Quote:
Originally Posted by Piwoslaw
I've been thinking about the freer flowing intake today. I don't know about the TDI turbo, but my HDi's turbo has variable geometry, and this lead me to a thought experiment:
The ECU is constantly checking boost pressure and tweeking the turbine's geometry to keep boost where it is supposed to be. So if the pre-turbo intake is opened up (less restrictive = lower pressure drop = higher pressure delivered to turbo) the ECU adjusts geometry to 'scoop up' less. Post-turbo pressure is the same (ECU made sure of that), but the turbo is doing less work, i.e. less work is extracted from the exhaust.
So, opening up the pre-turbo intake has the same effect as reducing exhaust back pressure. And is probably cheaper.
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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.
Some how we have to get Julian a 1985 Mercedes 300CD turbo. He would like that. The ECU controls an air recirculation valve and the EGR valve and that is it. Fuel is controlled by your foot and the amount of boost pressure you have.