JCooper
"If you have less dense air there is going to be less pressure in that air. If you have more dense air there is going to be more pressure in that air. The two are hand and hand you cannot just discredit one."
Not correct. Density = weight / volume. Take a sealed container (such as a scuba air bottle) and heat it up. The pressure will increase, but there will not be a single molecule more in that bottle, and its volume will remain equal too (for the purposes of this discussion anyway), so the density will remain equal. Now put the bottle in a freezer: pressure will drop with temperature, weight and volume remain the same, therefore so will density.
The limit that we try and set with the limit MP is really the maximum horsepower we want the engine to put into the drive train. The more WEIGHT of air goes into the engine, the more oxygen it has to burn fuel with, thus the more horsepower delivered. Higher OAT at equal ambient pressure means lower air density, so to get the same weight of air into the engine, more volume has to be delivered per second, which requires a higher (manifold) pressure.
Lower ambient pressure (higher PA) at equal OAT means lower density, but here the plot thickens somewhat. The pressure read by the MP gauge is the absolute pressure in the intake manifold (rather than a pressure differential). So to obtain an equal MP reading at higher altitude (lower pressure) and equal OAT, the throttle butterfly will have to be open quite a bit further than at the lower altitude. This means a larger volume of air will enter the engine per second than at the lower altitude, which means more weight per second, which means more horsepower. So to maintain the same HORESEPOWER output at higher pressure altitudes (lower ambient pressure), the manifold pressure will actually have to be decreased slightly.