How would you know? What would you look for? Often we get asked at what point do I need to upgrade my regulator, and why. Well if you have an 08+ STi you more than likely have read into the dreaded “GR Stumble”. The 08+ STi runs a parallel to series fuel system. It is when the switch between happens, fuel pressure drops. This causes is what you feel as a stumble. Here is an example of a replacement kit on a 08+ STi. We have used COBB’s fuel pressure differential to monitor and adjust for discrepancies in pressure at the rail.
Green is Boost
Red if Fuel Pressure
White is injector scale
What we see from this log is the regulator not bypassing enough pressure when fuel pump duty increases. You see this right before we hit peak torque, or our boost target. The pressure should scale identical to boost, it shouldn’t spike above the delta in boost pressure. This tells us the regulator isn’t big enough for our application. Which, in this example, it’s an older STi style OEM regulator. We simply swap it out for a larger aftermarket unit, and we are good to go!
Above is an example of a larger upgraded regulator. If you have any questions on what is needed for your specific application please let us know. We stock and supply many various aftermarket regulator options.