The blow through carburetor supercharger application is probably the most misunderstood forced induction application. However, by sticking to fundamentals, amazing results of more than 1700 hp are possible. Carburetors meter fuel based on airflow through them, or more directly, air velocity. To deal with high boost air flows, many aftermarket carburetor builders make modifications to increase the signal to the carburetor so it is able to deal with the additional fuel flow required when the engine is under boost. Modifications include annular discharge boosters, power valve channel restriction modifications, accelerator pump pullover enrichment mods, air bleed sizing, large needle and seat assemblies and several other tricks to provide a flat fuel curve under boost and still provide acceptable non-boosted drivability. Another necessary modification is solid nitrophyl floats as the brass floats can collapse under boost pressure.

To ensure adequate fuel delivery, the carburetor must see an extra pound of fuel pressure for every pound of boost to get fuel across the needle and seat. The fuel pump must be able to deliver adequate volume at a much higher pressure than before supercharging. With the typical street/performance system this can be anywhere from 14-24 psi of fuel pressure at max boost, or even more. The trouble is that most carb specific performance fuel pumps start to bypass internally anywhere from 12-28 psi. Many start significantly dumping volume well before they meet their bypass pressure setting. When the bypass valve opens fuel flow effectively stops and the bowls run out of fuel, dangerously leaning out the engine and losing power.

One fix is to run a high volume pump designed for EFI use. These pumps are capable of producing the required volume at higher pressure. The pumps that incorporate an internal relief generally don’t start to bypass until they reach over 70 psi. This allows the regulator to do its thing without the pump sabotaging fuel delivery. The use of a return style regulator is necessary with this type of setup. The benefit of the return system is that volume demand is met before the fuel is bypassed back to the tank so the regulator can react instantly to the changing fuel demand of the engine. A pressure reference line is run from the carburetor inlet hat to the boost reference port of the regulator to maintain the required fuel pressure. The reference line should not be connected to intake manifold vacuum as that would lower fuel pressure and volume in the bowl during vacuum.

 

 

 

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