Backpressure, and egor

[Bruce Plecan]

On a NA engine 4-6 PSI with a Converter would be a loud car.
6-8, is closer to normal, and a little higher for a Caddy/M-B..
These numbers are also true for downstream of a turbo.
Yes, in a stock oem/less than optimim system the crossover pipe
from the exhaust port to turbo can be twice as high as boost.  
In a "good" system, you can reach cross-over where at high boost the
crossover's pipe can be less than boost PSI.
  This pressure drops quickly as the exhaust cools, and is an average.
The pulse is high, and then there is a long low pressure event.
That's how they use reed valves for evacuating crankcases.
Jus me .02 worth
Bruce