Twin scroll downpipes, fitting a cat
 

At the moment I have a RCM decat twin scroll downpipe on my classic with a 6 speed gearbox. However we have no such thing as decat friendly stations when it's time for vehicle inspection and there's the risk of inspections on the road as well so I would like to have a good 100 cell cat fitted. I know there's Milltek and Hayward & Scott with 100 cell cats but I guess the Milltek won't fit my catback and to be honest the quality of the H&S looks far from impressing. I think the RCM decat downpipe might be made by H&S (not sure but it looks similar) and I had to let a friend better their welds and grind down some ugly bits at the turbo flange.

So I figured that the easiest way would be to weld in a good 100 cell cat in the downpipe, preferably one of those with a 5" core not to limit power too much. But the issue is that the RCM downpipe sits really close to the gearbox casing. It's about 15 mm between the downpipe and the 3 plugs on the gearbox meaning it would be tight even with a normal 4" core 100 cell cat.

Are the twin scroll downpipes supposed to be angled from the turbo flange to the right towards the gearbox casing like this?? Because there's plenty of room to the left. I think my old Invidia single scroll downpipe is more straight and other twin scroll pipes seem to be that way too. It gives very little room to fit a cat.


https://cimg3.ibsrv.net/gimg/www.sco...31d1d62c6e.jpg
https://cimg0.ibsrv.net/gimg/www.sco...c8539811d7.jpg

I don't know what would be the best and easiest way to fit a cat? Perhaps sell this pipe as it is and buy another catted one, or perhaps buying one of those twin scroll turbo dump flanges and weld onto my single scroll Invidia which is much better quality than that RCM pipe.

Just run a cat in the mid-pipe instead. The Litchfield / Milltek systems do this. They just use HJS cats in their systems.

Yes that was my first idea but I'm not sure if it will pass the emissions test with the cat so far downstream. Luckily EU rules have made the HC limit go up from 100 to 200 ppm in Sweden which makes it a little bit easier.

I believe Milltek have or had two versions where the other has the cat in the downpipe.

The cat in the mid pipe won't get hot enough to pass emissions; someone on here commented on this last week, on a Litchfield

That was what I suspected. It would be interesting to know how far it is from passing the emission limits though. Some cars you need to drive really hard to get them to pass. Also some E85 in the tank could help.

https://www.haywardandscott.com/suba...oll-turbo.html It looks like it has a big cat fitted but it's not the nicest downpipe I've seen exactly so I'm not sure if I feel like spending the money on it.

I have a Milltek full system on my Legacy

It looks like it has 2 cats

https://cimg0.ibsrv.net/gimg/www.sco...4590982314.jpg
Legacy 2.0GT spec.B Milltek system Mid Cat
https://cimg9.ibsrv.net/gimg/www.sco...9116716f03.jpg
Legacy 2.0GT spec.B Milltek system Front Cat
I do have trouble with emissions from time to time at MoT - but it has always got though - so far :D
I wonder if it is worth wrapping the pipe as far as the Mid section cat to keep more heat in?

I think it's a resonator in the downpipe so only one cat which explains your problems passing the emissions. The heat wrap could help I guess.

OK thanks - I will check it closer next time I have it on the ramp :thumb: