There is often a good deal of confusion regarding turbocharger boost pressure and engine power when comparing different turbocharger solutions for a given vehicle or engine. It is easy to fall into the trap of thinking that "a psi of boost is a psi of boost" and as long as two systems produce the same boost pressure, they should deliver the same power.

However, it is not uncommon to see two identical engines deliver the same power output but at vastly different turbocharger boost pressure levels. Or conversely, vastly different power levels at the same boost pressure level. How can this be so?

The internal combustion engine requires air and fuel to operate. The ratio of air to fuel must lie within a range of values for efficient combustion and that range broadly lies between 15 lb of air to 1 lb of fuel (15:1 Air/Fuel Ratio or AFR) through to 9 lb of air to 1 lb of fuel (9:1 AFR) depending on the engine's design and operating conditions.

Since it is the mass of air and fuel that determines the power output, the capacity and ability of a turbocharger to deliver a mass of air to the engine becomes the critical factor when considering engine power output. In fact the capacity of a turbocharger is measured by its mass flow rate (lb/minute). Some manufacturers specify a volumetric flow rate (CFM) however this may be misleading unless the volumetric flowrate is standardized (beyond the scope of this article).

So, as asked earlier, how can two identical engines deliver the same power output but at vastly different turbocharger boost pressure levels? The answer lies in the mass flow rate of air (lb/min) that a turbocharger delivers to the engine.

The higher the turbocharger's mass flow rate, the higher the engine power potential.

As we will see shortly, the mass flow rate of two different turbochargers at the same boost pressure can be vastly different. This is precisely where confusion arises. Boost pressure and mass flow rate of two different turbochargers can be totally different.

The following chart is a simplified version of what is commonly called a compressor map. It demonstrates the turbocharger's range of mass flow rates for a given boost pressure (or conversely, the range of boost pressures to supply a given mass flow rate of air). Traditional compressor maps also show efficiency curves however these have been omitted for simplicity.

The sweet spot of a turbocharger (where it is most efficient) is within the peak efficiency area as shown in the chart. For example, at a boost pressure of 9 psi, this turbocharger (turbocharger A) will supply anywhere between 12 lb/min of charge air through to 54 lb/min, however at around 30 lb/min is when the turbocharger is at its most efficient (at that boost pressure).

If we follow the choke flow curve (which shows the maximum capacity of the turbocharger), we can see that the maximum mass flow rate of charge air that this turbocharger can supply is 65 lb/min at around 21 psi of boost pressure.

Another turbocharger (turbocharger B) however may have the following characteristics. Again looking at the performance at 9 psi turbocharger boost pressure, this turbocharger may supply anywhere between 5 lb/min of charge air through to 36 lb/min of charge air - but in this case, the turbocharger will never operate within its peak efficiency zone at 9 psi. In order to operate this turbocharger at 9 psi as close as possible to its peak efficiency zone, the mass flow rate would be approximately 20 lb/min.

The maximum capacity of this turbocharger in terms of mass flow rate is 45 lb/min again at 21 psi of boost pressure.

Let's now summarize our findings in the following table:

	Turbocharger A	Turbocharger B
Peak Efficiency Output at 9 psi	30 lb/min	20 lb/min
Maximum Output at 9 psi	54 lb/min	36 lb/min
Maximum Output	65 lb/min @ 21 psi	45 lb/min @ 21 psi

From the above comparison, we can now see that turbocharger A delivers a greater mass flow rate of charge air than turbocharger B at both peak efficiency and maximum output. This greater mass flow rate of charge air is directly proportional to the higher engine power output of an engine when fitted with turbocharger A.

Hence, the answer to our initial question of how can two different turbochargers can deliver different power levels when fitted to identical engines. It all boils down to the mass flow rate of the turbocharger at a particular boost pressure - because it is the mass flow rate of a turbocharger that delivers horsepower.