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After 8 years of sterling service my "friendly" MOT tester has suggested that I consider having cats re-fitted as the powers that be are, understandably I suppose, looming large on all emissions tests and it's getting considerably more difficult to "fix" the test. He reckons I'm going to be ok this time (test this month) but going to be a lot more tricky going forward.

Anyone got ideas on what to go for? I'm running full Milltek with their race dp's fitted (so no pre-cats) and just rear boxes. Presumably, Milltek would be the obvious go to but anyone got other ideas? What are you running to get through the emissions test if you're not OE? Would the addition of 200 cell cats be enough to reduce emissions to a pass? Obviously, I really don't want the hassle and cost of fitting pre-cats....and can anyone give a ballpark figure on power/"torgues" losses?

I’m running miltek sports cats on mine and always flys though the MOT-not sure if there still doing them-only other thing you can do is get some custom ones made up that you can bolt on just for the test?i wouldn’t trust the cheap ones on eBay to last/or block.unless someone running them on here?


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Jayjay101 wrote: ↑

Wed Sep 18, 2019 10:58 am

I’m running miltek sports cats on mine and always flys though the MOT-not sure if there still doing them-only other thing you can do is get some custom ones made up that you can bolt on just for the test?i wouldn’t trust the cheap ones on eBay to last/or block.unless someone running them on here?


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Cheers Jay. No, I won't be going down the fleabay route.
You still running with OE dp's? Just wondering if sports cats alone will reduce emissions enough to pass.

Weiß-sechs wrote:

Jayjay101 wrote: ↑

Wed Sep 18, 2019 10:58 am

I’m running miltek sports cats on mine and always flys though the MOT-not sure if there still doing them-only other thing you can do is get some custom ones made up that you can bolt on just for the test?i wouldn’t trust the cheap ones on eBay to last/or block.unless someone running them on here?


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Cheers Jay. No, I won't be going down the fleabay route.
You still running with OE dp's? Just wondering if sports cats alone will reduce emissions enough to pass.

Sorry mate I should have explained it better-my downpipes are miltek with the sports cats built in the cats are massive which is why I wouldn’t got the eBay route as thay are small!-I’ve just got the sports cats and back boxs(powerflow) and it passes emissions with flying colours and I only live 10mins from the test centre so it’s not like there red hot when I get there. So I’d say 200cel is enff


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Cool. I've got 12 months to sort it

Being an older car you’d get away with the smaller cats most likely, my misses 03 plate GSI went through emissions with just the pre cat installed.

I’d just have a section cut out and flanges put in with cat sections made, so you can swap out nice and quickly for mot time, if that’s the case then just get a set of standard cats as you know they’ll work, 15 minutes under the car swapping the sections back and you’ll have a decat again.

I’m doing it for my s4 and TT as it seems the best solution, my s4 is currently running 2 x 1.6 Vauxhall cats at the moment for mot time, admittedly I struggled to pass but the map isn’t finished (running rich) and I had an exhaust leak on the v bands just after the turbos.

Pre cats generally have a higher cell count. I would imagine the C5 runs 400 cell pre cars and 200 cell main cats. If you’re going aftermarket and want more power but pass MOT’s, then 100 cell sports cats are the best option. I would think Wagners do them for the C5, as they do for the B5’s.

200 cell would give you the same effect as gutting the pre cats and keeping the OE main cats assuming the rest of the exhaust pipe work is the same diameter as OE. You could go to 3” pipes to generate more power on a turbo’d car too (like Wagners).

Thanks all, good info there. Marc1, do you reckon 100 cell would do the job with my race dp's or need 200 as no pre cats?

Out of interest, anyone know the bore of the Milltek system? 2.5"? (so I don't have to crawl under and measure ha)

Walton motorsport do a good selection of cats in various sizes , and cell counts . I'm currently trying too get a trade account sorted with them . I cant comment on there quality as yet and if your wanting a cat section building I to existing dp's or new larger bore ones with any options you need adding I should be able too help you out with it pretty shortly

Cheers Al, I'm not a million miles from you

I think theres only us in this area then bud , I did see another black c5 a couple of years back but I assume he was passing through . Wasmt you was it ?

Weiß-sechs wrote: ↑

Fri Sep 20, 2019 12:43 pm

Thanks all, good info there. Marc1, do you reckon 100 cell would do the job with my race dp's or need 200 as no pre cats?

Out of interest, anyone know the bore of the Milltek system? 2.5"? (so I don't have to crawl under and measure ha)

Should be fine. Not sure if Milltek are 2.25 or 2.5. Should be able to get from Google though

The miltek down pipes changed design and bore through their life. I have an early set with out precats and the replacement flexi sections from miltek didn't fit (too larger Dia). So you may have to measure your set to be sure. I fitted v-bands just after the engine mounts and after the cats do they can be swapped out in situ.

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Ajc wrote: ↑

Sat Sep 21, 2019 5:38 pm

I think theres only us in this area then bud , I did see another black c5 a couple of years back but I assume he was passing through . Wasmt you was it ?

Nope, not me pal. Mine's the exact opposite to black

Puggie wrote: ↑

Sat Sep 21, 2019 11:48 pm

The miltek down pipes changed design and bore through their life. I have an early set with out precats and the replacement flexi sections from miltek didn't fit (too larger Dia). So you may have to measure your set to be sure. I fitted v-bands just after the engine mounts and after the cats do they can be swapped out in situ.

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Good info mate, v-bands are the future