Vacuum in an engine occurs at many places,including valve areas,ports, carb drillings, cylinders etc..an engine uses vacuum to lower the boiling points of fuel molecules which in turn assists cylinders to create a pressure on the crank.
Vaporisation is the key to making power,smooth power,a pressure curve that has some grunt to it.
When you modify an engine you are at risk of generally drastically reducing the cylinders ability to vaporise the mixture,especially at the lower rpm levels of the engines operation.When you have this condition occurring it forces you to calibrate the carby and ignition to a compromise which in turn decreases the cylinders ability of getting to full power quickly when you WOT the carby.
When you increase the manifold vacuum you raise the vaporisation of the fuel mixture prior to entering the cylinders, its important to realise the importance of this.When you have a greater amount of fuel mix that is vaporised before the entry of the cylinders you end up with a more homogenous mix in the cylinder prior to ignition time,its very important to understand the need for this.
