Do dents in your exhaust manifold (header) rob the engine of power?

I'm not so sure.

The restrictions will act as venturis in which the gases will speed up (as in a carb) thereby allowing more gas to pass through so reducing the effect of the restrictions. With the restrictions affecting only a small part of the system their effect is therefore minimised.

Well, that is my stab at an explanation.
Remember it is a large v8 na engine which doesn't produce that much power per litre
If you tried this on a smaller turbo motor , say my straight 6 RB producing the same or more power but using higher revs I would suggest the dents would make quite a difference.

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