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EFI engines with Aeromotive bypass regulators:

All Aeromotive EFI Bypass Regulators allow intake manifold pressure, vacuum and boost, to affect the regulated fuel pressure. When a fuel pressure regulator is referenced to manifold pressure, it acts to mirror manifold pressure changes with equal changes in fuel rail pressure resulting in lower fuel rail pressure with manifold vacuum and higher fuel rail pressure with boost. The effect occurs on a 1:1 ratio, where the regulator raises and lowers fuel pressure in sync with the change in manifold pressure.

When calculating the injector size needed for a forced induction engine, it's critical to account for boost reference, or the lack of it. Failing to account for a shrinking injector flow rate (because a bypass regulator with boost reference is not installed) could cause an unexpected lean condition in boost. The argument for reducing fuel pressure with manifold vacuum is the same as for raising it with boost; it works to keep injector flow consistent. The use of a vacuum reference is applicable to both naturally aspirated and boosted engines, but works to reduce fuel rail pressure when the manifold is in vacuum. This prevents injector over-flow (injector acts larger than it's rated) when the engine need less fuel.

Installing an Aeromotive EFI bypass regulator and connecting the vacuum/boost port to the intake manifold allows the regulator to take dynamic control of the fuel rail pressure. This enables the regulator to monitor manifold pressure and change fuel rail pressure as needed in order to maintain the necessary difference, or what is called "differential pressure".

Differential Pressure:

Affects fuel flow through the injector and can change what size injector will be needed to support peak horse power and what fuel pump and regulator are suitable. It can even change how the fuel system is installed in the vehicle and how the ECU is calibrated to achieve optimal performance. It's simply the difference between two pressures, in this case between fuel rail pressure and manifold pressure and is found by comparing the two pressures and subtracting the lower pressure from the higher. It can get tricky when dealing with vacuum in the intake, as this is a negative number. For example, take an engine with base pressure of 43 PSI and -6 PSI vacuum (12" HG). The equation looks like this: 43 PSI - (-6 PSI) = ?? PSI. The rule is, when subtracting a negative number from a positive number, add the two numbers together (change the two minus signs into one plus sign) The answer is 43 + 6 = 49 PSI differential. Calculating the difference with boost is simple subtraction.

Calibration tables can be (and are) used to compensate for these significant variations in injector flow... but there's no ECU programming magic that can preserve the physical size of the injector, 100% duty cycle is 100% duty cycle. Differential Pressure is Differential Pressure. You either maintain it or you don't and live with the consequences. Without vacuum/boost reference games have to be played with injector sizing, base fuel pressure, and engine management calibration. Forced induction applications with a blower or turbo are especially disadvantaged without boost reference, but all engines are affected one way or another. The bottom line -- when compromises are made, consequences are inevitable.

Don't compromise fuel delivery to your engine, especially if high performance is your goal. The proper vacuum/boost connection for an Aeromotive bypass regulator requires a length of 5/32" vacuum line to be routed from the intake manifold plenum, after the throttle body, to the port in the regulator cap. Be certain to set base fuel pressure with the vacuum/boost line disconnected when the engine is running. For TBI engines where the injector is above the throttle blade(s), the vacuum/boost port should be left disconnected and open to atmosphere, never blocked or plugged.