What is the Free air Calibration and why is it so Important?

Free Air calibration is critical to proper oxygen sensor function and accuracy. Here’s a look at how to perform a calibration, how often to calibrate, and what the process accomplishes.

Just like micrometers and dial bore gauges need to be calibrated on a regular basis, wideband O2 sensors must follow a consistent calibration regimen to ensure maximum precision. The fundamental problem with most wideband sensors—whether they’re OE or aftermarket—is that their calibrations are preset for life from the factory. As the typical wideband O2 sensor becomes less accurate over time due to regular wear and tear, it can’t be recalibrated to compensate for wear. Surely, there must be a better way?  

To solve this problem, Innovate Motorsports developed a revolutionary new approach to O2 sensor design in the early 2000s. Instead of relying on an O2 sensor’s calibration resistor to calculate air/fuel ratio readings, Innovate’s sensors utilize a patented digital wideband controller that’s built into the sensor cable. This makes it possible to free-air calibrate the O2 sensor at any time in a matter of minutes. During this process, the O2 sensor measures ambient air readings, thus establishing a new baseline to compare against the sensor’s air/fuel measurements. This compensates for the inaccuracy typically caused by sensor wear.

Wear and Accuracy

Although wideband O2 sensors offer a far greater range of latitude than their standard narrowband counterparts, no sensor is immune from wear. Unlike IAT and MAP sensors that take their readings from a clean stream of filtered air, O2 sensors are constantly bombarded by particles of residual fuel, carbon, and soot floating through the exhaust system. Highly modified street and race motors are even less forgiving, often subjecting an O2 sensor to oil past the piston rings and potentially even coolant.

Furthermore, nitrous and forced-induction applications often subject O2 sensors to leaded race fuel and elevated EGTs. The growing popularity of E85 presents challenges as well. “In terms of O2 sensor wear, high EGTs are a concern, as is the high velocity exhaust gas in high horsepower applications,” Felipe Saez of Innovate Motorsports explains. “Ethanol and methanol will not affect sensor life, but these fuels tend to be tuned on the rich side, which does affect sensor life. Condensation can destroy a sensor very quickly as well.”

Maintaining a healthy engine with good ring seal and a proper ECU tune is the best way to minimize these effects, but O2 sensor wear and tear can’t be eliminated entirely. The consequence is a predictable decline in O2 sensor effectiveness. According to Bosch’s published test data, its wideband O2 sensors are accurate to .15 of a point when new, but that figure increases to .29 after 500 hours of use. After 2,000 hours of use, the variation between the measured air/fuel ratio and the actual air/fuel ratio increases to .59 of a point. This means that an indicated air/fuel ratio of 11.76:1 can be as lean as 12.35:1, or as rich as 11.17:1. To most tuners, that is an unacceptably large gap.    

Altitude

Granted that Bosch’s lab tests provide an interesting point of reference, they’ re a far cry from the rigors of real-world vehicle operation. When installed on a high-performance application, O2 sensors wear out much more quickly due to elevated EGTs, and richer air/fuel ratios. Detergents and additives commonly found in pump fuel accelerate sensor wear as well.

Even with a fresh, wear-free sensor, the environmental differences between lab testing and real-world driving throw another wildcard into the mix. Bosch calibrates the LSU 4.2 and LSU 4.9 wideband O2 sensors utilized in Innovate’s wideband systems under lab conditions that simulate 14.7 psi of atmospheric pressure at 68 degrees Fahrenheit. That means Bosch’s factory calibrations perform at optimal accuracy only if a vehicle is driven at sea level on a 68-degree day. Anything outside this narrow temperature and elevation window compromises the accuracy of an O2 sensor.

Easy Calibration

Considering that cars are driven in a wide range of climates and altitudes, free-air calibrating an O2 sensor is paramount in ensuring sensor accuracy. Just like Innovate’s digital wideband sensor controller can compensate for sensor wear, this same technology can be used to compensate for altitude changes by periodically re-calibrating the sensor. Fortunately, free-air calibrating an Innovate wideband O2 sensor is an extremely straight-forward process that only takes a few minutes.

The first step involves disconnecting the O2 sensor cable, then unbolting the sensor from the O2 bung. With the sensor disconnected, the vehicle’s electrical can then be powered up by turning the key to the “On” position. Innovate’s digital gauges will illuminate an error message, while the backlight on Innovate’s analog gauges will display a series of flashes. The system should remain powered up for a minimum of 30 seconds. The next step involves turning the key to the “Off” position, re-attaching the sensor to the cable, and then powering the electrical system with the sensor in free air (not in exhaust system).

Upon turning the key back to the “On” position, digital gauges will display a “HTR” message, while the backlight on analog gauges will steadily blink. This indicates that the sensor is heating up to operating temperature. After 30-60 seconds, digital gauges will display a “CAL” message, while the backlight on analog gauges will stop blinking, indicating that the calibration process is complete. Since the sensor is in free air, it will display the maximum lean value on the gauge.

Finally, after powering the vehicle’s electrical system back down, the sensor can be reconnected to the exhaust system. In a matter of minutes, any Innovate O2 sensor can be calibrated to compensate for wear, as well as for temperature and altitude changes. To ensure maximum precision throughout the sensor’s lifespan, Innovate recommends the following calibration schedule:

When to Calibrate Your O2 Sensor: