jmca

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

hackish

Emissions are not just a simple function of the amount of fuel burned. As far as emissions as a whole go it depends on whether you're trying to reduce NOx or *** emissions. There are really 2 strategies. Basically the higher the cylinder pressure the more NOx emissions you get. As far as I know the cats are 2 parts, one half takes care of NOx emissions and the other the ***.

I'm not an expert in this area but having done some engine management to get past tests here is what I do know. The ECU in our NA 2.5 engines here runs a lot of retard on idle. This does 2 things.
a) since the fuel is still burning as it comes out the exhaust port it lights off the cats faster
b) with 7-9degrees of advance most of the combustion happens later. This keeps cylinder pressures and also NOx emissions lower.

Running the same mixture settings and 19 degrees of advance increased NOx emissions by a factor of 5. I don't have my notes on this with the exact figures anymore

As for the stock computer I don't think there is much you can really do with it to alter emissions. Subaru did it a specific way to achieve compliance with different emissions laws. From what I can tell it is set up to run excessively rich at WOT so if you could lean that out ever so slightly you would probably achieve better emissions during accel. Who tests for this? I know here there is no AWD equipment for testing under load. Some states test for it but AWD vehicles are exempt.

-Michael