so I saw this chart in my plane that shows Maximum allowable Manifold pressure for any altitude up to 30,000. for instance: SL-->16,000 = 33.017,000 = 31.718,000 = 30.2 all the way up to 30,000 = 18.3 I was hoping that maybe some one could explain the reduction in MP intake? I know that a normally asperated non turbo engine will loose MP the higher you go, and even with a turbo there is a point where you cant make enough MP to run the engine, but the plane that this chart was affixed, (Cessna 340) could produce much more mp at those altitudes. So whats the difference of 25 inchs of manifold pressure at sea level or 25 inches at 25,000 feet? why must I reduce it to the said 22.4 inches? I hope someone understands this.

Basically because as you climb if you don't back the manifold pressure off, you will start getting pre-ignition and you can damage the engine. For this reason, on most turbochargers there is an exhaust vent which reduces the gas pressure available to the mechanism which spins the turbo so that the turbo won't overspeed and stress the engine, but you may have to control some of that youself on some enginbes, which is presumably what that placard is about. Al

The MAP vs. Altitude limit is due to the turbo bearing temperature and RPM limits. Cheers, - jahman.