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I'm selling a pair of downpipes, ripe for gutting: http://forum.rs246.com/viewtopic.php?f= ... 6&p=612775

Someone asked for a quick howto, so I thought it better to do it as a separate post. There's no photos or anything because I didn't really plan it.

This is just the method I used with success, sure there are other techniques

Warning: lot's of nasty stuff in the catalyst material so do this outside and wear a proper mask (and eye protection).

Tools required:
Workmate or similar
Electric drill
Drill extension
Largeish spade bit (25mm?)
Abrasive flap wheel
Hammer
Large old screwdriver/blunt chisel or similar
Hot air gun

1) Clamp the pipe in the workmate so it's secure and the precat is horizontal, to prevent too much stuff getting into the main pipe.

2) Using the drill, extension and spade bit make several holes in the centre of the catalyst material until you have one large hole, leaving just a couple of centimetres of catalyst material round the inside of the cat housing. Hold tight and take it easy at first. Regularly unclamp the pipe and turn upside down to remove material.

3) Using large old screwdriver and hammer, separate the remaining material from the inside of the cat housing. It's bonded in well so quite fiddly. Again, tip upside down and shake regularly to get all the shards etc out. Once you're done you should have a uniform but rough-ish surface around the inside of the cat housing where the cat material was bonded.

4) Using a reasonably fine flap wheel and the drill+extension, clean round the inside of the cat housing until it's nice and smooth and there are no bits which could come loose or burn up.

5) Now the cleanup. Very important. We need to make sure there are no bits left inside the pipe. Turn the pipe upside down and make sure you've got any loose material out (you'll be able to hear any thing rattling around inside. Get a hosepipe and thoroughly flush the downpipe i.e send the water down through the main cat to wash any bits and dust that may have got stuck, give it a good wash around. Shake out excess water and leave for a couple of minutes to drain out. You don't want water sitting in the pipe for long, so grab the hot air gun and *gradually* warm up the pipe (still in vertical position) until all the water has evaporated out and the inside is dry. You could instead, use some sort of solvent to wash it out (but definitely don't use the hot air gun then!).

Great stuff.

One daft question though, how do you get inside the precat? Does it come apart?

It sounds like a bit of a job!

toshers wrote:Great stuff.

One daft question though, how do you get inside the precat? Does it come apart?

It sounds like a bit of a job!

Use the drill extension.

here's an example with pictures
http://www.audizine.com/forum/showthrea ... ggies)-FAQ

Brooner wrote:here's an example with pictures
http://www.audizine.com/forum/showthrea ... ggies)-FAQ

Ah, yeah, I think I must have read that a while ago!

Found it tony, cheers great link from Brooner too, how have I missed this

IMO this should be in FAQ section

j1sgo wrote:Found it tony, cheers great link from Brooner too, how have I missed this

IMO this should be in FAQ section

Sorry had to dash yesterday. Glad you found it

Are you still able to pass uk mot emission test with out pre cats?

Yep, as long as they do the MOT properly i.e get the engine up to correct temperature first.

reading the audizine guide it mentions the following :
Q: Will this cause a CEL?
A: yes, you will need either O2 sensor extensions, or get a JHM tune, the programming ignores the O2 sensor and won’t throw a code.
further down in the guide it then mentions :

The sensors that get the extensions are the ones on the main cat, correct the ones on the precats stay intact.
Correct. Upper sensors on the downpipes don't need anything. Sensors on the main cats need the spacers.

Trying to understand why you would need extensions / spacers for the main cat ?

I had mine done recently at AMD and they said it would not throw any errors and a remap was not necessary. I did a scan recently after the engine warning light came on and there were 2 related errors. I assume this was the reason for the engine light, I did also have a hose blow off the radiator overflow tank at the same time though so this could of been the cause but still suspect is was the precats.

It hasn't happened again since though but I haven't really driven it since.

Yes, you will (eventually) get CEL caused by catalyst efficiency errors. The ECU compares readings from secondary sensors (the one's between the cats) to the primary (just before precats) and will register a fault if they get out of whack, which of course they will eventually now there is nothing in between them.

It's the primary cats that the ECU bases fuelling decisions off etc, the secondary is just for efficiency comparisons I believe.

What I've found though is that sometimes it can be weeks or months before the CEL will happen.

Putting spacers into the secondary sensors simply takes them out of the flow of the exhaust so that the ECU doesn't "notice" any different. I'm not sure if they're such a good solution. A remap is the most robust solution. I *think* a remap normally just raises the threshold for efficiency errors so high that it never triggers.