(Maybe dumb) question on headers?

[Yater]

Hi guys,

Not planning to put onese in my car,

But just wondering out of curiosity:

I see there are a lot of aftermarket headers leading to 4-in-1 exhaust manifold configuration on each side. I think many brands claim gains on horsepower.

However, since these are aftermarket (hence I assume "generic"), how do we know, apart from testing on the dyno, that the design (lenghts, etc) does not harm rather than benefit the performance?
I may be totally wrong, but in order to bring extra performance, it should be very well tuned with the valve timing of the engine, otherwise the pressure waves reflected at the junction could reach the cilinder exhaust ports at the wrong timing and penalize performance rather than help, isn't it?
The solutions being "standard" for different engines, heads, cams... i wonder about it.


Again, not willing at all to discredit the benefits claimed by manufacturers who probably know very well what they do, it's just out of curiosity.