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I built the exhaust on my 82 and until now didn’t understand why it ran so well,it had full size headers and a GS 1100 muffler,it had a flat spot in the rpm range so I had some different size holes water jet cut and fitted it where the muffler bolted to the header pipes,found the right one to clean it up,I think it was 1 1/4 inch.Mine sounded a lot different than a stock motor.
 

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I'd like to see a pic of this hot rod BMW you have. Many members here are proud of their builds and post their progress.
A little bit of knowledge can be dangerous. If you just get in there and start hogging things out odds are pretty good you will hurt performance. Matching the bottom of exhaust ports is never good as it increases reversion when the cam is on overlap. Bigger port sizes can hurt performance because it slows the speed of the air in the port. That causes fuel drop out and hurts low end torque. I bet Joe Average would have to be pretty lucky to improve over all performance with a grinder more than a couple of percent which would be negligible and not worth the effort. If you want to improve overall performance invest in a good 3 angle valve job and leave it at that. Most performance increase is right at the bottle neck where the valve and seat intersect.
 
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