Air / Fuel controllers and OBD-II
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Air / Fuel controllers and OBD-II
I see this question all the time so I sat down and wrote this so I can link to this thread when I see the question next time (enjoy) 
Air / Fuel controllers and OBD-II
Will OBD-II detune my A/F settings?
On-Board Diagnostic (OBD) systems are in most of today's cars and light trucks. These systems incorporate an on board computer to monitor and control engine functions. This was primarily to meet EPA emission standards. OBD-II, a new standard introduced in the mid-'90s, provides almost complete engine and emission control. Manufacturers started incorporating OBD-II in several models as early as 1994, and all cars built since 1996 meat the OBD-II standard.
One of the functions of an OBD-II computer is to keep the air to fuel ratio (A/F) at 14.7 under normal driving conditions. 14.7 is the ideal ratio for emissions and fuel consumption. The way it controls the A/F is by monitoring the primary O2 sensor, which is an indicator of the A/F. If the A/F is not at 14.7 it will trim the fuel levels until 14.7 is reached. Thus, if your car is running at 13 A/F it will adjust the fuel map until it is back at 14.7 A/F. You might ask “Won’t adjusting the A/F ratio with a controller just be offset back to 14.7 by the OBD-II computer?” The answer is not necessarily.
An important thing to know about OBD-II is it does NOT try to keep the A/F at 14.7 when under full throttle. At full throttle the computer ignores the reading from the primary O2 sensor and does nothing to try to trim the fuel. In fact most cars are set in stock form to run rich under full throttle and not at 14.7. Knowing this we can tune the A/F in such a way that it will not be “detuned” by the OBD-II computer. Don't set the A/F controller to do anything under part throttle because that will only though off what the ECU is trying to do. What's the point of trying to get anything other than 14.7 at part throttle anyway? If you want more power just push the throttle down more! You can set most A/F controllers to only adjust fuel at 90% or more throttle. This will give you max power at full throttle and the ECU will not try to “detune” it. Basically let your OBD-II computer do the A/F tuning for part throttle (it does a good job of that), and you do the tuning under full throttle via an A/F controller, and some time on the dyno.
Air / Fuel controllers and OBD-II
Will OBD-II detune my A/F settings?
On-Board Diagnostic (OBD) systems are in most of today's cars and light trucks. These systems incorporate an on board computer to monitor and control engine functions. This was primarily to meet EPA emission standards. OBD-II, a new standard introduced in the mid-'90s, provides almost complete engine and emission control. Manufacturers started incorporating OBD-II in several models as early as 1994, and all cars built since 1996 meat the OBD-II standard.
One of the functions of an OBD-II computer is to keep the air to fuel ratio (A/F) at 14.7 under normal driving conditions. 14.7 is the ideal ratio for emissions and fuel consumption. The way it controls the A/F is by monitoring the primary O2 sensor, which is an indicator of the A/F. If the A/F is not at 14.7 it will trim the fuel levels until 14.7 is reached. Thus, if your car is running at 13 A/F it will adjust the fuel map until it is back at 14.7 A/F. You might ask “Won’t adjusting the A/F ratio with a controller just be offset back to 14.7 by the OBD-II computer?” The answer is not necessarily.
An important thing to know about OBD-II is it does NOT try to keep the A/F at 14.7 when under full throttle. At full throttle the computer ignores the reading from the primary O2 sensor and does nothing to try to trim the fuel. In fact most cars are set in stock form to run rich under full throttle and not at 14.7. Knowing this we can tune the A/F in such a way that it will not be “detuned” by the OBD-II computer. Don't set the A/F controller to do anything under part throttle because that will only though off what the ECU is trying to do. What's the point of trying to get anything other than 14.7 at part throttle anyway? If you want more power just push the throttle down more! You can set most A/F controllers to only adjust fuel at 90% or more throttle. This will give you max power at full throttle and the ECU will not try to “detune” it. Basically let your OBD-II computer do the A/F tuning for part throttle (it does a good job of that), and you do the tuning under full throttle via an A/F controller, and some time on the dyno.
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