Target AFR Tables
Tuning the fuel tables relies heavily on AFR (WBO2 Sensor) values to determine how much fuel to deliver. All ECUs will reference a target AFR table. Some ECUs running a VE model will use AFR as part of a VE calculation to deliver fuel. AEM for example will allow you to, once the engine is tuned, adjust the AFR table and it will adjust the VE fuel delivery to match. However, most ECUs will adjust the fuel delivery table to match the target AFR. This comparison and adjustment feedback is part of the closed loop running. In my example below for Megasquirt, the AFR table is used as a manual feedback mechanism to setup the fuel delivery table. ( relationship of effective change with afr to ve , when to choose one or the other )
Normally, when I start from scratch on an engine I will simply set the AFR Target table to 13.5 everywhere. For the DIY tuner, This is a very safe AFR and the engine will be responsive and make good power. This allows you to get the engine running and the car driving to move onto the next phase. Once you have a good running engine you can go back and optimize. Now, experienced tuners will skip this test phase and go straight to an optimized AFR Target Table. Here are some key points that you need to consider as you optimize the AFR output. Please be cognisant that these are examples. Every engine is different and will need individual adjustment and tweaking.
The Fuel delivery requirements are broken down into four primary areas; idle, cruise, WOT and decel. This has been covered before. Here are my typical targets. YMMV.
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WOT. I like AFR 13.1-13.3 up to peak torque. Then drop to 12.8 to redline. This should be anything above 92kPa and 80% throttle on most cars.
The fuel delivery curve (VE table) at WOT should look just like a torque output curve on a dyno. There are many engine parameters that have known performance metrics. Look one of these up and you will see bump in the torque curve. The center of this bump is peak torque. Fuel requirements usually drop off after this bump as engine efficiency also drops. ( yes supports my #’s)
Cruise. Highway steady should be as lean as you can run. Most 911s like to be 14.3-14.5 to run smooth. It will depend on timing. The more lean you go, the more timing you need to advance. If everything is dialed you can go to low 15s. You will have to find what conditions you run highway (MAP and RPM range) from gauge or datalog. ( tps and inches of vac)
Transitions. Between the two states (WOT and Cruise) transitions should be non linear gradual. Meaning, drop from 15.2 to 13.8-14 as you are cruising and need to go up a hill or change lanes. Drop down to mid 13.3’s when you really need to accelerate but you are not WOT.
Off-throttle Decel. I am usually at AFR16 for off-throttle. These are usually the lowest 2 rows in VE table. If you are running a full fuel cut, the AFR can be 18-20. This can be challenging to catch idle depending on what RPM you bring fuel back into the tune. ( no need to go here)
Idle. Most 911s are happy to idle about 13.5-13.8AFR. This is timing dependent. You can go leaner if your injector dead time is dialed. There are times when the injector size is too large and the small pulse widths for idle get really short that it forces a more rich mixture.
Now, here is the most important part. Make sure the Load axis on the AFR table mimics the fuel table. Otherwise, you will not be able to adjust with the same resolution. Meaning, Don’t have an AFR table scale from 0-100 when the fuel scales from 25-105. They need to match or be very similar. You may also notice that your AFR table is 12x12 and your VE table is 16x16 making an apples to apples match impossible. That’s ok, just get the basic scale similar. ( pics and demo)