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How much fuel you get is directly proportional to the voltage out of the MAP sensor. That's how the SAFC controls the gain/decrease in fuel delivery - by modifying the incoming MAP voltage. The ecu sees "so much air being seen by the MAP sensor, requires this much fuel delivery". The higher the voltage, the more fuel delivered. Of course there are other parameters, but the map sensor voltage is key.
Lets make sure we all understand this: If you get more than 4.79volts out of your map sensor, nasty things will happen. Its all in the service manual for your car (at least for OBD-2 cars). Check engine light (i believe code 34?), fuel cut at 4k rpm, rough and stalling idle, etc.
The VC-1 and HRC FCD both do a fairly good job in preventing you from going over the higher-than-allowed voltage limits, but one fault that they both share is that they are "soft" and "transient" voltage clamps. Meaning that when the voltage from the MAP sensor starts getting close to the 4.79 limit (lets say at 4.3), these "black boxes" will start holding and clamping the voltage. But because they are soft clamps, they dont stop the voltage at our example 4.3 volts. They will vary every single time what voltage the clamp stops at. So there are times that the clamp will stop your MAP sensor voltage from getting high enough to yield optimum fuel delivery (i.e. at a low 4.3 volts) or not stop it by the 4.79volts limit (i.e. at 4.89 or 5volts even) which will yield nasty stuff such as the check engine light or even fuel cuts.
In addition, by being a "soft clamp", the voltage "holding" or clamping action is variable or "transient" i.e. the clamp starts at 4.3volts, but will continue to clamp at an unknown and unpredictable rate until X voltage, where X could be 4.7, 4.5, 4.9, etc. Like said before, you play "voltage roulette" everytime you run your car and try to tune it with something such as a SAFC.
The advantage of the VC-2 like many others have mentioned above is that 1) its a hard clamp. If you set it to stop the MAP sensor voltage at 4.78volts, it will always and ONLY stop it at 4.78volts and 2) its ajustable! set it as close as you can to 4.79 to get the max voltage out of your MAP sensor.
Hope this helps. I'm hanging on to the last piece of reason for keeping my HRC FCD. At least until vc-2 comes out.
_________________
Vinnie Lima
vlima@csc.com
AIM: Kiku888
1997 Chrysler Sebring "Silver XTC"
I4 420a w/ HRC Stage 2.45 w/ FMIC
14.7@97 mph@6psi
Proud member of the
ASOG BoOsT JuNkIe Crew
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