To Cat or not to Cat

[Standaman94]

My Williams failed it's MOT on emissions because, unbeknownst to be, it was fitted with a decat pipe (it all makes sense now!)

So the garage is getting a Cat and putting it on to pass the MOT.

The question is, however, after the MOT do I put the decat back on or not? I know it will sound "better" but I'm more interested in power and fuel usage.

[andreas395]

If your not bothered leave it on mate saves swapping it over every year

[16v_paddy]

Or find a more friendly MOT tester & leave it de-catted

[Standaman94]

What about fuel efficiency... Does a decat use more or less fuel?

[16v_paddy]

Fuel use is entirely dependant on how heavy your right foot is, the lambda sensor is before the cat so it wont make any difference if it's got a cat or not, removing the cat gets rid of a restriction making the gases flow more efficiently

[mrodz]

Bumpitty. Any issues with fitting a decat? Will the car need any mapping after or is it a "good to go" job?

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[owen87]

Technically mapping it would be best but the cost would be extreme for little or no difference, the actual benefits would be negligible. It's basically good to go. The restriction from a cat is unnecessary and I would always remove even if that means taking it in and out for tests.