2008+ STi fuel stumble
What is the fuel stumble?
The fuel stumble is an issue that primarily plauges the 2008+ STi. Subaru made some changes to the fuel pressure regulator setup that caused a strong stumble or hesitation in the 2700-3200 rpm range. Subaru attempted to remedy this with some load based compensation tables but this is honestly an incorrect approach to fix the issue they created. The fuel pressure regulator design itself is unique for the 2008 and newer STi. It attempts to feed fuel to the rails in both directions which does not work well in practice. The regulator also references the vacuum/boost directly off the cylinder four intake runner. Cobb Tuning did some testing and proved this is also a big part of the issue. Using one cylinder for reference leads to a fluctuating signal which can cause fluctuation fuel pressure. As you change parts, injectors and fuel types on the 2008+ STi, its easy to notice the stumbles more and more and they can change rpm as well. The best approach is a mechancial change to the fuel pressure regulator and reference line. With this done, the tuning becomes more normal rather than having to use the load based compensation tables that should have little to no use anyway.
What do you need?
Cobb offers a complete plug and play solution to cure the stumble issue. The first thing is a kit that inlcudes an older-style fuel pressure regulator (FPR) like that found on the 2007 and older STi. The kit comes with plug and play fuel lines to make the install easy. The other importnt thing their kit includes is a fuel pressure regulator reference relocation kit. By changing to an older style FPR and referencing vacuum/boost from a central source on the intake manifold, the stumble is effectively removed naturally and the car will tune much better. It is recommended to do something like Cobb's kit or set up any 2008+ STi in the same way using the same parts. If you choose to change your fuel rails & lines, that will also eliminate the stumble as long as you use an aftermarket or older style FPR. It is recommended to reference the new FPR with a central intake manifold source as well.
Cobb's fuel pressure regulator kit: