Fuel pressure

How does fuel pressure affect the engines operation?. Is it only a way to compete with the air pressure in the manifold or does it directly affect the combustion process excluding rich lean characteristics ?. Does fuel under pressure change the rate at which it combusts?.

Fuel Pressure affects the injector flow for a given open time (the injector pulse width). The injector flow is determined by the differential pressure (ie, the fuel pressure relative to the manifold pressure). Ideally, you keep this constant, by using a fuel pressure regulator that is referenced to the manifold pressure. Critical for forced induction, it's still a good idea of all engines, since it makes the injector flow more predictable so correction factors (% trims) will produced desired results regardless of the manifold pressure.