Widebands/Narrowbands (O2 Sensors)

Updated: Jun 18, 2020

Chemistry is incredibly important in gasoline engines. Oxygen sensors, both narrow-band and wide-band help us measure that fuel chemistry. Fortunately for gearheads like us, we don't have to get very deep into the science of it to make power.

Air Fuel Ratios

As an enthusiast, you've probably heard about air-fuel ratios, and understand to some extent that you need the right mixture of air and fuel to make power. To complicate things, the exact ratios needed for a complete burn will vary based on fuel composition.

For example--

Gasoline has a complete (Stoichiometric) burn at 14.7:1

... That's 14.7 parts air, and 1 part fuel (by mass) ...

Stoichiometric for Ethanol (E100) is at a mixture of 9.1:1

If I were to say a motor was running at "11:1 AFR":

- That would be rich if we were running Gasoline - That would be lean if we were running Ethanol - That could be a complete burn on an Ethanol / Gasoline blend...


If I've confused you, that's OK. There is an easier way to look at fueling, which is "Lambda"... While Air Fuel Ratios were a common benchmark during the last century, the prevalence of Gas / Alcohol blends (E85) has changed the way we talk about air and fuel mixtures, into something that makes more sense.

Lambda, simply put, is the ratio of how rich or lean a motor is running, compared to complete burn (Stoich), regardless of fuel composition.

Lambda of:

> 1.0 - Lean (1.05, 1.10) = 1.0 - Complete burn < 1.0 - Rich ( .92, .85, .77)

When tuning, we target different lambda ranges depending on conditions, and depending on what each individual motor wants.

  • Cruise / Idle / low load - Lambda of 1 will give good fuel economy

  • WOT, Naturally aspirated - Lambda of .85 (+/- .03) will make good power

  • WOT, Forced Induction - Lambda of .77 (+/- .02) will make power safely

These targets vary, but should give you a general idea what we are looking for.

Measuring Lambda

Since the inception of the gasoline motor, there has been an understanding that to achieve the best power and fuel economy, we need to hit specific lambda targets. Without precise measuring devices, that can be a difficult task.

Have you heard of spark plug reading? Early racers would literally cut apart spark plugs, to look at the soot deposit on the porcelain of their spark plugs, to ascertain whether they wanted to add or remove fuel. Based on close observation, along with drag strip data to measure whether their cars went faster or not, they could make informed decisions on what to do with their carburetors to get the right fueling for a faster pass.

Clearly, spark plug reading is imprecise, and lacks any means of knowing real-time lambda values under different conditions. External factors, such as fuel additives, combustion chamber efficiencies, spark plug heat ranges, and more, can throw off the results of a plug reading.

In 1993, a company by the name of NTK developed a zirconia dioxide array that could be immersed in the exhaust stream, which would instantaneously tell us what equivalency ratio (lambda) our motor was running at. This development has evolved into the modern day wide-band o2 sensor, and is indispensable as a tuning and diagnostic tool.

Wide-band vs Narrow-band sensors

Simple zirconia dioxide sensors have been around since 1976. These early Narrow-band o2 sensors were only able to tell if a mixture was "Rich" or "Lean" compared to a complete burn. While they could tell that you are running rich, they couldn't tell you exactly **how** rich.

Due to their simplicity, low cost, and reliability, the narrow-band sensor was used on the majority fuel injected vehicles up until around 2010. This provided for closed-loop feedback to keep the car at a lambda of 1 for efficient cruise operation. As the narrow-band sensor is low resolution, it's not effective as a tuning tool. At wide open throttle (WOT), the narrow-band is simply going to read "Rich", which is fairly useless.

More commonly, we are seeing wide-band o2 sensors equipped on factory vehicles. These on-board wide-bands allow the vehicle manufacturers to program optimum lambda targets, and consistently hit them under any number of conditions, including WOT.

Why would I want an aftermarket Wideband?

A wide-band is not just a tuning tool. It is also a great tool for ensuring your vehicle is running correctly. From a glance, you can easily see if your car is hitting the right lambda targets, or whether your fueling is wildly off. You can see data on your wide-band that helps both you and your tuner troubleshoot mechanical issues as they come up.

One night I was troubleshooting a Corvette with an on-board wide-band. I didn't have a laptop or data-logger with me, and the car had an intermittent stumble, with associated rich spike on the wide-band. Because of the rich spike, I knew that the Vette was commanding extra fuel from a bad sensor input... I checked over the MAP, TPS, and IAT, and found out that the intake air temperature had a short, and was causing air density calculations to be incorrect, adding extra fuel.

I've had other customers call me and tell me that their car, tuned with a WOT lambda target of .85 was suddenly hitting around .9, and not running as well. With this data, I was able to suggest some items to look over. This led them to discover a vacuum leak, allowing un-metered air to bypass their mass airflow sensor.

What wideband does OnKill Tuning use?

OnKill uses a Ballenger AFR500v2 lambda controller, with a calibration grade sensor. The AFR500v2 has many sensor options, calibrations, and is easy to read. I've found that the large numerical display is convenient to quickly glance at and read during a pull. The AFR500v2 does not have a serial (digital) output, which means you have to be sharp at calculating voltage offset when adding it to your data logger. Since I work with a lot of tools that do not accept serial input to start with, this is OK with me.

What sensor do you suggest?

We have had good results with AEM's UEGO x-series gauges. I've personally put around 70,000 miles worth of use on various AEMs. It's cheaper than the Ballenger, has both an Analog and Serial output, and I know I can trust them to last a number of miles before the sensor probe wears out.

For newer vehicles that run on a CAN bus (usually 2008 and later), there is a special version of the AEM that can output directly to the CAN bus, allowing you to bypass the need for serial or analog wiring entirely. It's pretty slick if you ever want to pull high-speed accurate data. I haven't attempted, but you should be able to pull the CAN WBo2 data into apps like Torque.

https://www.onkilltuning.com/product-page/aem-x-series-wideband-uego AEM 30-0300 for NON CANBUS VEHICLES AEM 30-0334 for CANBUS VEHICLES

Where do I put the Wideband?

Most WBo2 sensors come with a stainless steel exhaust bung for the sensor to thread into. This bung should be welded as close to the front of the exhaust stream as possible, and before any catalytic converters. For the sensor's accuracy, it's crucial that you do not have any exhaust leaks up-stream of the sensor.

If you are turbocharged, it's also important to know that these consumer grade sensors are not intended to be installed before the turbo, as the exhaust back pressure will affect the readings, and the extreme heat will quickly burn up the sensor.

Do these replace narrow-band o2 sensors?

Nope. Your car relies on its narrow-band o2 sensors for closed loop operation, and without your narrow-band sensors, you will not be able to learn fuel trims. Factory ECUs aren't able to understand the output of a WBo2, and would not be able to take advantage of the wealth of data provided by them.

There are some WBo2s (Innovate, for instance), that can emulate a narrow-band o2 sensor and take their place. I personally would recommend the simplicity of running both the factory narrow-band sensors, and an on-board WBo2.


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