How do. Another moment of bad luck for me... On my way home from work i was waved in by a police man and then i was given an emisions check... Which i promptly failed as i havent got a cat... So in short, i now need to fit a high flow cat, Im using a 3 inch down pipe and decat onto a miltek system. Ive looked around on gaybay and found a few universal ones ( imhappy to fit one into my cat bypass section) The thing is... they all list diffrent CPI figures for the cat... what should i go for to avoid too much restriction but to pass an MOT type test??