Sports cat: 100 or 200cpi?

[D1andonlyantman]

Hi. I'm looking to buy a sports cat downpipe for my mk4 180. I've found 2 that are both the same price but 1 of them is 100cpi and the other is 200cpi. I've had a look on google and found out the cpi is cells per inch which determines the life of the cat and how well it filters. Firstly, does more CPI mean it is more restrictive because it filters more and secondly, would 100cpi be enough to pass an MOT?

Thanks. 

[Wazzzer]

Yes the higher the number the more restrictive and I doubt you'll pass MOT on a 100 cell cat

[D1andonlyantman]

Oh ok, thanks. Think i might have been persuaded to go decat anyway cheers

[Wazzzer]

Be aware you may have the EPC light come on with a decat

[D1andonlyantman]

The light is already on. I had a look earlier and he cat is pretty bodged as far as I can tell and I think it's down on power but I'm not sure why the light is actually on. Would the light being on affect performance if I did go decat?

Here's a pic

[Wazzzer]

the light itself doesn't give you limp mode or anything like that. You have 2 lambdas so you need to find out what one is flagging up the code before you think about going for a decat or you might end up masking a problem

I feel your pain with that snapped stud, I did two on my last downpipe swap. It looks like there is still some stud left poking out so get the downpipe off and give it some heat and you might be able to get it out with some stillsons

[D1andonlyantman]

I thought you had 2 lambdas but couldn't see the other. My mate actually hooked it up to his diag computer and said the warning light was because of the dodgy cat, so couldn't I replace the downpipe and get a decent look at both lambdas and see what's what? In retrospect I should have got my friend to go into more detail....also, if I can get the stud out then I guess I put a bolt in its place, if I can't, what then? Replace the bit the stud is coming out of (whatever that is). Sorry, I'm a complete beginner on cars but know my way around a bike pretty well so it shouldn't take too long for me to get the idea

Thanks

[D1andonlyantman]

i was just looking at a fiche to work out what I might need to buy and it occurred to me that if I replace the cat, whether it's with a sports cat or decat, it would replace the front part with the snapped stud wouldn't it?? So couldn't i get the decat and it would replace the broken bodged bits and worse case scenario need a couple more lambdas or have I got it wrong?

Thanks

[D1andonlyantman]

After speaking to a mechanic friend of mine I just happened to bump into, he's said that's not the front pipe with the snapped stud, that's where it connects to the turbo. Apologies for the confusion. He said it should work with 3 studs and some sealant though and is happy to do the work for me at a good price and also happens to be an MOT tester who's isn't too picky

[dom]

The downpipe/cat is one piece so if you replace the cat or fit a decat you'll need to sort the broken stud. As Wazzzer has already said it looks like you'll be able to get the broken one out easily enough.

As for your lambda sensors, the sensor after the cat just monitors emissions. This is the one that can cause issues with the engine management light after fitting a decat (and some sports cats) as the emissions are too high. If the light comes on you can either get it coded out or fit a spacer to trick it into thinking the emissions are ok.

The lambda sensor before the cat is responsible for controlling the fuelling and is much more important - if this one is faulty it needs to be replaced regardless.

It's worth noting that lambdas rarely fail so I'm guess they are not the reason for the engine management light coming on. Have you scanned for fault codes?