it's what regulates fuel pressure, it's job is to keep it at a constant "head" pressure in reguards to vacuum manifold or boost, I.e. fuel pressure falls when you back off, or cruis along, and it goes up when you boost. basically it raises or lowers fuel pressue to keep it 44psi(?? close to) above the inlet manifold pressure, so if your boosting 15 psi you get 44 + 15psi, 59 psi of fuel pressure, works the same for vaccum to. I dont own an xr6 so I cant tell you where it is, most likely on the end of the fuel rail. if I were you I wouldnt worry about trying to get more fuel in, I doubt you'll make anymore power, just use more fuel and make the back of your car dirty. Anyway I was wondering if anyone could tell me the size of xr6t injectors, thanks,.