Assumption - stock FPR.
Logical - Fuel increases with load (approach zero vacuum or atmospheric pressure); ignition timing decreases with load

The VAFC works by "tricking"/altering the MAP signal so the ECU thinks its at a different load level on the fuel & ignition map (which hypothetically can add or subtract fuel indirectly). It does not directly alter the AFR or injector pulse. (hypothetically, if the VAFC was tricking the ecu into thinking it was at WOT when it was really at idle, it would be adding a lot more fuel and cutting ignition timing a lot compared to idle). In closed loop mode, the ecu will constantly adjust to get 14.7AFR no matter what load level or MAP input regardless of the MAP hacking or trickery (the o2 feedback will constantly fight with the VAFC [which provides different signals] and reduce its effect).

In open loop mode ie. WOT, the ecu will just read off the table and not consider o2 feedback. The problem is at WOT, the ECU is at the top of the table. Attempting to trick the MAP to see more load than 100kpA or zero vacuum (negative vacuum = boost) means the stock ecu won't add anymore fuel because it doesnt know what to do. That's why they say fuel can only be removed from WOT and not added.

The game changes when the stock FPR is replaced by a 'mechanical' FMU rising rate regulator. With a mechanical regulator pushing more fuel through the injectors (at WOT, positive pressure, increases with RPM as SC builds up pressure), you could create a rich condition that can be leaned out (VAFC tells ecu that it's at 95KPa load instead of 100KPA, also means the ignition is timing increases...see logical). Ignition timing is not independently tunable from fuel and adverse conditions may occur.

I used a FPR with my NA 2005 since it runs leaner than AP1 when i had a VAFC2. The EMS is so much easier...no trickery, you just set the injector duty cycle, AFR, and ign timing degree per load (or tps) and rpm and that's the end of it.

the GEMU, pretty much works by tricking the ecu into thinking it is in open loop at all times except for idle, decel, and other situations. Not sure if it hacks into (overrides) the injectors and coils directly (and simulates normal injector and coil function back to the ecu), but it's better than the VAFC since it can modify both independent.

 

Excellent description!

 

The EMU _can_ force the stock ECU into open loop sooner than normal using a synthesized TPS signal, but you don't have to use it like that. The EMU intercepts some signals and "T"s into other signals. As with any piggyback (AEM FIC, VAFC, etc.), tuning fuel during closed loop is pointless. The stock ECU will use the short and long term fuel trims to compensate and obtain a stoichiometric AFR during closed loop operation. Timing, however, can be tuned in closed loop and open loop portions of the tables. All of this said, I'm not a big fan of the EMU.

Here's a wiring diagram I made a while back, showing correct EMU wiring per recent discussions in the R&C forum.

 

Apex'i is selling AFC harnesses discounted for $80 direct right now from their website.

 

i have an ap1 with js header 70rr exhaust and aem v2 intake and 70mm test pipe.and 76mm skunk2 throttlebody and alot of heat wrap i make 227 on dyno jet tuned on vafc2 before tune 207

 

Sweet! That is what I want to hear! Thanks for the input guys!