It's a bit wierd on some cars though,
for example, a completely standard 306 gti6 must pass an emissions test on an mot and the only way to do that is using some sort of cat in the exhaust,
I know people who have put the 306 gti6 engine in there 205 (most of them have e,f,g reg cars) and they aren't running a cat and they go to the testing station (without a cat) and pass, why is that? They have declared all there mods and engine swaps so why are they still tested as a non cat 205? I thought the test was based on what engine it has rather than the car