do I need to retune if I add a cat converter?

[crispy]

I bought my fd a while ago and found out it doesn't actually have a cat or air pump from one of the previous owners and it's coming up on it's mot. From what I've read a lot of people just throw in a high flow cat to get rid of the smell and don't bother with the emissions but I'm debating putting one in with an air pump again since the exhaust smells terrible and I thought I may aswell make it legal to save hassle

I'd prefer a high flow cat but I have no idea what ones will make it pass smog test and would it be as simple as putting cat in with air pump and attaching the belt to the air pump again or will it need tuning? engine is basically stock except from an apexi ecu and no clue if it has a custom or base tune or whatever on it

my fd is also a 1998 so I'm pretty sure it's not exempt from any emissions laws

 

[JoeLanger]

Even with a cat you'll struggle to pass emissions. Is it a single or twin turbo?

And will likely still smell

And yes would need a tune as these engines very sensitive to any changes to exhaust