What are these defect devices trying to achieve? More engine power at the expense or pollution and worse fuel efficiency?
The downside is that engines used for short trips at low power (city driving) rarely get to high enough temperature to burn the carbon in the filters. Filters/catalytic converters get clogged and cause engine warnings / errors and bad performance.
no, it's the opposite. Emission controls result in worse fuel efficiency.
After Dieselgate, Volkswagen brought their diesels into compliance by software patch. People did dyno runs of all VWs before and after the patch, and in many cases (e.g. the 180HP Passat) got something like 10% less horsepower and drastically worse torque curves (max torque at 1900 rpm instead of at 1500 rpm) after the patch. Fuel economy also went down by around 10% because of the patch.
Makes sense from a thermodynamic perspective: your engine runs with more power/torque and more efficient if you run it hotter. But running a diesel hotter makes much more NOx.
The extra intake may seem pretty dramatic - though of course full data for comparison should be in order... It should be possible to compare euro-4 vs euro-5 model consumption through spritmonitor.de (through selecting by year) - I have no time right now but may do it later, and possibly report.
Then there's the real jackasses (like my cousins) that delete all that stuff along with the muffler and deliberately blow clouds of black smoke because deep down they are deeply insecure people and suffer from undiagnosed depression and anxiety.