Secondary cat delete - costs and issues?

[c3nturi0n]

I have a 3" MIJ custom-fitted exhaust on mine and it passed last 3 years' MOT w/o an issue.

Secondary cats were replaced with straight pipes (as per Ian's picture above) and rear silencers with given a new look with a couple non res boxes. It is pretty much 'std practice' for Stage II RS's.

Road legal, passes MOT w/o a problem and it sounds amazing (especially at cold starts ).

Hope that helps.

[cammmy]

I discovered the "secondary" ones behind the gearbox on my car were removed in 2013. The guy i bought the car from got it in 2014 and never knew. He never had an issue with MOT.

[welwynnick]

It sounds like it's established and proven practice to remove the second catalysts, but I'm still at a loss to understand how it's possible.

Most cars have a single catalytic converter (per bank) but the RS6 and a few other "ordinary" cars with particularly powerful engines have two in series, but it's not for flow - they do two different jobs. If you do a google picture search for "reduction catalytic converter" or "oxidation catalytic converter" you'll find pictures like this:

https://gaukmotors.co.uk/post/catalytic-converter

Modern cars have 3-way catalysts, meaning that they convert three pollutants - NOx, HC & CO. NOx are reduced to N2 & O2 in the reduction catalyst, and HC & CO are oxidized to H20 & CO2 in the oxidation catalyst. Also see SSP 941003 page 20. Now here's the point - there are always two catalysts, and the reduction catalyst always comes first.

Therefore in the RS6 the small cat after the turbo is the reduction cat, and the large cat after the gearbox is the oxidation cat. And it's the oxidation cat that removes the HC & CO that the MoT tests for. The MoT doesn't test for NOx.

So how is it possible to pass the MoT without the oxidation catalyst? Well I'm scratching my head there. I can only guess that the reduction catalyst also does some oxidation as well, though perhaps not as efficiently as the oxidation cat.

Nick

[Hadies]

Got my secondary cats removed about a month ago.
Cost 180 quid. Local exhaust place with a good rep.

Were they the ones behind the gearbox? Have you had an MOT test since then?

Yes, the bulbous ones.
MOT not due for a while but it won't be an issue.

[Stiggles]

Secondary cat removal is absolutely passable for MOT

My car with just primary cats passed last year, and our MOT system is government run so much stricter than in England, Scotland and Wales.

[bal]

Can someone confirm if o2 spacers are needed to remove the secondary cats?

[Doug_RS6]

I have only found a handful of mentions and so for everyone has said no need for spacers and no EML on.

[Fatneck73]

changing the ARBs gave a much better result than i was expecting, i went H&R, was expecting to also need to change the DRC/Springs etc, but for me found just the ARBs gave the feel i was looking for so not gone any further.

Just had mine done and ‘ditto’...great ‘upgrade’.

[Fatneck73]

What’s the benefits of removing the cats? Performance or sound or both.

[pcorpe]

cat removal? helps with stage 2, and a little more 'throaty'

[IanH755]

EML On/Off - As the vast majority of people get a stage 2 tune at the same time as they get the cats removed I don't we really know if there is "something" in the MRC stage 2 tune which prevents the EML from coming on, or whether the EML won't come on even without the remap.

[Doug_RS6]

I found some references to a stage 1 tune being suitable to keep the light off also. However only time will tell. Mine will be removed in the next few weeks and will see if it needs a further tune to keep the light off. Will most likely get the map checked anyway.

[IanH755]

A stage 2 map usually bumps the power/torque figures up a good bit too over a stage 1, which is always nice - adds an extra 40-70hp and an extra 100-150nm.

[Doug_RS6]

I must admit I'm slightly ignorant to the exact spec of the map on my car as it was done prior to my ownership. I did have some vauge info from the company that mapped it.

[IanH755]

As a rough guide you can tell what level of map you have from the hp/tq figures, these are the most common -

Stage 1 (MRC/Revo/Litchfield/AMD etc) - Usually 660-700hp and 830-900nm - just an air filter and a map, you still get the same power levels from a stock or an after-market cat-back exhaust.

Stage 2 (MRC) - Around 730-750hp and 1000-1050nm with gearbox map or 950nm without - air filter, secondary cat delete but, while a exhaust still not "needed" yet, its advisable.

When MRC do their Stage 2 map, gearbox remap and secondary cat delete to an completely stock car it costs roughly £2200 combined with the 2 maps costing the most (nearly £1600 total) and thats probably the best "value for money" upgrade on any car I've ever owned - 30% more power and 50% more torque than OEM for a little over £2k!