Originally Posted by
JohnC
Putting a #9 resistor on a pump that should have (say) a #4 tells the PCM that the pump delivers less fuel for a given command than it really does, so, when the PCM commands a specific rate the pump actually delivers slightly more. This is the basis of the "hot rodding" idea, but, as Dmax said, it's barely a measurable amount. What it does do, however, is make the PCM's commanded fuel rate transitions more dramatic than they should be, hence (I believe) the surging issues.
It is my recollection (could be wrong) that the resistor isn't actually part of any control circuitry. Rather, the PCM reads the value of the resistor to determine which fuel map it should use for a given pump. That value is stored in the PCM's memory and stays there until you perform a TDC offset learn, even if you change the resistor or pump. The proof is, you can remove the resistor, without any effect on performance, and a month later the PCM will still report the same resistor value.