As far as I know, most boost pressures are measured at the plenum, because that's the last place it can be measured b4 it goes into the engine... A turbo may be making 10 psi at the turbo, like you say, it might end up being 6psi at the plenum, but that air is hot.. With heat comes detonation ( also with not enuf fuel comes detonation ) . Add more fuel, raise the boost, off you go. If you can increase the efficiencies along the way, like intercooler and shorter/bigger inlet tract ( pipes ) then the boost at the plenum will be closer to the amount made at the turbo.. More turbo boost = more heat = less efficient. Bolt an exhaust on, you will make more boost, because your taking a load off the turbo.. ( backpressure ) Then you wind the turbo boost back down. Still the same boost in the end, but the turbo does less work to get there, and spools up quicker in the process, etc etc.. <{POST_SNAPBACK}> Thank you, all received and understood ... Looks like a worthwhile investment then.