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Fitting a Decat Downpipe

Won't pass MOT. Getting caught without one by VOSA could result in car getting crushed. MUST be remapped to run safely and avoid running lean.

If you really want to go down that route, don't bother buying a decat pipe. There's some good cheap race manifolds out there such as the japspeed which will net you much bigger gains than a simple decat ever will. Still requires remap.
 
Wow thanks had no idea it was that serious. Why wont the car pass MOT without the CAT, is it an emissions problem? The Decat downpipes i have looked at have the 2 sensors in them the same as the CAT so i am surprised it causes the engine to run lean.

So I guess most people stay on the original CAT? Surely changing the manifold and exhaust but keeping CAT causes performance issues? Should I have the car mapped after fitting a new exhaust?
 
To get the most out of an exhaust a remap would be required, but it won't be running lean without a remap.

CAT is there soley for emissions on all cars.

The reason they run lean is that the CAT is a huge airflow restriction, removing it allows a signifcantly more air to move through the engine. More air = leaner mix. There's only a small amount the OEM ECU can correct for, removing the CAT takes it out of that range. Hence you need a remap to add more fuel and avoid running lean.
 
I ran ukdm ep3 for over a year with manifold decat and backbox not a bother on it ! 3 of them together made bit of difference
 
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