Can someone explain why we need a stock 2 speed fuel pump setup, and what are the pro's/con's of having 12 volts to the pump full time? Looks like my crossover to high speed is at about 2700 rpms, and while still under vacuum during light/normal accelaration, so it's hard to believe that the low speed is to keep fuel pressure from skyrocketing at idle, since light cruise doesn't consume much fuel through the injectors, and the fpr is still having to bypass basically the same amount of fuel. I do see a fuel pressure rise when this happens, and I'd like to have a more stable fuel pressure (relative to intake manifold pressure) for tuning purposes. Has anyone run their pump in hi speed all the time?

Also, there was a thread somewhere, that discussed the idea that the built in venturi (necessary for the saddle type fuel tank we have) on the fuel return to the tank caused a lot of backpressure (ie could cause high fuel backpressure, effectively making a fpr useless at low engine speeds). This venturi needed to be hogged out a bit when running dual pump setups. Can someone point me to that thread, I can't find it anywhere.

Thanks

 

You answered your own question already. It's to help the FPR control fuel pressure at low IDC's (i.e. idle and low rpm cruising)

 

the 2 stage fuel pump is mainly for noise and wear controll , lets say you need 10% of available fuel flow at idle why work fuel pump at a 100%

 

also keeps the fuel cooler