even with a non turbo engine, when you got to Wide Open Throttle you are effective opening up the inlet manifold to atmospheric pressure (essentially 0 bar or psi or whatever on a gauge).

you should get about -14 psi with a closed throttle (with the engine running it is effectively sucking against the throttle to get the air in hence a negative or vacuum pressure) but this should rise to close to 0 as you open the throttle. a turbo gives you what is termed positive boost, i.e. above atmospheric pressure. so 15 psi boost pressure is 1 bar above atmospheric, or 2 bar absolute. hence the '2 bar Manifold Atmospheric Pressure' sensors you might have heard of that were standard for a cosworth. if you want more than 1 bar boost, you need to go up to a 3 bar MAP sensor or higher if you're really mental.