Guide; HDi De-Cat

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De-Cat Guide

Removing the catalytic convertor (cat for short) on a HDi is an easy and common way to more power. The exact gain depends on how clogged your current cat is and what stage of tune your car is in anywhere from 2-5bhp is possible on a standard car, possibly up to 10bhp if you have a re mapped car. You will also notice that the turbo spools sooner giving you a more responsive drive around town.

Many people will warn against this telling you that it wonâ??t pass its MOT emissions test but the truth is that it will still easily pass without a cat as all 306 diesels in the UK, even the late HDis with cheap road tax, are only given a smoke density test, this is the same test that applies to all diesels from 1973 onwards. So when you sit down and think about it your modern efficient High Pressure Direct Injection engine in a small car only has to meet the same standard as a smelly and un-refined 3.5 tonne diesel van from the early 1970s.

If you de-cat your car and fail the MOT on the smoke test do not instantly blame the de-cat, it means there is something wrong with your car, probably the MAF sensor but possibly a fuelling issue.

My car comfortably passed on the fast pass system (half the normal smoke limit) last month on a stage 2 remap with no cat. The only issue you may come across is the MOT tester being a real stick in the mud and being aware of the rule that states that if a car came fitted with a cat from the factory then it must be fitted with one for the MOT. Many 306 owners get away with having a straight pipe welded in place of the cat and still pass the MOT like that but do you run the risk?

If not hereâ??s how to remove the restriction in your cat but look like you still have one fitted.

First you need to jack the car up and remove the cat. The cat is the front section of the exhaust. It will have a exhaust clamp at the back end with either 2 x 13mm nuts or a single 13mm or 15mm nut. The front end where it bolts to the turbo has a 10mm nut each side that youâ??ll need a deep socket and 2 long extension bars to get to. Be careful to catch all the springs nuts and washers when you undo these as theyâ??re important to allow the exhaust system to flex when the engine moves.

Many people will tell you that you can smash the insides out of your cat to remove the restriction and while this is true to a certain extent it does not fully remove the restriction. Here is all of what I got out using this method.






This isnâ??t all the rubbish in there by any means so I used an angle grinder to cut a 3 sided square along the top and prised it open so I could clear it out properly. This is what I found.





These are what Id missed while smashing with a pole. You can see how the cat is a restriction when you think the exhaust gas has to pass through 2 of those.




So once youâ??ve cleared the insides out hammer the flap back shut and take it to the welders for them to weld up. Mine charged £20 and even painted it for me.





Re-fitting is the reverse of removal.