Guide; HDi De-Cat

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Guide; HDi De-Cat
#1
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.

[Image: EstateDe-Cat01.jpg]




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.

[Image: EstateDe-Cat02.jpg]
[Image: EstateDe-Cat04.jpg]


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.

[Image: EstateDe-Cat03.jpg]


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.

[Image: EstateDe-Cat05.jpg]
[Image: EstateDe-Cat06.jpg]


Re-fitting is the reverse of removal.
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#2
What backbox would you recommend using with a decat? Great guide by the way!
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#3
Some people use the middle section. But anything that isn't restricted like the standard boxes.
#2 Moonstone Blue HDI
[Image: 6b0c7309-5184-463c-9f73-2a7b96601418_zpsfdf041fe.jpg]
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#4
Any modification needed to make the middle box fit where the back box should be?
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#5
There's plenty of guides and things around if you search for them. Dum dum has done plenty.
#2 Moonstone Blue HDI
[Image: 6b0c7309-5184-463c-9f73-2a7b96601418_zpsfdf041fe.jpg]
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#6
Nice work here, I forgot about this Big Grin

Just one question Chris, do you think with this technique it'd be fairly easy to weld a pipe inside the cat chamber, so it was basically a straight through pipe with a cat body around it...

I've never had the amount of gear to test it properly but I always thought my HDi sounded a bit more like it had a pipe with running water going through it with the de-cat fitted, but I wonder what the difference is with a hollow cat. Ie, louder, resonance, flow issues etc.


Obviously it's a bit more hassle doing a straight-through stealth cat, but it'd be interesting to do a test on them all hehe...

If I get the chance I will I think, would be interesting to see Smile

Cheers

Dave
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#7
Having an empty cat on mine certainly accentuated the turbo-spool noise, not sure it was particularly good for flow though as it didn't make as much difference as i expected.
306 HDi Deathtrap - 130bhp / 220lbft
...UPGRADING...



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#8
do you think you could do the same with the middle box and back box? cut and gut them
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#9
You could but the mid box isnt a restriction, the back box is.
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#10
straight through pipe it is then lol
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#11
Just de cated mine and the decat knockson the sub frame...

Short of grinding a bit of the sub frame down and a bit of the exhaust... im struggling to think of what i can do to stop the banging!
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#12
(15-10-2012, 09:17 PM)sean-306 Wrote: Just de cated mine and the decat knockson the sub frame...

Short of grinding a bit of the sub frame down and a bit of the exhaust... im struggling to think of what i can do to stop the banging!

Did you cut out the cat?

If so, cut the weld and adjust the angle. If not, check the exhaust is hanging correctly.
Welding and fabrication projects undertaken, contact me for more information.

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#13
no, its a proper de cat pipe i bought of hdi kyle of here... all exhaust is hanging properly
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#14
are your engine mounts alright? If theyer loose/worn it could be causing the nock
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#15
Engine mounts are good, exhaust is hung properly... still banging. Cut a small section of the subframe away for clearance...

Doesn't knock when in neutral and revving it, only when its in gear... really stumped to whats banging now...
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#16
For any newbies reading this: Sean's problem is one you get when your engine mounts are knackered or you buy a decat pipe that hasn't been made up properly, done properly there is no downside to a decat on these engines (unless you're an eco-warrior and genuinely concerned about your cars emissions).
306 HDi Deathtrap - 130bhp / 220lbft
...UPGRADING...



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#17
(22-10-2012, 08:09 AM)Poodle Wrote: For any newbies reading this: Sean's problem is one you get when your engine mounts are knackered or you buy a decat pipe that hasn't been made up properly, done properly there is no downside to a decat on these engines (unless you're an eco-warrior and genuinely concerned about your cars emissions).

Solution... BakerBM Engine mounts!
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#18
personally, if it was a genuine cat i would weight it in (they are worth around the £200 mark atm) and buy a decat pipe for £30!

even if it was a after market jobby there are still worth £20/30 so your not really spending anything on a proper decat which will be far better than a empty cat any day of the week...
Stealth 3door Dturbo - In progress
http://sphotos-d.ak.fbcdn.net/hphotos-ak...6076_n.jpg
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