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The exhaust downstream of the cat will not have any effect on emissions other than accoustic :)
The emissions tests are done at part thottle where the engine management is controlling the mixture with the O2 sensor in closed loop mode. So the emssions upstream of the cat are correct. If the cat is in good working order, the emissions downstream of the cat will be fine. For ODB2 controlled vehicles, some have an O2 sensor downstream of the cat to detect that the cat is working ok.
Normally asperated and turbo vehicle usually run open loop at full throttle, so they are not low emmisions when operated like that. They run off the ECU fuelling map. Both types of vehicle can run rich at full throttle to inhibit knock. Turbo charged engines need to run rich while on boost to prevent knock. That is why the tail pipe can be sooty. You need #6 heat range plugs to burn off plug deposits that occur while running rich on boost. The cat also gets carbon deposits while on boost. So while on boost, lots of CO is created. And while off boost, that carbon is slowly burnt off, which produces elevated CO for a while.
So, when you report for an emissions test, be aure that things are well heated up, and that you have not be on boost on the way to the testing station. A highway run would be ideal to burn the cat and other bits clean.
If your injectors are dirty and there is uneven fueling or poor spay patterns, things can go wrong. So use some fuel system cleaner in a tank prior to the test. You plugs should also be in excellent condition prior to the test. Uneven fueling, coupled with closed loop O2 control, will have some cylinders lean, creating NO, and some rich, creating CO2. The cat is designed to deal with such things, but there is no point in challenging it during the emissions test.
If you have never used a fuel system cleaner, perhaps two treatments would be a good idea. You can't get Techron Concentrate in Canada, but there are plenty of knock-offs and copy cats on the shelf at CTC. I think that it would be wise to not be burning this stuff in your fuel during the emissions test.
Modified ECU's do not change the off boost fuel maps or O2 sensor closed loop controls. So there is no impact on emissions. The changes involve increasing the fuel cut pressure, increasing the boost and extending the range of the fuel maps. Sometimes the RPM limiter can be increased and some further retard of the ignition at the higher boost pressures may be mapped in as well. Mixture richness could also be increased.
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