Hagerty did a you tube on adding headers to a 440, not that long ago. Mild Comp XE Cam, factory intake, factory TQ carb. I do not remember the exact amount of the HP increase, but it was substantial, with a big increase at lower rpms, good for the street.
I can run a dyno test and prove that headers pick up a lot of power when switching from manifolds. I can also run a dyno test showing that headers don't pick up much power from manifolds.
Just because you see a video of something doesn't mean that you aren't being lied to.
The trick is the camshaft. If you want the header advertisers to be happy then you use an engine with a big cam. Then the engine works better with headers and people buy headers and everyone is happy. Nobody buys ads for factory manifolds so the dyno tests aren't designed to make them look good. But, if a person is racing in a class like FAST where manifolds are required, they can be made to work. I've used manifolds on a few builds and it is fairly easy to make 500+ hp with stock HP manifolds. The FAST guys are making way more than that but those engines are crazy expensive. A normal performance build can use HP manifolds if the cam is chosen with care. That is really all it takes.
PRH on this board is the person to talk to if you want a camshaft ground to work with manifolds.
