Project GD STI: Making More Power with Cobb Tuning and Precision Turbo Part 2!

Project GD STI: Making More Power with Cobb Tuning and Precision Turbo Part 2!

by Mike Kojima


When we last left off with Project GD STI, we had just installed our drop-in ball bearing center section upgraded Precision Turbo. In our opinion, this is one of the best drop-in, non-rotated mount turbos on the market for these cars. 

However, the turbo left us unable to turn the boost higher, and we were stuck at running 2 lbs less boost than we were able to run with the stock turbo, or 14 lbs falling to 11 lbs. With the stock turbo, we were at 16 lbs falling to 14 lbs on top. So, we would have to address this.

Another thing of relevance is that in our area for the first time, E85 was becoming easy to find with several stations now selling the wonder brew within a few miles of our shop and home. Because of this, it was time to make an effort to be able to run the ethanol mix, not so much in search of more power, but to help preserve our fragile stock bottom end EJ257 engine. Hopefully we could do this without too much effort. 

Read more about Project GD STI !


Since we are lazy and want convenience, we wanted our E85 conversion to be seamless and to be able to switch back and forth between E85, regular pump gas and any mix of the two at any time. 

Cobb Tuning had just the right solution for us with their flex-fuel conversion for the STI. The Cobb system uses this OEM grade fuel sensor that uses electrical conductivity to determine the amount of ethanol in the fuel.

Cobb's Subaru flex fuel system is very comprehensive. It has provisions for error checking and reporting of specific errors via the Accessport for issues such as water contamination of fuel (which can occur when ethanol sits for long periods of time), sensor failure, module failure, or a standard wiring fault (such as the unit being unplugged).


The sensor also comes with a plug and play wire harness that makes installing the kit a snap.

By using the Cobb Access Port to do some clever reprogramming to retask some of the OEM plugs, the flex fuel system plugs into the OEM wire harness via the TGV valve plug.  Everyone who builds the EJ27 gets rid of the TGV system anyway. 

The harness also goes in line with the stock rear O2 sensor, which is replaced by an AEM Electronics wideband sensor. 

No cutting or splicing of wires is needed at all for the Cobb flex fuel system.


The Cobb flex fuel system has a sophisticated way of measuring actual fuel pressure via differential fuel pressure, so the system comes with a fuel pressure sensor as well.

Turbo Subarus, like many forced induction vehicles, use boost referenced fuel pressure regulators, which can require a calculation in order to know what your fuel pressure should be at in any given moment. The COBB system calculates differential fuel pressure based on the MAP (manifold absolute pressure) sensor readings and the fuel pressure sensor- reading the actual line fuel pressure.

Since the differential fuel pressure should always be about the same as the non-referenced “base” fuel pressure, this makes the monitoring of the actual fuel pressure easy. The differential fuel pressure should always be around 43-44 psi under any condition (except post start when it's higher, since the fuel pump runs a 100% duty cycle for several seconds, and it overruns the return regulator). The fuel pressure can be monitored and logged on the Acess Port Gauges screen to check for fuel starvation, steps in pressure on multi pump systems, etc. It's a great monitoring and diagnostic aid.

For the best real time estimation of fuel needed, COBB has added fuel injection compensation based on the current differential fuel pressure. This way if the differential fuel pressure goes either under or over the normal fuel pressure target, the ECU will compensate by increasing or decreasing injector pulse width, keeping the fuel mass flow and air/fuel ratio consistent if the fuel pressure fluctuates.

The tuner can enable or disable this compensation system, set their own differential fuel pressure target and adjust the fuel compensation table based on their needs and preference. COBB also pre populates these functions with reasonable values to help calibrators out. 


To install the unit, Cobb picked a place on the firewall where it would be easy to access the stock fuel lines and fuel pressure return regulator.

Since there is not an OEM hole here, we would have to drill one to mount the Cobb bracket. This was quite simple and literally took only seconds.


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