my fuel pressure stays at 39-40 at WOT with a peak boost of 10 psi from the supercharger. some people say that is right, some say it should go up 1 psi over 39 as boost increases, but my gauge stays at 39-40. your thoughts?
I believe the following to be correct but do some corroborating research to be sure:
The goal of the OE design was to ensure a relatively constant pressure
drop across the fuel injector, a drop of approximately 39psi. The rail pressure is adjusted with respect to the manifold pressure to attain a constant drop of something around 39psi. So if you measure the absolute manifold pressure and see 10psia, then the fuel rail pressure must be 49psia to maintain 39psi across the injectors. In that regard, I'd expect fuel rail pressure to increase one psi per psi of manifold boost.
It's important to know the "base" point when measuring pressure and that the gauges being used are using this same base point. There are "absolute" (psia) and "gage" (psig). A "boost gauge" that reads '0' when exposed to one atmosphere (no boost, no vacuum) is showing a gage reading. If it read 14.7, it would be showing absolute. So you need to make sure when taking readings that all gauges and meters are starting with the same base.
As well, the PCM isn't aware of manifold pressure, per se. It attempts to obtain a 39psi reading from the fuel rail sender but the sender itself is modifying the signal seen because it's referenced off the manifold pressure. If you used a scan gauge, for instance, that reads the fuel pressure from the PCM using the OBD-II port, you'd probably only ever see around 39psi shown because what the PCM is showing is really the pressure drop from the rail to the manifold pressure (it's reference point...) You'd need a rail-mounted transducer/sender and separate gauge to show the absolute fuel rail pressure. Which do you have?
If you're using a separate FP gauge and sender, you might want to have your system checked. As I said earlier, I'd expect the fuel pressure, referenced to the atmosphere (not manifold pressure) to change with engine running conditions, whether at idle or under boost. In those conditions, I'd expect to see 49psi at 10psi of manifold boost. But again, it all depends on what's being used to make the measurement.
