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Old 03-14-2018, 06:33 AM   #1
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Wideband O2 sensor and piggyback controller

Has anyone installed and used a wideband O2 sensor/gauge and a piggyback controller like the Apexi AFC Neo? I just finished installing the 987 airbox, 74mm throttle body and the 997 plenum and am thinking that using the Apexi to tune the afr may be a more precise approach vs using an off the shelf modified tune (Softronics or maybe the 996 tune). It would also be less expensive. I would be interested in anyone else's experience installing these pieces and using this approach. I'm also wondering if this is going to modify the afr during open loop or would it only help closed loop? Any downside?

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Old 03-14-2018, 06:45 AM   #2
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piggyback can't modify closed-loop - ecu will trim everything back to 14.7:1. so, piggybacks are only good for open loop - lean out the mixture a bit - closer to 14.7:1 more more power and fuel efficiency. i think it was insite who did some work with this? there is also the resistor method - install a resistor on the maf to modify the signal; again, ecu trims car back to 14.7:1 in closed loop so you are only setting afrs for open loop. most guys use a potentiometer; put the car on a dyno with a wideband, and adjust the potentiometer until they get the open-loop afrs where they want them.

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Old 03-14-2018, 08:59 AM   #3
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Using a potentiometer makes sense. At what point is the system in open loop? Is it rpm or load dependent or both? Once you adjust with a potentiometer at a specific point (rpm or load) in open loop will the afr then remain constant throughout the open loop range? I ask as the Apexi gives you a number of points (16?) that you can make adjustments.
Is the reason that a potentiometer or a device like the Apexi doesn't help in closed loop due to the fact that the ecu is already making adjustments? Is there a range in closed loop beyond which the ecu can't compensate (such as using a 987 maf in place of the 986)?
Just when I thought I was getting me head around this stuff.
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Old 03-14-2018, 09:22 AM   #4
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there's a variety of triggers for the move from open to closed loop - temp, throttle position, rpm. once the ecu decides to move to open loop based on these triggers, then it basically goes by maf reading. realise that the apexi is just a sophisticated, programmable potentiometer. and yes, the process is more complicated than i've stated - the ecu is looking at different maps depending on iat and other variables, but fundamentally, you are just intercepting the maf signal and modifying it. there is a good video on youtube of a guy doing the potentiometer thing on a 996.

the apexi i think also has the ability to intercept/modify other data such as temps so as to impact change-over points, etc., but that is above my pay grade.

in closed loop, the ecu is monitoring products of combustion, so if the apexi intercepts the maf signal and modifies it (with the intent of sending an incorrect airflow reading and impacting fueling) the ecu will see things are out of whack via the o2 sensors and trim to correct. why would you want to modify closed loop regardless - it is already as good as it gets.

presumably if you tweak the maf reading too much (with the apexi or a potentiometer) the closed-loop fuel trims could so high that a cel gets triggered. but note that the cel goes off at about a 30% error (.7 to 1.32, if that is what those readings mean) so if you've modified fueling that much you would be pushing your open loop fueling from 12.5:1 to well past 14:7:1 and be running way too lean anyways.

and i think that is the biggest gut check - how lean do you want to make your open loop operation? you are going to have a baseline of 12.5 or so to 1; do you want to change it to 13:1? 13.5:1? how risky do you feel? presume there is some safety margin in the porsche settings, but how much? how will the leaner afr affect timing? that's why i went with the 996 tune - if you can find a shop with the tool that knows what they are doing then it is a 5 minute job. if they don't then you have to pay to train them and are looking at, in newarts case, 3 hours labour.
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Old 03-14-2018, 10:29 AM   #5
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Great explanation. Thanks!

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