Great news! All new Stage 3 Stage Power Packages and parts for USDM 2008 – 2017 Subaru STIs are available now!
For several years tuning an STI beyond the Stage 2 level has typically required cherry picking parts from several different companies with varied results. While we previously offered Stage 3 Power Packages, earlier this year we recognized that there was room to improve those packages along with opportunities to implement some fresh new parts in the process. After several months of testing and tuning we are excited to release revised Stage 3 Power Packages which provide a solid foundation as the next big step for your STI build.
Key Updates and New Products:
New COBB OTS Maps for 1050x Injector Support – Shiny new OTS maps revised specifically for use with the new Stage3 Packages
Reverse Compatibility with previous COBB Fuel Pressure Regulator Kit, Flex Fuel, and Fuel Pressure Sensor if already installed to the car.
Expected Power Gains
Installing a complete Stage 3 Power Package on your 2008-2017 STI should net peak gains of 24% HP and 40% TQ over stock. Maximum gains upwards of 68% HP and TQ are found earlier in the power band.
Lets get technical: The how and the why for our Fuel System Redux
One of the reasons Subaru is so popular with the enthusiast aftermarket is they rarely change components. This allows bolting a 2017 WRX STI engine right into a 1991 Subaru Legacy Sport Sedan if you’re so inclined. The downside is occasionally they stick with something that is not ideal. One example is the fuel system on the 2008+ STI which causes erratic fueling behavior, you can actually feel, in the form of a stumble. (Click Here: For a more detailed diagnosis of the stumble issue)
In the stock 2018 STI engine calibration we can see that after 10 years, Subaru continues to tweak the calibration around the issue rather than resolving it by updating the fuel system. While the calibration tweaks allow the car to run well in stock form, the problem persists once you install higher output injectors.
High flow injectors can supply small amounts of fuel common to daily driving in very brief bursts. Each burst of fuel can cause a rapid change in localized fuel system pressure in the rail. Because the factory fuel rails and lines between them are all metal, any pulsation of pressure quickly propagates through the system and can affect operation of all the injectors. The concern here is fuel pressure at each injector inlet has a massive effect on its fuel output. Sometimes the pulsations interact in a manner that all injectors receive less inlet pressure than normal so they all burn lean. Other times some cylinders receive too little pressure and others too much, causing some cylinders to run lean and others rich. The more stable we can make fuel pressure across the fuel rails and lines, the more consist and reliable the engine will run.
Now that we have briefly rehashed the problem, let’s talk about solutions! A few years ago COBB found two simple to install changes which generated a marked improvement in fuel system behavior. The first was relocation of the fuel pressure regulator (FPR) manifold reference hose, from a single intake manifold runner, to the plenum. The factory FPR air pressure source is a port on intake manifold runner #4. It was likely convenient for packaging, but proximity to the cylinder head causes large oscillations in pressure due to valve events. If the air pressure fed to a fuel pressure regulating diaphragm is oscillating, the resulting fuel pressure will mimic. Using a stable plenum source settled pressure down a bit but fuel system oscillation was still present. COBB’s other improvement was our Fuel Pressure Regulator Kit (pictured above). This includes a Subaru OEM regulator with retrofit lines that allow for plug and play installation. Again fuel pressure stability improved and was able to support 725cc injectors.
Recently the 725cc injectors became unavailable in favor of new 1050x fuel injectors. With that change we decided to engineer a fuel system capable of not only supporting current Stage3 needs, but also our future plans for additional stage packages. New ethanol safe 1050x injectors were chosen and we were happy with their performance but found their higher output was bringing fuel system limitations back to the forefront. Additional improvements would be required to achieve excellent fuel pressure stability, drivability, and engine safety.
After a great deal of research including testing of several leading aftermarket fuel pressure regulators, various fuel rail configurations, several different fuel hose types, routing, lengths, and pulsation dampers over a period of several months, we arrived at the new COBB fuel package. Here are a few details of that journey and how we arrived at this package. We of course monitored fuel pressure in multiple locations at high speed, but we also monitored other factors such as fuel temperature, manifold pressure, and more.
Some of the FPRs we tested simply did not manage fuel pressure as well as the COBB unit. If an FPR can not regulate fuel pressure well, there isn’t much more to say.
Through the course of our testing we also found the existing COBB FPR kit was the only one that maintained consistent differential fuel pressure as fuel temperature changed. Every other unit we tested dropped 0.5 to 2 psi (depending on the FPR) over the course of a mild temperature sweep found during normal driving behavior.
Shown Above: The COBB Subaru Fuel pressure Sensor Kit was used throughout the testing process. (it is included with some packages)
While the high speed fuel pressure data from an external logger was useful, capturing fuel pressure data at the injector inlet wasn’t possible without modifying the fuel system in a manner that would change its behavior. It turned out that logging fuel trims via the Accessport was the best way to gather data which matched what the driver felt in terms of the stumble. During testing COBB’s custom Differential Fuel Pressure Compensation was disabled and Load Compensation (the bandaid) tables were zeroed out to see how well each regulator could do on their own. For those less familiar with fuel trims, values closer to 0% are ideal, and indicate the amount of air and fuel entering the cylinder has been accurately represented. Incorrect fuel pressure would cause more or less fuel mass than intended to be injected, causing error in the resulting air/fuel ratio, which requires fuel trimming by the ECU in an attempt to correct.
Here are some examples of the fuel trim data collected. The colors represent how much data was gathered in a given area, so focus on the darker yellow and green areas where lots of data was collected. Total fuel trims are short term + long term (correction + learned values).
This first graph shows a leading aftermarket FPR with a leading upgraded fuel rail and line package which many shops suggested:
As you can see, under some conditions, fuel trims are nice and close to zero. However around 3200 RPM, fuel trims were up over 15% at certain loads and the stumble was easy to feel from the driver’s seat.
Updates and Results:
While we tested many many combinations of parts; as we honed in on the final kit, line lengths and construction were the icing on the cake. Since the fuel line itself can act as a damper, we chose a hybrid system of lines that use two different styles with specific lengths to best tune the system for optimal pressure stabilization.
Here is what fuel trims look like with the all new COBB Stage3 Power Package with 1050x fuel injectors installed. This package includes the existing COBB FPR kit (with manifold reference relocation) and our new system of hybrid lines, upgraded fuel rails, fittings, and adapters. Dozens of iterations were tested, but I will skip to the end of the process now and show you how the final product performed. Results on our in-house 2011 STI and 2017 STI as well as external 2012 and 2013 STI test vehicles were extremely similar using the final kit. This is in stark contrast to results on other test systems which showed different behavior on each car. This is the type of thing that has had Protuners pulling their hair out for years. They get one car dialed in to band-aid the stumble, put the same settings in the next STI they tune and it doesn’t run well so they have to start over. We are happy to say that those days should now be over!
As you can see, aside from a few stray values during brief throttle transitions (shown in boxes with white background which illustrate a low hit count) fuel trims are +/- 3% under most conditions. Despite using injectors with almost double the flow rate of stock, fuel trims are now better than stock on this car. Attempting to “tune around” the issue, which has mediocre results, is no longer necessary!
As mentioned previously, this kit was designed with headroom in it to meet the needs of future COBB Stage Power Packages. Those wishing to go beyond Stage3 power today are welcome to add on our Flex Fuel and Fuel Pressure Sensor Kits and an adapter line and contact their Protuner for a flex fuel tune. Our fuel components with the AEM 320 lph fuel pump we include in the Stage 3 Power Packages is designed to go even further, supporting up to about 400 wheel HP on E85, and 600 wheel HP on E0-E10 gas .