318i cat bottleneck?

[JohnA]

Hi,

Im doing a m40 -> m42 swap on a '92 convertible,

I changed the exhaust manifold, but I was planning to keep the rest of the exhaust system, and adapt it accordingly. As I removed the catalytic converter I noticed a bottleneck just before the cat. As i am playing with pipes an my welder , i was wondering if this is beneficial for something, or would it be better to remove it and fit the same diameter as the rest of the exhaust system?

thx,

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