When Does A 5" Cat Become A Restriction

[rob82]

I had a decat pipe and bolted on a FIA approved 100 cell metal substrate race cat on our R32 GTR with twin high flowed HKS2540 turbos.

Yes at top end, say 5600rpm onwards you can feel some loss of power and turbo response, the higher you revved, the more you lost.

Now heres the funny part after i spent all my money on it.......its still not street legal, it will not pass emissions testing, it does not meet the requirements, its just a race cat for classes that specify a cat must be installed.

I was better off staying with a gutted out cat with flanges and keeping the most restrictive/best scrubbing cat on the side in case i needed to go for emissions testing.

Learned the costly way.

C63 amg's run two 7" cats to be emissions compliant and still make decent power - you will probably find that they are over 400cpi though. Btw they only make around 280rwkw with a tune.