Originally posted in drivetrain - should have probably posted it here.....

There's been a couple of posts on the board regarding emissions for MOT etc. Some people have the centre CAT replaced for MOT & get through. Some have a downpipe with centre CAT etc.

Without thinking of these variations as much as possible & just thinking of fuel, octane booster ECU's etc.

Could anyone shine some light on which would be best. I read on Power Engineering's site about running lean or rich - it seems for more power you run leaner but this increases the chance of engine meltdown under certain conditions.

With SUL & octane booster the engine would have a more efficient burn. However, even if the burn is more efficient & the amount of fuel in the exhaust emissions is less wil the ECU not compensate after the lambda sensor gives it's reading & squirt in more fuel. Hence bringing up the fuel in the exhaust emissions.

So maybe the only way to reduce emissions would be to have CATs after the lambda sensor (wouldnt affect the lambdas reading or the ECU's interpretation) or have a differently maped ECU (wouldnt affect the lambdas reading but would affect the ECU's interpretation). This wold still take the readings of the lambda sensor but maybe not squirt in as much fuel as a normal ECU & run a little leaner.

Maybe I've got it completely upside-down, sideways. I'm not sure at all but would love anyone who has knowledge to post & put my mind at rest either way.

Cheers

Johny