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All cars equipped with oxygen/lambda sensors can run in 2 modes.
At constant throttle opening and (mostly) constant load the ECU will look at the signal coming from the oxygen sensor and use this feedback to adjust the mixture to be as close to stochiometric (14:1 air fuel) as possible, so the cat-converter gets just the right gas mixture to work best (it actually 'swings' around the optimal point constantly).
When using varying load and throttle (eg. accelerating) the ECU disregards the oxygen sensor input and falls back on the pre-programmed maps for fuel, ignition and boost.
Full throttle is a bit of a special case.. At that point the ECU will also disregard the oxygen sensor (even though it's a steady-state) input and deliberately enrich the mixture a little more. This extra fuel has the property that it cools the valves and also lowers combustion chamber temperatures.
But.. There is a tradeoff because running rich also causes the power to drop off a little.
If you back off the throttle just a little bit then the ECU will no longer see the 'full throttle' condition and start to look at the oxygen sensor again and you get a 'good' mixture, but the throttle butterfly is open so far that it hardly makes a difference for the air-flow going into the engine (some may argue that the added turbulence of the throttle is even a 'good thing').
This bascially results in just a tad more power available to you. I could definitely feel it on my T4 and the 2.0T shows the same sort of behaviour. Very odd to feel the car accelerating when you ease off the throttle just a bit. Friends driving behind me also mentioned that it stopped emitting blackish smoke (from the rich running) at that time.
Hope this helps..
Bye, Arno.
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