log the information that will give you immediate and accurate drive-time results that you can use to help tune your engine. The system uses a five-wire Bosch heated oxygen sensor that is intended for use only with unleaded fuels to create the air-fuel ratio readout. At this time, the system is capable of data-logging only the air-fuel ratio, but by the time you read this, Innovate should have released auxiliary components that will allow you to data-log several other inputs, including rpm, coolant or inlet air temperature, and pressures.
How's It Work?If you study the included graphs (6A and 6B), you should be able to see that the narrow band oxygen sensor is really only accurate at 14.7:1 air-fuel ratio. This is because that's the stoichiometric air-fuel ratio where all three major components of emissions-hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx)-are at their combined lowest point. This is where the new-car companies need their engines to run at part-throttle both for emission and fuel mileage reasons. That's why these narrow-band sensors were built this way.
These sensors operate by measuring the free oxygen content in the exhaust gas and converting that amount of oxygen into a specific voltage. As you can see from the graph, a narrow-band sensor will deliver a 0.45-volt reading when the air-fuel ratio is at 14.7:1. Because the voltage line is so steep, the higher and lower air-fuel ratios create tiny voltage changes in this kind of sensor, making this type of sensor notoriously inaccurate at air-fuel ratios on either side of 14.7.
Eventually, the new-car companies realized that it was possible to build a sensor that was capable of accurately measuring oxygen levels roughly in a range from 9:1 to 18:1. This is evident with the graph that shows both a wider voltage band from 0.95 to 2.11 volts (with 1.47 volts at 14.7 air-fuel ratio) while the slope of the curve is not nearly as steep, meaning that this type of sensor is far more accurate in measuring air-fuel ratio within the range of gasoline-fed engines. These ratios will change when using fuels like methanol or propane. We'll limit our discussion to gasoline-fed engines.