Ok I was just asking myself the following qeustion why does my SC boost gage displayes -10 and +10 psi of boost, does that mean until it gets from -10 to 0 there is no boost being produced from the SC or is it something else? Also as i understand the SC is always on so therefore it must produce some levels of boost as long as the engine is running, but -10 psi for some reason confused me and made me think it over. Please explan. thanks in advance.
Our supercharger is a blower - specifically a roots type blower.
An engine creates power by mixing air and fuel. The amount of air and power is a factor of the displacement of engine and volumetric efficency. The displacement is total potential volume of air/fuel that can be mixed in the cylinders. The efficiency is essentially how much of that potential actually gets filled with mixture.
The efficiency is determined by how well the engine breathes across the rpm range. The engine breaths by creating a vacuum in the intake manifold which sucks in air.
The best an engine without forced induction can do is create a complete vacuum - which would show up as 0PSI on your boost gauge. The boost is relative to the outside atmosphere. Hence, the 0PSI actually represents 14.7PSI across the intake manifold. This indicates a complete vacuum because air is rushing into the engine at the same pressure as the atmosphere - due to a zero net back pressure.
The negative boost indicates there is back pressure - as the PSI is less than the outside air. A -5PSI indicates 5PSI of back pressure reducing the positive pressue and thus intake flow from 14.7PSI to 9PSI.
A blower essentially pushes air into the engine - increasing the volumetric efficiency.This creates positive pressure and show up as positive PSI on your boost gauge.