Clio RS mk1, from 2 sport cat's back to one and still pass MOT?

[Stefan86]

Alright here it goes, I have to translate my whole story from dutch to english so wish me luck:

Ladies and gentlemen I have a question regarding my Clio RS.

Since februari 2011 I 'feel' myself a proud owner of a Clio RS. There was some work to be done to the car so I'm not yet finished that project.
After the pipe to my rear silencer was raptured, I decided to get myself a custom made Stainless Steal exhaust at Maxi Performance at Hengelo. They have a great price for the the complete system so I took a gamble that everything would be all right.
They mentioned an awesome increase in power if I would go for a sports cat, but I knew this was just sales talk and far from realistic (they mentioned 30-40 hp increase because a Golf VI GTI got 70hp). If I was a sales guy I'd probably tell the same, I think it's your responsibility to know what is realistic and what's not.
The price went a quite a bit up because I was one of the 'lucky' ones to have myself a Clio RS with double cats... Now I had to buy 2 instead of one which obviously raises the price. They made me an open system as to obtain the most power from this system.
After about a month and a half it was time to optimize the engine a bit more (and after I bought my Clio a K&N replacement filter, thanks for the filter test which made me go for this one). I choose to chiptune the Clio at BullPower at Dronten. The first reading told us the Clio possesed an acceptable 164hp, if it weren't for the new exhaust system I'd be happy with only 5hp lost in 10 years. After the dyno run Marco was busy tuning the software and after about 3 runs the final score was 180hp and torque has gone from 204nm (sorry I don't know the lbs/ft figures...) to 224nm.
After a while I wasn't happy with the starting figures because I had put quite a bit of money in an improving exhaust system which didn't seem to do a lot... Besides, I'd like to go up to 200hp for my Clio. The torque figures were a bit higher than according to the specs of Renault so maybe the exhaust and or filter did it some good.
It's beginning to be quite a story so I'll go straight to my question:
- Could it be that the double or dual sportscat configuration is holding the Clio back?

That's the idea of the Maxi Performance so they offered me to go back to 1 sportscat. I like the fact that they do care about aftersales and keep the customer happy. I think I might even be able to arrange for some money in return if they remove 1 sportscat.
One last addition to my main question:
- If I go back to 1 sports cat instead of 2, will I still pass my MOT?

If I take a look at the K-Tec site (made me wish I'd live in England aswell) I see only single sports cat for the Clio and no double or dual ones. On different forums I saw my Clio is quite a special one since it's the last of it's kind (Clio 2 mk1) which has a double cat with 2 lambada sensors.

I look forward to your replies concerning 1 sports cat vs 2 and if I will still pass my MOT.

(jeez what a work to translate it all, I hope my grammar is ok)

 

[Gardus]

A good sports cat is better than both a single or a dual stock cats.
There's no sense in fitting two sports metallic cats, it's just more espensive! A single sports cat in the right size is the best solution.

Don't worry about MOT, if a single metallic cat is from a reliable source and it's in good condition you'll pass it without any trouble.

Obiously you won't get more than 5-7 hp or so just with an exhaust, even you fit the 182 header which is a common mod. A 172 in good form will give you 160/165 hp stock, with a full exhaust, an air filter and a good map it can probably get close to 180.
If you want 200 hp you need deep pockets as you'll need individual throttle bodies or a JMS intake, cams, a custom remap, a ported head and a RS matched inlet.

 

[Stefan86]

Thanks for your input, just to be sure, the question is a single vs double sports cat. I'm not talking about a single or double OEM cat.
The 5-7 bhp is exactly what I expected to 'gain', but according to the Dyno diagram I have 164 while I expected it to be more towards 170. At the moment, according to the diagram afterwards the chiptuning, I have 180 bhp. So I hope to obtain around 185bhp when I go back from double sports cat to a single sports cat. The tuner thinks my Clio might hold back because of the double setup, could be it has to little backpressure right?

 

[Gardus]

The only advantage of the twin cats are that they leave more room under them so more clearance.
A single cat has a better flow because there's no y-pipe before and after, but the performance gain also depends on the density of the cells and the area and lenght of the honeycomb matrix.
For example, for our Supersprint system we use a single cat, the in and out pipes are 60mm in external diameter, then we have two cones, 75mm in lenght and 61 to 108 mm in diameter. The cat itself is 108x135mm, with 200 cells per square inch.