Hilborn Technical Support - Mechanical Systems - By Pass Valves
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Adjusting The Hi Speed Cut-Off Valve
The Hi Speed Cut-Off Valve controls maximum rpm fuel flow curves by maintaining the air to fuel ratio required for good power throughout the whole rpm range. It is used mainly on special applications; usually large cubic inch engines that operate above 6000rpm.
The body is marked with the letter "C" and an arrow. When the valve is installed, the arrow should point toward the fuel tank. See the Installation Schematic for location of this valve in the fuel system.
The principle behind the Cut-Off Valve is to match air flow with fuel flow in the higher rpm ranges. Since both fuel flow and air flow increase at the same time with engine rpm, a problem occurs when the engine reaches maximum air intake and quits breathing. The fuel pump is unable to determine that the engine is no longer taking in additional air, but continues to increase the fuel. Once the engine has reached its' capacity to take in air, it no longer needs additional fuel, so the valve allows for the fuel supply to fall off or be "cut off".
Fuel starts to bypass when it reaches the valve’s preset pressure, usually beginning around 6500rpm, and as the rpm increases, so does the amount of bypassed fuel. The point at which the valve begins to open can be adjusted by changing the various spring and shim combinations inside. Although fuel flow is controlled in other valves by changing jet size (affecting the total upper and lower fuel curve), the Hi Speed Cut-Off Valve controls the amount of fuel the engine receives at above peak power levels only.
The Hi Speed Valve can also be used as a simple pressure check or a check valve for additional nozzles. It can be used to tailor a very rich bottom end fuel supply and a leaner top end curve. This valve can control the maximum fuel pressure the engine will receive, conserving fuel for longer races.
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