I actually got into an arguement with someone over this same issue. It basically came out to you don't really need one as long as you can tune around it.
Precision control of fuel flow, through the injector and into the engine, is the primary purpose of your electronically controlled fuel injection system. The actual rate of flow through the injector depends on 3-key variables, including:
1.) Nozzle size (lb/hr or cc/min rating, i.e. how big is the hole in the outlet end).
2.) Pulse Width (how long the hole is held open by the ECU, may be termed “duty cycle” when open time is expressed as a percentage of the time it takes a 4-cycle engine to make two revolutions at a given RPM).
3.) Delta Pressure (the difference between the pressure at the inlet end of the injector, i.e. fuel rail pressure, and the outlet end of the injector, i.e. manifold pressure).
A key part of the equation is earth’s atmosphere has weight. As measured in Pounds Per Square Inch (PSI), at sea level, the average is 14.5 PSI (also called 1-Bar in metric terms). With that in mind, the injector in a port fuel injection engine lives in a two pressure environment, one side in the fuel rail, and the other in the intake manifold runner.
Once the injector nozzle is selected and installed, the flow rate per millisecond of open time is fixed. The same goes for fuel pressure, once the base pressure is set, and the regulator vacuum/boost port connected to the intake, the Delta Pressure is fixed. And this is the point of using a 1:1 regulator with vacuum/boost reference. As the engine runs at light load and during deceleration, vacuum builds in the intake manifold, acting to make fuel flow through the injector easier, but the referenced regulator drops fuel pressure in the rail in the exact same amount, at the same time, preserving the delta pressure across the injector and assuring the flow rate per millisecond of open time does not change. Conversely, when the intake manifold sees boost the, the air pressure in the intake acts to appose fuel flow through the injector, reducing the flow rate per millisecond, but the boost referenced regulator increases fuel rail pressure, once again preserving the injector flow rate.
With this in mind, the only thing that actually changes the injector flow rate into the engine is what? Injector open time, which is the only thing the PCM can actually control. Using a referenced regulator then puts the PCM in full control of engine fueling.
Today’s returnless fuel systems often do not have the benefit of a vacuum/boost reference to the regulator. When this is not available a recalibration of the PCM must be made in an attempt to compensate. Unfortunately, there is a limit to how much compensating can be made for an injector that gets larger in vacuum and smaller under boost. This is a particular problem in a boosted engine because the injector flows less and less as the boost goes higher and higher. For this reason, beyond 500-650 HP forced induction, a boost referenced fuel system is virtually a must. Boosted or naturally aspirated engines will all drive better and cleaner, with less programming fuss, using a vacuum reference.
Brett Clow
Aeromotive, Inc.
7805 Barton St.
Lenexa, KS 66214
913-647-7300 Ext. 109
