*MAXXD* Ollie's Nissan 200SX S14a

[Roger_Sausage]

Yup....once the blow off valve is removed, there's nowhere for the compressed air to go once the throttle closes, so it comes out back through the turbo. If you're accelerating at full-chat, then the turbo can be doing anything up to 100,000 RPM, so when the throttle butterfly closes there is still some residual boost pressure created that is now sandwiched between the turbo, and plenum. At this stage, the blow-off valve would see this excess pressure build up and vent it out (either to atmosphere or back in to the inlet tract). Without a blow off this pressure builds up to the point where it has enough force to travel backwards through the now-slowing compressor and turbine blades. The noise is the air being disturbed by the force of travelling against the direction of the turbo. That's why the over-fuelling happens, because the AFM just sees a load more air so dumps fuel in to the manifold.

The wastgate doesn't chatter, it either opens or closes, it's purely a device to bleed of excess boost from the turbo. That's what electronic boost controllers do, they just control when the wastegate opens. Usually the wastegate sees a set pressure and opens up to relieve it. What the boost controller does is fit a solenoid in-line with this, and bleeds off the pressure that would normally activate the wastegate, preventing it opening before the desired boost level is met. Once the controller gets to it's determined value it stops bleeding off the pressure and the wastegate opens. This is why uprated wastegates are required are cars running high boost, because once at a certain level, despite the boost controller solenoid bleeding off the signal to the wastegate, the pressure is simply too much and forces it off it's seat anyway. S14a's usually need an uprated actuator and wastegate at around 1.2 bar.