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Home Forums Customer Support Pressure Regulators Bypass or Return Style Regulators How a Bypass Style Regulator Works

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  • August 19, 2015 at 12:34 am #5549

    Key Master

    Please note, while the following describes use of a carburetor, the regulator functions in the same way with EFI systems.

    Below is an example of a FUELAB bypass style fuel pressure regulator. Please note the cut-away image shows the fuel regulator in the valve closed position.

    51505-c-L-L - Low Press Article - Labelled copy Design

    With a bypass style regulator, fuel enters through the inlet port (A) and travels past a fuel bypass valve/fuel return line port (which governs fuel flow and pressure) (B) and then is distributed through an outlet port to the carburetor (C). Opening and closing of the bypass valve is limited by a spring (D). Fuel pressure (psi) to the carburetor is set with a threaded adjustment mechanism (E). A vacuum/boost reference port allows the regulator to compensate for boost pressure with forced induction applications (F). Bypass style regulators are characterized by a fuel return line from the regulator back to the fuel tank.


    Fuel travels into the regulator and flows out to the carburetor. As fuel pressure in the carburetor float bowls increases, so does pressure within the regulator, causing fuel to push upward against the fuel bypass valve. As the fuel pressure reaches the maximum pressure to which the regulator has been set (this pressure setting is usually based on the maximum which the carburetor manufacturer has specified will provide optimal performance for the carburetor), the fuel bypass valve is progressively pushed open to bleed off fuel flow and pressure. This fuel is sent back to the fuel tank via a return fuel line. As the engine demands fuel from the carburetor, the float chambers begin to empty causing fuel line pressure to drop. As line pressure drops the fuel bypass valve descends, progressively closing it off, thereby increasing fuel flow and line pressure. By using the threaded adjustment mechanism to increase tension on the fuel bypass valve, it becomes harder for pressurized fuel to push the valve upward; more fuel pressure must build to push it open. Thus, increasing tension on the bypass valve spring with the threaded adjustment mechanism is how the regulator is set to increase fuel pressure to the carburetor. Conversely, decreasing spring tension sets fuel pressure lower.

    The vacuum/boost reference port operates in the same fashion as the blocking style regulator.

    The advantages of the bypass style regulator

    • Return style provides constant effective fuel pressure to the outlet port – pressure overage is bled off through the return port as needed.
    • Constant effective fuel pressure enables fuel pressure to be set more accurately, and should remain constant regardless of load. It should be noted that the engine is not required to operate to accurately adjust fuel pressure, however, the fuel pump does need to be energized.
    • Longer pump life and quieter pump operation as the pump is operating just hard enough to maintain pressure, instead of maintaining typically 10 to 20 PSI higher as is the case of blocking style regulators.
    • Bypass style regulators are preferred for forced induction blow through applications that required high fuel flow and pressure.

    The disadvantages of the bypass style regulator

    •  Added expense, complexity and weight of additional fuel lines and fittings
    • The return line is very sensitive to pressure drop, especially with these very low pressure ranges. Large return lines, 1/2″ or higher, must be used with limited bends and direct return to unpressurized tanks or reservoirs.
    • Not applicable when multiple regulators need to be tied together (set at different pressures, such as with a nitrous oxide system use) and fed from one pump, as the entire fuel system will be limited by the regulator with the lowest pressure setting – negating those set to higher pressures
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