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In the '80s I was using an engine dyno to extract maximum HP from my two liter carbed 911 race engine. I tried many variations of the bolt on "HP increased" parts available then. I played with engine and cam timing (906 cams), different height and shaped intake manifolds, factory vs. aftermarket headers, choke sizes and velocity stack heights (I made adjustable ones).
Every change in stack heights (longer and shorter) hurt the numbers. It seems that the Porsche engineers spent a lot of time doing their homework and getting it correct. Another thing that impressed me was that when a factory intake manifold run was slightly bent (the two outside runs), the inside diameter went from round to curved. None of the aftermarket manifolds did that and lost some flow.
My biggest gains were in compression and more modern exhaust systems (pipe sizing and controllable back pressure).
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