This picture shows how fuel pressure is supposed to follow the boost curve. You can see the blue line hits a peak of 70psi and holds that all the way to redline following the boost curve hitting 19psi and holding that all the way to redline.
When making more than stock power, maintaining the correct fuel pressure under boost/full throttle becomes very critical. Too little fuel pressure and your fuel injectors are left not injecting enough fuel which causes your engine to run lean. Potentially causing major damage. Cobb Tuning has added the ability to monitor fuel pressure and make real time adjustment based on fuel pressure differential to the Subaru ECU logic. It definitely doesn’t get the attention it deserves but it’s a great feature.
With the Cobb Custom Features we can add fuel pressure to be monitored on the Accessport and also set up the tune to make real time adjustments from fuel pressure. We input the base fuel pressure and if the fuel pressure is more or less than expected from the formula above, the ECU can make the needed changes to keep the AFR at its target. This feature is very helpful when doing flex fuel tunes as we typically raise the boost on the E85 portion of the tune and fuel pressure doesn’t always follow exactly since it’s controlled with a mechanical spring. This also has the benefit that if you have a fuel pump/fuel pressure regulator issue the ECU can compensate and potentially save your expensive engine from blowing up. The system can only compensate so far and shouldn’t be relied on if you are having issues. It’s really to keep your AFR’s at the tuned targets as fuel pressure varies slightly under full throttle. If your car is constantly making large adjustments from large fuel pressure differential errors it’s a good idea to get your car checked out, find the issue, and have it repaired.
Below are some pictures illustrating what happens. The graph shows fuel pressure, boost pressure and fuel pressure compensation. The blue line is losing fuel pressure from a failing fuel pump and the red lines hold fuel pressure. You can see that as the fuel pressure drops the fuel pressure compensation starts rising and ends up at 50%! That would have left your engine running dangerously lean but as you can see in the second picture the AFR stayed almost identical.