There seems to be quite a lot of misunderstanding when it comes to the differences and similarities between boost pressure, flow rates, and overall power production. Hopefully this article can clear some of these up. All the numbers quoted and assumptions made wherein will assume that the motor in question is properly tuned, and that things such as boost pressures, compressor efficiencies, and intercooler efficiencies are within the normal realm of gasoline powered street and strip cars.
First of all, it is important to understand exactly what quantities are being measured in each case.
Pressure, which is the amount of force per area exerted on a surface, will in this case be a measure of the pressure within the intake system. This pressure is generally caused by the larger than normal (normal referring to atmospheric conditions) amount of gas molecules per unit of volume, but is not limited to only this. Pressure can increase or decrease depending on other factors, such at temperature, volume, and waves of different pressure within the charge pipes.
Flow, on the other hand, refers to the passage of a number of gas molecules over a period of time. Flow rates give no information as to the condition of the gas in question, they simply tell you how much of it is going by. The two most common units of flow are CFM (cubic feet per minute), which measures the volume that flows over a period of time, and lbs/min (pounds per minute) which measures the mass of the flow over a period of time.
With this, we have already come across a common discussion, and a common misconception. There is a lot of confusion with regard to which measure of flow is “better,” and what to do when people use both almost but not quite interchangeably. The problem here is not that either unit is wrong, but that they are measuring the same thing in a different manner, which tends to be a bit difficult to grasp.
In reality, whether a turbo, motor, or head is spoken about in CFM or lbs/min, what we really care about is the same thing. In order to make power, we require that molecules of fuel and oxygen be burned. More molecules creates more power, and many many things depend upon the exact amount of molecules entering the engine. CFM and lbs/min use different units, but define the same thing: how many molecules of gas are going through the turbo, or down the intake runner, or into the head.
Now, back to the original discussion. A motor obviously simply takes the “stuff” you put into it, and combusts it (chemical reaction) in order to exert a force on the piston, and thus to make power. Now, the amount of stuff that you burn directly correlates to how much power you need, so that is what we are concerned with.
As you can see from the descriptions above, pressure does not describe the amount of air that is flowing into the motor, it simply describes the pressure that it is exerting on everything around it while it sits and waits for the intake valve(s) to open.
On the other hand, flow describes the rate at which air is entering the motor, in terms of a mass or volume over a time interval (lb/min, CFM, g/rev, etc). This means that it tells us DIRECLY how much “stuff” is going in. This is what we want to know!!
It is also flow that we need to use for all fuel calculations. The computer (ECU) in a car has to inject the correct amount of fuel to provide the proper air to fuel ratio (A/F ratio), and in order to do this, it has to know how much air is entering the motor. Pressure gives close to no indication of this, while mass flow or volume flow is halfway to injecting the right amount of fuel. Once you have either of those, you just select a target A/F ratio (saved within the ECU) and then find the right amount of fuel to satisfy the ratio.
That’s it! That is why pressure is almost meaningless, and why airflow is what really matters.
Kyle Tarry - 2003