STP Technical Class : Fuel Pressure Compensation

STP Technical Class : Fuel Pressure Compensation


Simple Techniques Performance is happy to bring you another technical blog article, for your reading pleasure, this one is brought to you by Pavlotech, you can click this HERE to check out more articles from them. 

An injector flow rating is accompanied by a static fuel pressure value.  This may look something like 500ccs@43.5psi (300kpa).  This means the injector is rated for that flow at 43.5psi.  If you raise or lower the pressure then the flow will change.  There are some online calculators that can help you estimate the fuel flow – but generally, if you want to run a different pressure you should have the fuel injector re-tested.  


Many ask what is fuel pressure differential measurement?  


It is the measurement of the effective pressure.  Not only is it one of the most misunderstood functions of a properly functioning fuel system, but also seems to be disregarded, overlooked, or ignored frequently.  Essentially you want your fuel pressure differential to stay the same no matter what is happening so that your EFI system can properly estimate the fuel flow.  Maintaining the fuel pressure differential ensures the fuel flow stays the same.  This is true for boosted applications especially.  


The following are important :

Fuel Pressure – The raw measurement of rail pressure

Fuel Pressure Differential – The live effective pressure (should stay close to the fuel pressure spec to maintain flow)

Differential Fuel Pressure Offset – The amount the Fuel Pressure Differential changes (either positive or negative)

    You must remember that when there is pressure inside the intake manifold, the fuel injection system must overcome the pressure to inject fuel into the engine at the specified flow.  This means that fuel pressure must rise with the intake manifold pressure to maintain specified flow.

    Here is an example that explains what happens in a boosted engine without a rising rate fuel system (returnless).


    • 500ccs@43.5psi injector rating
    • Static fuel pressure is set at 43.5psi
    • Intake manifold pressure = 20psi
    • Actual fuel pressure = 43.5psi
    • Fuel pressure differential = 23.5psi
    • Differential fuel pressure offset = -20psi
    • Actual fuel flow = 280ccs@23.5psi (estimated)
    • 500ccs@43.5psi
    • Fuel Pressure = 43.5psi—>>><<<— 20psi = MAP
    • Fuel Pressure Differential  = 23.5psi


    Theoretically the injector would be flowing almost 50% less fuel than it is rated for in this situation!

    Imagine what kind of spray pattern the injection system is producing with the fuel pressure that low.  How about how much it affects atomization/evaporation?  Also, the injector latency is affected by fuel pressure as well (similar to battery voltage compensation).  

    Now a properly functioning rising rate fuel system.


    • 500ccs@43.5psi injector rating
    • Static fuel pressure is set at 43.5psi
    • Intake manifold pressure = 20psi
    • Actual fuel pressure = 63psi
    • Fuel pressure differential = 43psi
    • Differential fuel pressure offset = 0.5psi
    • Actual fuel flow = 490ccs@43psi (estimated)
    • 500ccs@43.5psi
    • Fuel Pressure = 63.0psi—>>><<<— 20psi = MAP
    • Fuel Pressure Differential = 43psi


    I added in -0.5psi of differential fuel pressure offset on purpose to demonstrate that is never perfect.  Some regulators work better than others though.  

    Here is a plot of a rising rate fuel system functioning properly.  


    This car had static pressure set at 300kpa (43.5psi) and was regulating manifold boost pressure at +200kpa.  The fuel pressure rose about 215kpa as you can see (subtract the static).  This makes the differential fuel pressure offset +15kpa (it is actually rising more than it should slightly).  If you look at the Diff Fuel Pressure reading (red in the third column – Fuel Differential Pressure), you can see how flat this is.  This is showing the effective pressure is maintained, and therefore the fuel flow.  

    Many modern cars come with returnless non-vacuum referenced fuel pressure regulated systems now.  This is for packaging and emissions/economy reasons.  Returning the fuel right inside the tank keeps the fuel cooler which prevents excessive evaporative emissions (and also keeping the fuel cooler raises the fuel density).  Many of these cars have variable speed fuel pumps as well, but generally, this is more to keep the fuel cool (so it doesn’t bypass more than it has to the return), and in most cases NOT designed to raise the pressure as many believe.  The fuel pressure is governed by the regulator, which in that case is set to a static pressure that cannot rise.  

    The OEM overcomes this situation by planning the mapping around the situation and generally they oversize the fuel injectors to compensate.  Modern high flow injectors with good control systems are pretty good at negating effects on drivability and mixture.  Installing the larger injector lets the ECU command a larger pulse width to compensate the fuel flow dropping.  

    Generally, a stock vehicle with forced induction doesn’t produce massive amounts of intake manifold pressure, so say the differential fuel pressure offset on a stock turbocharged car is -12psi, it can be managed with the extension of the pulse width.  

    However, when you start increasing the intake manifold pressure, the results can become catastrophic.  If a proper rising rate fuel pressure system is not part of your upgrade plan, then think again!


    What Pavlotech blog post would be complete without plugging the amazing advantages of aftermarket EFI solutions though?  

    Modern ECU systems like Emtron have the ability to take a fuel pressure input (which should be mandatory on every installation) and calculate the Fuel Pressure Differential (and offset) on its own.  Not only are these runtimes loggable (as in the plot above), but the ECU can be set to adjust the fuel model with this information.  This makes the system capable of adjusting the Fuel Model to ensure the specified fuel is being metered as perfectly as possible.   


    From the above example, you can see here the Fuel Model is correcting the fuel flow -2.5%.  This is because the differential fuel offset is slightly positive/higher than normal (it is rising slightly instead of true 1:1).  What this does is ensure the calculated fuel is as perfect as possible.

    It also will correct for the opposite direction as well!  If your car has fuel starvation issues, or a fuel regulation issue (say it is not rising), the ECU will do its best to compensate the fuel flow to maintain the target mixture.


    This is a very extreme example of fuel starvation on a car with a completely equipped Emtron solution.  If you pay attention to the bottom two columns, you will see the fuel pressure dropping off almost completely (this car had a fuel pick-up issue), but the third column shows the ECU extending the pulse width to its maximum (20%) to attempt to compensate.  The second column shows the mixture leaning out (there was almost no fuel pressure…), however you can see the mixture returning to target even before it comes back to the specified pressure.  The injector on-time returns to normal as the fuel pressure comes back as well.    

    This kind of compensation operates and adjusts continuously on the Emtron ECU when the Fuel Pressure input is included in the fuel model (highly recommended).  This instantaneous adjustment is much more functional than relying on typically closed loop mixture adjustments that many other ECUs use alone.  

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