Looking at doing a cat back stainless upgrade. Can someone advise as to the optimum diameter ?
1.8 mpi running at about 120 bhp

I am not a scientist in this field or anything like that, but I would go for something in the 2-2,5 inches range depending on tune. that should be enough regarding flow.

Think there's an exhaust section on dvapower with manifold size/lengths and exhaust size