What era car is outside the Euro 1,2,3,4,5,6,7 etc emissions nonsense. In other words at what age can I tune my car how I want?
You can't. Even on an old crock you have to avoid visible smoke
Mind you if you can adjust your own ECU I guess you can retune for the MoT test just like some people remove the cats and replace them for the test. Some bikers take that approach with noisy exhausts and funny plates though I have never bothered.
The idea of 'tuning' an engine and ending up with poor emissions is a bit of a contradiction in terms. To extract the most power you can out of any given engine, you need to get it burning fuel as efficiently as possible. A well-tuned engine will always run as clean as possible, and a catalytic converter works to clean up the emissions even further. They do tend to absorb a bit of power doing so, hence the need some people have to fit Cats which are not as efficient, as they think they can make use of the extra power.
There is an approach to tuning which tends to go for maximum power at any cost, so you go for more capacity than you need, chuck in more petrol than you need, and go for the biggest bang possible. Engines 'tuned' like this tend to be the ones than run dirty.
