1.9L VW/Audi/Skoda/SEAT Diesel Mod alert! :)

[Piwoslaw]

Originally Posted by ConnClark

On some cars that would be totally correct. Unfortunately for modders, some emissions control units utilize the turbine vane control to keep exhaust back pressure high to promote EGR flow. When the ECU tries to maintain X amount of boost with less pressure across the turbine, the EGR percentage drops and this change is picked up by the O2 sensor. Thus the ECU throws everything out of whack to try and keep emissions down and you can't always be sure what the results will be.

You may still get a little better mileage as Julian noted, but you could brainwash the ECU and make it better.

To increase EGR flow, certain HDi (and maybe TDI) engines use a device to restrict the intake air between the turbo and manifold. This may be a small throttle plate style thingy or part of a larger gizmo. My engine doesn't have it, maybe because it was built to satisfy EURO 3 emissions, but slightly newer EURO 4 engines do. With one of those the probable scenario would be: Switch to new, more open intake -> more air to turbo -> turbo geometry changed to keep boost at set level -> less exhaust backpressure -> less EGR flow -> restrict post-turbo intake to suck more EGR -> screw new free flowing intake.

I think that the bottom line is this: If you have an ECU controlled engine, then the stock intake is good enough for the low-rpm driving we do. Unless the pre-turbo intake has other faults (like the way mine snakes around the engine causing intake air to warm up), or you are constantly red-lining, then replacing it won't make a difference.