One other thing to remember, a car can make more power and torque even when boost or PSI drops with changes to the exhaust system such as the headers, manifold, heads, or a multitude of other areas.
BimmerBoost members, I have been getting tons of questions regarding Psi and power output lately and it seems there is a ton of misinformation out there and it is spreading like wild fire. Below I will try and explain to the best of my knowledge how this all works. So Here it Goes!
Things to know before reading:
The boost (psi) you have come to know and love is being read at the intake manifold only. PSI does not tell you how much air actually makes it into the intake ports during their short open interval. It's only a measurement of force exerted on the intake plenum. With that said Lets get started!
What is PSI ?
First you need to understand 1PSI = 1LB force per square inch and not Pounds of air per square inch. A square inch is a unit of area, and not volume. 7 PSI = Seven pounds of force exerted on every square inch of internal surface area of the intake manifold and intake ports only. This says nothing about how much air is actually getting into the engine If this were the case it would be read as pressure per cubic inch. Psi is just how much force the air is exerting as it gets force fed from the Supercharger compressor.
So to sum it you can calculate the air density based on how much pressure is exerted, but PSI is not a measure of volume.
There are many misguided comparisons regarding blowers floating around on this forum so to start lets talk CFM "Cubic feet per minute." CFM is a non-SI unit of measurement of air-flow that indicates how many cubic feet of air pass by a stationary point in one minute. Or to simplify it is a unit for measuring the rate of flow of air volume into or out of a space.
The wheel size and outlet volume of a supercharger compressor has a great impact on the CFM ( speed the air is actually traveling through the manifold" A large supercharge can flow much faster and requires less psi to make the same HP. This large volume of air leaves the Supercharger and enters the bottleneck which is the intake tract speeding up just as water speeds up just like in a river when you hit a bottle neck. You white water rafters know what Im talking about.
The air flowing from a smaller supercharger on the other hand is flowing into the same size river, but this time the river is large in relation to the charge volume so the air just creeps along and will require more psi to hit the same hp.
Notes: A larger supercharger can show less manifold psi compared to the smaller blower which needs higher Psi but the flow into the engine will be the same and make the same hp. All things being equal in this example temps,tune ect..
Heat soak is the systems lack of ability to get rid of excess heat much like having too small a radiator for a car.
We measure Heat soak from the manifold in the form of Intake air temperature "IAT". When a gas "air" is heated, it's molecules get farther apart and it's density decreases while it's volume increases. However, if it is heated and has no room to expand, density will go down and volume stays the same, but it's pressure will increase. This is a generic property of all gases."
So basically if you get heatsoak, and your boost doesn't increase, you're losing efficiency and in turn flow. This will result in lower hp numbers until lower Iat temperatures return.
The air coming from the smaller blower will travel forward into the intake ports with a lower velocity than that from the larger blower for the reasons that we established in our "river bottlnecK" example above.
So while both superchargers are exerting the same amount of force on the intake ports/ manifold the air from the larger blower is approaching the intake ports at a much higher velocity and therefore more will get in before the port closes.
--And this ladies and gentlemen is why you cannot compare Psi from blower A to blower B.
If you have any questions or if I left anything out please let me know.