Originally posted by bpenrod
Why would the presuure go down as the box was turned up?
the short answer is that whats being measured is the pressure-fooling behavior of the box, not the actual rail pressure. the DRB is not capable of measuring actual rail pressure when a pressure box is installed.
The box-off behavior of 18,000 psi does sound low -- but may be because the box (in the off position) is boost fooling the ECM without adding any fuel. 18,000 psi is low for a stock 2003 HO (305) truck running WOT, so I still have some uncertainty here. Maybe there was some variability in the measurement, or maybe the DRB didn't capture actual maximum pressure during the experimental driving run. The Volumizer caps maximum boost at a figure that is BELOW stock maximum -- so the ECM may not fuel as hard I don't know. Note that I'm not completely certain about this, as I haven't made pressure measurements myself. I have characterized how the Volumizer boost fools, however.
Note that If these are low side pressures (line pressure, as has been clarified), and the comma is european for a decimal point, the pressures are way too high for a stock truck (there is no pusher pump installed). I'd expect much lower than 12 psi at WOT, certainly, but beyond that I didn't even know that low side pressure was even available from the ECM. is it? rail pressure is definately available via DRB -- because it is spec'd by DC/Bosch. But I didn't think that low side pressure was - it is not spec'd by DC.
So I don't think these are low side line pressures.
I think they are high side pressures, (rail pressure) as reported by the ECM. Note that its natural that they would go down at the higher HP settings. thats what pressure boxes do! They make the ECM think that rail pressure is low. so the DRB reports low fuel pressure, because it only reports what the ECM sees -- and the ECM is being fooled by the box, and the DRB is only as smart as the ECM is. ahem. note to DC mechanics: don't expect to get the truth from a device that is being fooled

.
I get a chuckle out of reports where a DC tech goes riding in a truck with a box on, and verifies that all is within factory specs according to DRB. What they don't realize is that they are reading the ECM, and the ECM is fat dumb and happy because an external box is lying to it.
A fooled pressure of 13,000 psi, to the ECM, to me means VERY agressive fueling. it suggests that the CP3 is going all out. It may be that the Volumizer at 100HP (its not a 120HP box) might be pulling the rail too low, I don't know. that of course will cause the box to fuel harder, and lower the fuel pressure signal as seen by the ECM even more.