Am in the process of building my exhaust for the car as I am going to be putting the car through an iva test I need to keep my emissions down as its a 1998 2.0 turbocharged engine.

I've noticed that the cat's am looking at are rated to cell's per square inch. Is there anyway of working out what would be the best cell rate

400cpi is tin-top standard. Will do the job easily but sap power.
200cpi should do the job but will have to be hot.
100cpi... could work but marginal. It won't take much power.

Power loss isn't really an issue tbh. Much rather have a happy iva tester

I went for a weld in 400 cell item - can't be any worse than the original item and like you I don't want a fail!

[Edited on 13/9/15 by ianhurley20]

This is what I used to get my 1997 BMW 2.8 straight 6 powered Viento through that part of its IVA test:

2.50” 63mm Centre 200 Cell Universal High Flow Sports Catalytic Converter

I spoke to the supplier and they said that their 200 Cell ones worked as well as other 400 cell ones as they had more catalyst in them per unit surface area (which is why they cost more). I was a little sceptical but gave it a go. There was no e-mark on it and the tester didn't mention it.

Test results were as follows:

Fast Idle CO = 0.04% (needed to be <0.2%); HC 25ppm (200ppm); Lambda 1.013 (target 0.97 to 1.03)

Idle CO = 0.1% (target <0.3%);

So easily within limits.

Guys, thanks for your very useful replies. Will check out these examples.