You do not want to match the header inlet to the head. Let me clear up my earlier post. On the many exhausts I had seen there was a weld on the inside of header pipe which was very restrictive. This is what I was talking about. Now the perfect header pipe will be slightly smaller then the exhaust port. This is what you want. If you notice the header pipe starts with a smaller tube then opens into a larger tube. This creates scavenging as well as I believe the correct term is exhaust reversion. This slight mismatchwill aid in the removal of the exhaust from the cylinder allowing more room for the fresh mixture. I would not remove the sleeve completely which is used to seal the donut ring. If so you will not get a proper exhaust seal. If some have removed this you will need to go with a flange exhaust gasket which will have to be changed much more frequently. Most do not realise that an improper exhaust can actually hurt performance.
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