Kinsler_Handbook_#32 December 2017

Kinsler Fuel Injection, Inc, 1834 THUNDERBIRD TROY, MICHIGAN 48084 U.S.A. www.Kinsler.com Phone (248) 362-1145 Fax (248) 362-1032 136 PRESSURE RELIEF VALVE PLUMBING See EFI PLUMBING on Page #154 regarding the physical placement of the valve in relationship to the fuel rails. All hose sizes must have large enough inside diameter so that they will not cause any restriction, which could cause the system’s fuel pressure to vary with the change in flow. VACUUM/BOOST REFERENCE Some pressure relief valve models have a port (fitting or barb) to connect a manifold absolute pressure (M.A.P.) hose to the top side of the diaphragm in the valve. At idle or partial throttle the vacuum pulls up on the diaphragm, lowering the fuel pressure. Under boost, the fuel pressure will be raised. This feature broadens the usable range of the injectors. Ideally the valve should raise or lower the fuel pressure at a 1:1 ratio. See Fig. 2. EFI PRESSURE RELIEF VALVES AND ACCESSORIES ADJUSTABLE PRESSURE Some pressure relief valves are available with an adjustment screw to allow the fuel pressure setting to be raised or lowered. Adjustment of the pressure relief valve allows the fuel pressure to be increased or decreased to change the injector’s flow (see ORIFICE THEORY on Pages #202-203), thus allowing the user to add more or less fuel without changing injectors or the fueling program in the ECU. To clear up a bit of confusion: a pressure regulator limits the pressure going into a system, such as a regulator feeding fuel to a carburetor... any vapor bubbles in the fuel are vented out through the float bowl. All EFI systems use a pressure relief valve to control the fuel pressure. Most bypass fuel after the fuel rails which purges the vapor out of the system. If a pressure relief valve is used on the inlet of the fuel rails there would be no way to purge the vapor out of the rails. This is very important, since after a hot soak any vapor trapped in the rails would flow out to one or two cylinders, causing those cylinders to be lean at start up. Fig. 2 Fig. 1 PRESSURE RELIEF VALVE BASICS Fuel rail pressure is controlled on an EFI system by a pressure relief valve which returns excess volume back to the tank thereby maintaining pressure at the injector’s inlet. If the pressure relief valve does not do it’s job properly the system fuel pressure may rise or fall which will cause the fuel flow to do the same. Should the valve allow the fuel pressure to flutter, then the fuel delivery will flutter. These conditions could cause difficulty in starting the engine, inconsistency in the engine’s performance, difficulty in programming, or engine damage. Pressure relief valves used with EFI systems have a diaphragm assembly with a pintle or ball that sets on a seat. A spring(s) above the diaphragm assembly will force it to set on the seat at a desired pressure. Fuel pressure from the pump will exert force on the diaphragm and lift the sealing device off the seat allowing fuel to pass through the valve. In order for the valve to open and allow fuel to flow, the spring must be compressed. Compressing the spring, causes a rise in pressure. When larger volumes of fuel must be bypassed, the spring will need to be compressed even further to allow the flow through the valve. The best designed valve will pass the desired amount of volume with very little pressure rise. See Fig. 1 It is desirable to maintain a constant pressure differential across the injectors for consistent fuel mapping (see ORIFICE THEORY on Pages #202-203). When the injector outlet is installed where it would be subjected to vacuum (under the throttle plate) or boost (on the pressure side of blower/turbo), the flow of the injector will increase or decrease as the resulting outlet pressure changes. The vacuum and/or boost in a manifold runner is called manifold absolute pressure (M.A.P. see SENSORS FOR EFI SYSTEMS on Page #146). Referencing the manifold absolute pressure to the pressure relief valve causes the fuel pressure in the rail system to be adjusted by the change in the manifold absolute pressure. This reference will maintain a constant pressure differential across the injector and consistent flow. © 2017 FILTRATION: It is very important the pressure relief valve has filtered fuel. The mechanical seat of the valve is very sensitive to dirt. A proper fuel filter is vital to maintain the operational integrity of the EFI system components. We recommend the Kinsler fuel filter assembly #8170, which has a 10-micron replaceable paper filter element #9031 - see FUEL FILTERS on Pages #162-166.

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