[quote="RedkopFrom my experience with this.... if you have for example a 93/94 car but have had another engine put in it with a build date pre 1st August '92 - the MoT/emissions test will be based on the engine manufactured date.[/quote]

pretty much it... the test is done on whatever is the older - engine or car. hence why in my mk2 escort the zetec only needs to meet 1980 emissions regs... hence why i can run dirty big carbs rather than forking out on throttle bodies etc. by the same token you could stick my old crossflow that was fitted in my escort into a modern mondeo (god only knows why though!) and it would be emissions tested according to the engine's age, not the cars.