Ok, that was part of what i was struggling with.

so say you take an engine of 2.3 litres and 5 cyls, making say 280bhp at 2 bar absolute, subject to everything else being suitable around it, and you wanted 400bhp, you could specify a turbo that would do the job, say at 3 bar absolute, simply because lbs/min is a known constant, regardless of capacity?