Hi guys ive heard many opinions on this subject but it needs putting to bed im looking to upgrade my exhaust on my focus 1.8tdci so as to get the best out of the remap im guna have but iver heard mixed opinions on Decatting sum say they only do a smoke test (MOT) on older cars mine being 2004 i want more power but i also want it to pass an mot can anyone solve this argument ?

On a diesel engine they only do a smoke (particulate) test, so you can decat ok and still pass this test (petrol engined cars are a different matter)

But there is also a visual inspection, on some cars you cannot see the cat/ decat (due to undertrays/ engine covers etc) so a decat pipe can be fitted ok, on others, where the cat is more visible, it may be nessesary to remove the inside of the original cat, to decat the cat

as carbon can sit in/on the surfaces of the cat, it can actually increase smoke, so removing it can reduce smoke (along with things like a solid EGR plate)

As well as a dacat/ cat innard removal you may need to remove/ upgrade the exhaust back box, as this can be restrictive (it can restrict flow more than the cat) - so no point in just doing the cat without upgrading the back-box too

Im thinking of upgrading the exhaust for a remap mines 1.8 tdci mk 1 focus would it be better to remove the cat and get a decat pipe or just get a holesaw to the cat ? What kind of power increase can i expect from just the exhaust upgrade ?