to add to Narnians great explanation:
headers also assist in scavenging. what is scavenging? I will try and explain....
exhaust is not dispensed in a continuous flow, it is in pulses. W/O scavenging the exhaust pulses are pushed into the exhaust by the piston moving towards the top. With scavenging the previous pulse sucks the exhaust from the combustion chamber, this frees up HP!
each of these pulses as it flows down the exhaust tubes has a low pressure area behind it in the tube so as the exhaust valve opens the exhaust in the chamber will flow to the low pressure area behind the previous pulse....and another benefit to this is...due to the overlap in the cam(intake and exhaust valve open at same time)this scavenging will also help suck the intake charge into the combustion chamber.......!
so you can see how an exhaust manifold is not as good as a header because all these pulses from all the cylinders are dumped into the manifold and mixed up together so they can't help do the job of scavenging.....check out the Thorley Header website and they will explain how Tri-Y headers take this theory oone step further...
I hope this made sense to someone out there!
360 2v, 400 QT with low
33x12.50 BFG KO's
Thorley headers, 3" exhaust
Infiniti power leather seats