So many trucks in my town obviously going around with highly bypassed exhaust now and there's no enforcement so they get away with making life toxic for everyone else. Many states don't even have inspection anymore and even if they did they just switch it off.
Why is that?
It's possible to tune a diesel without making smoke - normally this is done by leaving the wastegate closed for longer to make more boost and then adding more fuel.
Any smoke is too much extra fuel.
Here is more explanation: with the diesel emission devices, you are trying to control soot and NOx gasses. For example the EGR (exhaust gas recirculation) system reintroduces a bit of exhaust gas back to intake to reduce combustion temperature to reduce NOx gasses. However, this means that you are introducing sooty gas back to the intake, where it messes with other stuff like swirl flaps.
Swirl flaps are there to create a small tornado inside the cylinder for more efficient mixing and burning. But these are mechanical flaps and, in the best case, they stop working when they soot up. In the worst case, however, they go inside the cylinder and cause catastropic damage, requiring an engine rebuild/replacement.
Then another emission device for diesels is the particle filter. This is a very fine filter to catch soot exiting the exhaust. The engine puts a bit more fuel every now and then to burn them off properly. But the engine needs to run under a certain load and operating temperature for this operation to succeed, and if not, it gets permanently blocked and you need to visit a garage to force it or to have it cleaned/replaced. This happens if you only drive short distances and never let the engine to warm up, or if you idle for too long, or if you always drive in stop and go traffic. And sometimes, the filter collects ash instead of soot and that can't be burned off. This happens when you use incorrect sort of oil in a modern diesel car or if the fuel is bad. The only cure is cleaning or replacement of the filter.
Then you have another emission device: the ammonia. Newer cars spray a bit of ammonia (AdBlue) every now and then on the exhaust gasses to reduce NOx emmisions. This adds one more thing to refill every now and then and the car reduces power if you don't comply.
Now compare this to the gasoline car emission device: the catalyst. This one fails if you overheat it, or if it goes too old by age. The overheating happens if the combustion is too rich (too much fuel for the amount of air) for a long time but the engine controller prevents it anyway. Catalyst is there to burn any unburnt fuel and turn carbon monoxide into carbon dioxide.
Some newer cars also have EGR in them like a diesel car AFAIK, but I am not aware of the relevant failures of that bit.
Don't confuse the above with zero gain from custom tuning a gas engine, just that the gains are not nearly as great as you can get with a diesel. If you are running a gas car on a track you can get measurable results from custom tuning - but it isn't nearly as noticeable in day to day driving.
In USA are humungous engines (5.0l +) without a turbo In EU are mini engines (1.0l ~ 3.0l) and almost always with turbo. Is it better fuel economy?
Even when talking about gas engines, if you are towing (this is about diesel where towing is more likely than normal) your turbo is running all the time anyway and there is no savings over a bigger turboless engine, and the bigger engine will last longer. The bigger engine can also run at lower rpms which brings up power. In the end for a lightly loaded gas engine a small turbo is more efficient.
However, when you run your fueling extremely rich like someone that is rolling coal, NOx is going to be higher than in any reasonable tradeoff, because if the irreversible damage to the catalytic converter. It will be far worse than any conceivable lean air mixture.
Under lean conditions, the Rh2O3 layer in the cat reacts with the Al2O3 to create an inactive oxide. This degrades the ability for NOx reduction.