Originally Posted by projekteg

the only reason you can use an fmu with stock injectors is because it gives you more fuel by raising fuel pressure, the afc does it through voltage, and since your stock injectors are almost at full duty cycle (or whatever it's called),


Ok, once again, an AFC works by scaling MAP signal input to the ECU. What this means is that by changing what your ECU thinks your manifold pressure is, it will compensate with either more or less fuel. We use it to lean out 450cc injectors so when higher manifold pressures are generated (boost), the modified MAP sensor signal read by the ECU will not exceed ~3.0v, throw a CEL, and put your ECU into the undriveable "limp mode".



you can't get them to put out more fuel, just the fmu can push more fuel through them.

Yes, you can. It performs the exact opposite procedure as leaning out your injectors to richen them. I.E., it will scale your MAP sensor voltage signal up, placing your base manifold pressure higher, thus plotting a richer part of your fuel map and, and less aggressive part of your timing map. The problem with doing this with higher boost levels (3-4psi+) is your puts so far at the top of your fuel and ignition maps your injectors become saturdated (never closing), and your fuel flow doesn't increase. But, on a naturally aspirated setup, you CAN increase fuel flow on stock injectors with an AFC. That was actually it's original design! They call it an "AFC Hack" when using it with a turbo because it's just that, not designed to work this way, but does.

Originally Posted by toupe95

If this is the case, how can using the hack lessen drivability? Thanks in advance.

Originally Posted by projekteg

can you really increase the amount enough for boost? i always though that stock injectors (240cc) were ALMOST maxed out in stock applications, therefore you couldn't really get them to provide enough fuel for boost.

Yes, you can use stock injectors fo minimal boost applications. But, saturation becomes a factor as boost increases. That is why I stated it doesn't work for higher than 3-4psi+ and that even depends on the cfm of your turbo.

Originally Posted by toupe95

This is kind of off topic, but regards the "hack." I was talking with some friends and they mentioned that drivability with the hack might be compromised.

When people talk about "loss of driveability" they're talking about 3 things which happen with an AFC. The severity of each greatly differes between applications. The problems with an AFC are characteristics that accompany the MAP sensor scaling.
First is ignition timing advance. Since you're scaling the signal down in a boost application, this increase the amount of ignition timing advance because your ECU thinks you're at a lower manifold pressure, therefore you're at a more radically advanced part of your ignition map at any given RPM.
Second problem is vacuum to boost transition. Depending on your scaling device, be is a hardware AFC, a DIY, or (in the case of GhD, a digital scaling code routine), the signal must be interpreted by the ECU correctly, and the scaling must be accurate, or the ECU could cause fuel and ignition cuts which lead to jerking during transistion.
Third is irratic idle. BEcause of the scaled sensor code, and the ECUs programmed ability to try to find the proper air fuel ratio in closed loop, people have stated that with larger injectors, tuning idle is next to impossible.
Now, for my personal findings on all of these problems? They are fixable or can be tuned out. An AFC is a reliable, and proven, method of fuel management for midrange boost setups. Personally, I run 10psi SOHC, daily driven, with my V-AFC and 450cc injectors. No irratic idle, no jerks during boost transition, with premuim gas (which you should be running anyway with a turbo setup) no detenation. Still get ~27mpg.

It has been repeated that the hack's main purpose is to scale the map sensor voltage, but it also controls the output of the injectors. If I understand right, ECU still controls the sequence of the injectors and plots the fuel maps, but the AFC can alter the amount of output for the injectors on that map.

This is incorrect. The AFC scales, "controls" if you will, the MAP sensor signal, that is it. ECU still dictates injector pulse duration by the values in the tables. The MAP sensor signal value itself is one of two key factors that dictate which cell the ECU uses on the table. The other is RPM. From those two values, a point on a 2D table is chosen from the 15x17 table, this is the injector pulse value.