Decat and Fuelling

[VTRelite]

Now I see a lot of people saying how they've decatted their car and have seen a difference in MPG.

Now I believe this is mostly down to people revving harder and higher due to the noise change.

But the thing that gets me is that obviously on the MK2 it will have some effect regardless of driving style simply due to the fact of the second lambda post -cat but surely on the MK1 it won't make any different if driving style changed as there is only 1 lambda sensor on and that's pre-cat so removing it will not make the car richer or leaner. Surely?

[SteveWC]

Now I see a lot of people saying how they've decatted their car and have seen a difference in MPG.

Now I believe this is mostly down to people revving harder and higher due to the noise change.

But the thing that gets me is that obviously on the MK2 it will have some effect regardless of driving style simply due to the fact of the second lambda post -cat but surely on the MK1 it won't make any different if driving style changed as there is only 1 lambda sensor on and that's pre-cat so removing it will not make the car richer or leaner. Surely?

The increase won't be so bad when if you're still using the standard manifold, but you have to think the greater flow you give your exhaust, the ECU is gonna match that with more air through the inlet this goes especially with a manifold meaning you will be burning more mix overall and higher fuel costs.

Fuel costs are increased loads with a manifold though I found, the first lambda will struggle to get the mix as economical as before usually because its in a different position i.e. further down the pipe and/or it doesn't get as hot as the standard cast iron manifold. Also although the lambda will adjust fueling in general there is much more unburnt fuel in the exhaust anyway without a nice cat element to soak it all up as well as a big increase in flow compared to standard manifold meaning more increase at the inlet again.