Boost referencing the blow-through carburetor (4150-style) and fuel pressure regulator

Boost referencing the blow-through carburetor (4150-style) and fuel pressure regulator

By V. Moore: Conventional carburetors operate at prevailing barometric pressures, from oxygen-rich sea level to oxygen-lean Denver, a mile high in Colorado. Blow-through carburetors, which work in conjunction with centrifugal superchargers and turbochargers, operate with boosted pressure. It’s a contrasting concept that accelerates the induction process. It impels higher velocity fuel flow through the main jets and robustly emulsifies the air-fuel mixture in the metering passageways, as it makes its way to the carburetor’s boost venturii. The boost venturii are carefully positioned in a choked area, an area with a reduced cross-section to increase the air speed. Increased air speed lowers the air pressure at the small discharge orifice in the booster, extracting the air-fuel mixture into the air stream on its journey to the cylinders. When boost pressure enters the fuel bowls, via the two vent tubes, its force lowers the fuel levels and quickly becomes so potent with increasing engine speed that the needle-and-seat valves cannot keep pace with demand. To keep the carburetor flowing relative to boost, the float bowls must be pressurized. As boost pressure increases, fuel flowing through the metering systems increase proportionally. Further, as the float bowls are pressurized, the fuel pump must overcome that boost pressure (as opposed to the naturally aspirated condition of pumping fuel through the needle-and-seats into float bowls that are vented to atmosphere). If 7psi of fuel pressure were employed to a normally aspirated engine and 7psi of boost applied to the float bowls, the pressure in the float bowls and the pressure from the fuel pump would be equal, causing zero fuel to flow through the needle-and-seats. So,...