But you also get rid of the O2 sensor surely.
Carby models would have no 02 bung, no bump/ no cat.
Next model had O2 bung, Bump and Cat horizontally mounted.
Then it had an O2 bung and a Bump/Cat mounted Vertically and closer to the exhaust port thereby allowing the Air injection to be deleted?
That's my understanding anyhow!
One thing they all have in common is their purpose.
Which is to allow the exhaust Gasses to escape ( I am disregarding the emissions and noise concerns) and the better the Gasses go out the better you can get them in.
We have seen modified Air filter covers that are available at a price, the first iteration was an OEM cover trimmed down on a Lathe.
Then the super metal one.
Oh nearly forgot, the other thing the header pipes have in common is the wonderfully useful restriction ( I wait to be informed of its importance) do the Math.
OD of header is 43mm the OD the end that accepts the Muffler is 35.5mm about a 17% decrease in diameter of the OD which is both directly relative to the ID and when expressed in Square Area is considerable