Differential Fuel Pressure Measurement - And Why You Need It

by Lance Lucas

Over the last 18 months or so, I have become increasingly interested in very high level fuel system knowledge, design and implementation. Firstly, because I'm a geek and I find it personally enjoybale, and secondly because of the sharp rise in both the customer-demanded power levels and the variety of fuel types being used. Ethanol based fuels have presented a whole new challenge due to the sheer volumes of fuel that must be delivered to make the 400, 600, 800 and even 1000 whp goals that many COBB customers have. Granted, the 600whp+ goals aren't happening on Subarus very often, but the time spent working with GTR's at this power level is knowledge that can be applied across all power ranges.

How many posts have you seen with people claiming that they had perfect fuel system performance without any actual data to back it up? "XYZ fuel pump was awesome...no fueling issues at all even on E85!" The devil is in the details, folks. Just because the tuner was able to manipulate the tune well enough to get through full-throttle dyno pulls with a suitable tailpipe AFR ratio, they have little to no actual guarantee that the fuel system was performing perfectly, just that it was working well enough based on the data they have available.

Now, to the interesting stuff. "Differential Fuel Pressure". Huh? Right! Differential Fuel Pressure is an unofficial moniker used to describe the effective fuel pressure available at the fuel injector inlet. We're all familiar with the concept of a manifold referenced fuel pressure regulator -- as manifold pressure goes up, fuel pressure goes up, and vise-versa, so that the injector's inlet/outlet pressure delta is always the same. This allows for injector flow rate to remain constant even as manifold pressure swings from high vacuum to full boost and back. "Differential Fuel Pressure" is the quantified measurement of that phenomenon. 

So, how is differential fuel pressure calculated? Quite simply: Actual Fuel Pressure - Relative Manifold Pressure (boost) = Differential Fuel Pressure. More verbosely, Actual Fuel Pressure - (Manifold Absolute Pressure - Atmospheric Pressure) = Differential Fuel Pressure. In order to begin monitoring differential fuel pressure, I added an AEM fuel pressure sensor and barometric pressure sensor to Surgeline's Mustang AWD-500-SE dyno and utilized a "math channel" that does the aformentioned algebra. Differential Fuel Pressure is now reported in realtime by the dyno and on graphed outputs just like Boost, AFR or Torque. Of course, we don't use this data for every vehicle, but we are using it an ever-increasing amount as we spend more and more working on cars with wild fuel system upgrades. Here's a sample of a "loaded" set of dyno chart from a recent tune that includes WTQ, WHP, Boost, Wideband, Turbo Pressure Ratio and Differential Fuel Pressure. What's on YOUR dyno chart?:

Why is fuel pressure so important? Many reasons, but this is the key takeaway: the key metric for ensuring adequate fuel is available to the fuel injector is fuel pressure. When fuel pressure behind the injector is correct, the injector will do it's job. When fuel pressure is not correct, fueling errors result. The real struggle comes from the fact that we must keep raising feed-side fuel pressure (via the fuel pressure regulator) in order to keep differential fuel pressure constant as boost rises. However, due to the simple physics of a fuel pump, as pressure demanded goes up, flow rate decreases. For example, here's Radium Engineering's flow rate results for the Walbro 416LPH pump at various line pressures:

http://radiumauto.com/blog-page.php?Radium-Engineering-s-Ultimate-Fuel-Pump-Test-73

What does this all mean? Fuel becomes serious business when we're looking to run 20psi+ on E85 cars. It gets really wild when you add another turbo, 2 more cylinders and 200-400whp on a GTR. The total flow demanded is enough to make most all of the intank fuel pump options beg for mercy. When calibrating cars at this level, especially when attempting to develop a correct Volumetric Efficiency table for cars utilizing Speed Density, it is mission critical to know that the fuel system is performing on-par. Here's a graph of differential fuel pressure from two recent "big" fuel system cars. NOTE: I use Excel to chart the data from the dyno to get more graphic display options. One is a ~400whp 2011 STi running on a Dom 1.5XT-R, E85, FIC1100 injectors, stock fuel rails and ~52psi base pressure. The other is a ~475whp 2008 STi running on a EFR7670, pump gas, ID2000 injectors, Perrin fuel rails and ~43.5psi base pressure. Both utilize a Radium Surge Tank containing a Walbro 416LPH fuel pump, though the 2008 STi also features fully upgraded feed and return fuel lines (pump gas was a warm up test for the E85 tune coming soon):

Note that the differential lines are not absolutely flat. This is due to the fact that fuel pressure regulator utilizes a simple coil spring. While coil springs are fantastic little devices, they aren't perfectly linear across their entire range of travel end to end (suspension guys can feel me on this one!). The key point of interest is that the differential pressure line does not sharply rise or dive at any point and maintains a virtually identical position even acrost widly different boost and power levels. The regulator and fuel supply components of the fuel system perform perfectly on these cars, making the tuning process a much more straightforward process. Both of these fuel systems are working great!

Now, to demonstrate some of the various quirks and factors that must be taken into consideration on an elaborate fuel system. This is the GotBoost Performance shop 2009 GT-R, featuring a built motor, upgraded Spray-It Racing turbocharders, ID2000 injectors, T1 Race Development 3 pump intank hanger and ~43.5psi base pressure.

Notice the sharp rise then correction in differential fuel pressure at ~2400 RPM, with a much smaller second rise and correction at ~3400 RPM? Each of these events directly correlates with the triggering of the 2nd and 3rd fuel pumps, respectively, as boost pressure passes pre-defined levels for triggering the Hobbs switches controlling the 2nd and 3rd pumps. Despite the regulator's best attempt, the additional fuel pump's additional flow forces the regulator into a much different spring range, and a small change in differential fuel pressure is the result. The effect is lessened as the engine's demand for fuel begins to catch back up to the newer much-higher incoming flow rate from the pump side. With this knowledge, it's much easier to see how precise control over fueling in those transitional areas will be difficult. The ECU does not have a fuel pressure compensation, but fortunately the GTR does run full-time closed loop fueling, so temporarily fuel system performance issues have at least a minimized effect on overall fueling. On a Subaru, this effect can create significant issues.

Many thanks for your time. I hope you hope this post informative and enjoyable.

Best regards

Lance Lucas
COBB Tuning Surgeline

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