Is there a performance difference between these manifolds? Is it work exchanging a bb manifold for a avenir manifold? Any and all comments are greatly appreciated.

I would say that they may have made improvements to it because it is obviously different, specialy the lenght of the number 1 cylinder runner is longer. I don't know but in this case I would say newer is better and it doesn;t hurt that the Avenir is rated at 230hp but thats just my opinion.