|November 4, 2009 - Time for some intake manifold testing.|
Since the big torque dip wasn't affected at all by the cam change or the fuel pressure change, I'm thinking it's something with the intake manifold.
The 1999-00 intake manifold I'm using has VICS, which stands for something like Variable Intake Control System. It doesn't change the length of the intake runners as you might expect, but opens and closes a resonance chamber in the manifold that's placed about halfway down the runners. It's been proven to be fairly effective on the dyno, and the butterflies flip at around 5200 rpm.
The manifold is in two pieces. The lower contains the butterflies and the upper has the chamber - you can see it here with a nice tan color. The passage inside the chamber is for the EGR gases, which are routed from the back of the manifold to be injected just behind the throttle body.