Why More Boost Doesn't Always Mean More Power

If two engines have the same boost, and the same displacement, why does one make more power than the other? The answer is complex, but ultimately it comes down to how much air (specifically oxygen) can you get inside an engine's cylinders. The more air you have, the more oxygen you have, thus you can inject more fuel and produce more power. But manifold pressure is only one measurement that will determine how much oxygen you'll have within your combustion chamber.

Temperature and humidity also play a role on the engine's performance. You need to look at manifold density, instead of simply boost, to really know how much air the engine is getting. Not to mention ignition timing, air/fuel ratios, and the efficiencies of the turbos you are using. This video will break it all down from beginning to end!

Humidity Effects On Performance:
I spoke with a Ford engineer about how humidity affects horsepower. He stated that humidity acts like EGR. For example, combustion with 1% water by mass (not to be confused with relative humidity percentage) is nearly equivalent to using 1.7% EGR. As humidity increases, you increase your mass flow rate, which increases heat capacity, which causes cooler combustion and reduces combustion pressures.