EFI fuel pressure regulator explained

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!

Injector performance

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.

If you liked this article, please explore more articles though this webpage.

My 240Z ITB EFI build and other car related videos can be watched on YouTube here:

I am also on Instagram:


Share on facebook
Share on twitter
Share on pinterest

Leave a Reply

Your email address will not be published.

On Key

Related Posts

Datalog recording of induced low load knock

Knock control tuning

Knock control tuning is important for any modified EFI engine. In this post I explain theory and practical verification of knock control tuning.

Ignition coil dwell calibration

Why calibrate ignition coil dwell? I am using a distributor based ignition for my EFI build and ignition coil dwell calibration will help optimise various

Megasquirt battery voltage calibration

These days injector behaviour at different voltages is well understood. The speed of opening and amount of fuel delivered vary with voltage. Corrections are made in fuelling calculations to account different performance for differnt voltages at the injector. If what the Megasquirt ECU understands is the voltage at the injector is different in reality, then it is likely fuelling calculations will be incorrect. This is why the battery voltage calibration needs to be correct in your Megasquirt ECU.

Manual Idle Air Control

The ITB idle air control conundrum Idle air control is generally a challenge with individual throttle bodies.  This is from the sensitivity of throttle position