Swapping from carburettor to EFI on my L28 stroker build required a different fuel pressure regulator, not only to accomodate a higher operating fuel pressure but in how the pressure is regulated. I know how a ‘referenced’ fuel pressure regulator works, but do you? If the answer is no, read on for some theory and a practical demostration! (or watch my YouTube video below).
Why is constant differential fuel pressure important?
The short answer is so you EFI system works as predicted. The long answer is a little more technical which I’ve dvided into logical sections. Read on!
The differential fuel pressure really is making sure your EFI system functions as predicted. This is ultimately governed by injector flow rates as tested by the manufacturer or EFI specialist. For my EFI build I am using Siemens Deka 446cc (42ld/hr) high impedence injectors (P/N FI11420). These are flowed at 446cc at 300kPa. As an example I have included the data sheets below.
The injector flow rate is required for an EFI ECU to calculate the required opening times for the injectors. The purpose of a referenced EFI fuel pressure regulator is to maintain a differential fuel pressure across the injector. In my case that is 300kPa at one atmosphere (101.3 kPa). OK, so how does the fuel pressure regulator do that? Read on again!
How an EFI fuel pressure regulator works
We first need to understand the conditions the injector is operating in. The fuel injector has the end the squirts the fuel into a runner or manifold to be drawn into the engine.
The pressure inside that manifold varies from less than one atmosphere (eg. vacuum under idle) to one atmosphere (for NA engines) or higher (for forced induction engines). When in vacuum there is a tendency to ‘pull’ fuel out of the injector and when in boost more effort is required to inject the fuel into the manifold.
A referenced EFI fuel pressure regulator is connected to the manifold with a vacuum line to understand the pressure environment the injector is operating in. The manifold pressure is used to alter the fuel injector rail pressure through the fuel pressure regulator to maintain a constant pressure differential at the injector. If the manifold pressure goes down, so does the fuel rail pressure. Injector fuel flow rates become constant and predictable when the differential pressure at the fuel injector is maintained.
A practical demostration
I have used my own EFI build to demonstrate the operation of a referenced EFI fuel pressure regulator. I have a fuel pressure sensor in the fuel line from the fuel rail and a manifold air pressure taking recordings. The demonstration was completed without the engine running. I simulated boost and vacuum manifold conditions with my lungs through the disconnected vacuum line from the manifold.
Below are outputs from the datalog entries. You can see the fuel pressure differential between the manifold air pressure and fuel pressure behind the injector remain approximately constant.
Summary: EFI fuel pressure regulator
I hope this article was helpful in understanding the purpose of a referenced EFI fuel pressure regulator. Maintaining a constant fuel pressure differential gives consistent fuel flow from the injectors.
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