Yes, You Can Be Too Rich

Silly me. For the longest time I thought “Stoichiometric” was the name of some hockey player from Quebec, or a small village in Russia. Well, it turns out stoichiometric is a really important automotive term having to do with air, fuel, and junk.

We all know that an engine needs to burn air and fuel to make power, right? Well, stoichiometric is the scientific term that indicates the ideal ratio of air and fuel for complete combustion (meaning no gasoline or oxygen is left in the combustion chamber). That stoichiometric air/fuel ratio is 14.7:1. When the ratio gets lower than that, the mixture is rich, meaning there’s a greater proportion of fuel.

When the ratio gets higher, you have a lean mixture and a greater proportion of oxygen. Normal, unmodified cars are set up to operate as close as possible to that stoichiometric ratio, since it delivers an optimal combination of fuel economy and low emissions while still producing adequate power. To maintain that ratio, the engine’s control unit depends on feedback from the oxygen sensor, which is mounted in the exhaust stream. The sensor monitors the amount of oxygen left in the exhaust gas and reports that back to the ECU. The ECU can then adjust the amount of fuel delivered to the combustion chamber to enrich or lean out the mixture as needed.

So, who cares about normal, unmodified cars, right? Not us. But air/fuel ratio is important in the quest for horsepower. A mixture that’s a little rich can help make power and keep the whole combustion process cooler. A mixture that’s too rich, though, just dirties engine parts with unburned gasoline. What’s more critical to watch for is the lean burn, especially with cars that are stuffing a whole bunch more air into the engine with a turbocharger or a supercharger. If you introduce a lot of air into an engine without also adding more fuel, you will blow up the motor in no time.

There are many products on the market that monitor air/fuel ratio. Most hook up to your car’s stock oxygen sensor and use the sensor’s voltage signal to light up LEDs or bar graphs to indicate when the ratio is running rich or lean. The Wideband Oxygen Sensor Controller from FJO Enterprises is more sophisticated than that. For one thing, it monitors the air/fuel ratio with a dedicated oxygen sensor that has a sensitivity range greater than most units. It can read ratios from 10:1 to 20:1. The FJO system also gives you several options for monitoring the ratio, from a straightforward display box you can mount on your dashboard to the ability to map the ratio in real time on a computer or a PDA. And unlike a line of LEDs or a bar graph, the FJO Controller gives you the actual air/fuel ratio number as the sensor is reading it, so when it changes you know exactly by how much.

One thing the FJO Controller won’t do is control or alter the air/fuel ratio. It is a diagnostic tool only, to be used to check the ratio when you’ve made mods or are tuning for competition. You’ll still need to reprogram the fuel and ignition maps separately. But the Controller will give you peace of mind—or a clear warning—after you’ve installed a turbo and you stab the throttle for the first time.

To see how one of the FJO systems work, we watched as Elton Lo at Raceline Development installed one in Todd Kanashiro’s Drag turbo–equipped 2000 Civic Si. We were glad we could stand around and watch a pro like Elton do the work instead of trying to fumble this kit on ourselves. The installation requires mounting a second oxygen sensor into the exhaust flow. That meant cutting a hole in the Civic’s downpipe and welding in a bung for the sensor. And welding is waaaay beyond our screwdriver-and-hammer-in-the-kitchen-drawer mechanical abilities.

By Drew Hardin

Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!