Posted: September 12, 2016

We recently got our hands on a brand spanking new 2015 Subaru WRX and couldn’t resist the urge to do some testing. This exciting new platform features a 2.0-liter engine like the WRX of old but with quite a few major differences. The new car features a twin-scroll low-mounted turbocharger, direct cylinder injection, a completely redesigned engine, and a 6-speed transmission. Considering this platform is likely the future of Subaru we decided it would be a great vehicle to do some serious testing.

In this article, we test this car in its purest “stock” form. We explore the capabilities of the car exactly as Subaru designed while making changes to only the tune.


It’s always a good idea to get a baseline when modifying anything so you know that your modifications are working as intended. To baseline our new 2015 Subaru WRX we decided first to put around 1000 miles on the chassis to break it in. After that, it was off to the dyno for initial testing.

On the stock tune, the vehicle put down 197.7 whp and 228 ft-lb tq. A little shy of what we had expected from drivetrain loss over the advertised 268 horsepower. We decided to reset the ECU as often Subaru likes to learn lower timing. This yielded slightly higher numbers on the first pull but on subsequent pulls we were back down to the previous numbers. The thing that was the most shocking was the boost curve, or rollercoaster rather. The boost came in spiking to nearly 20psi on the dyno, then back down to xx psi then back up before finally tapering off towards redline. This boost inconsistency was present on all pulls done with the stock tune. Regardless it was time for some tuning. This thing needed it bad.

Quick references: Actual vehicle weight 3,307 lb. Weight with driver 3,600 lb even. The fuel used is 92 octane e10 pump gas. The WRX has absolutely no electrical or mechanical modification for stock tests and only a COBB Accessport tuner for tuned tests.


For tuning, we are using the COBB Accessport and COBB Tuning Access Tuner Pro software which is a tried and true tuning tool for Subaru and other platforms. They also have excellent customer support as well as full data logging capability which is extremely important for tuning and making changes, especially in a racetrack environment. While laptop loggers and tuning solutions are available, the Accessports ease of use and feature-rich interface make it the ideal choice.

To get the party started we loaded the off-the-shelf (OTS) 91 octane stage 1 map onto the ECU. After letting it run for a bit and running it through some gears on the dyno to allow the ECU to learn we did the first pull. Immediately the IAM (dynamic advance multiplier) dropped down below 1 which is an indication of timing and knock issues. Another pull and the IAM stayed the same. 3rd pull and the IAM climbed back up a little. Horsepower on the COBB OTS Stage 1 map was 223.0 whp and 249 ft-lb, a healthy gain over the stock rating. Additionally, the boost was much more stable which translates into a smoother power and torque curve. Driving the car on the COBB Stage 1 map is much more exciting and predictable than the stock tune. Power comes on smooth and pulls hard to about 5500rpm. The throttle feels much more linear and less bottom-loaded than the stock tune. All-in-all the car is actually easier and significantly more fun to drive.

On an interesting note, while the ignition advance multiplier (cars adaptive timing) defaults to 1.0 in the Stage 1 map, it quickly dropped to 0.500 while driving indicating over advanced timing. After a couple of full-throttle pulls the IAM climbed up to 0.680 where it wanted to stay. It seems these direct injection cars are especially prone to knock at part throttle, more on that later.


Since we have full tuning capabilities with the Accessport using the Access Tuner Pro software we wanted to see if we could make any gains over the COBB off-the-shelf stage 1 map. Adjusting the air-fuel ratio slightly and making pretty significant timing curve changes we were able to squeak out 7.6 more horsepower to the wheels for a total of 230.6 wheel horsepower and 257 wheel torque.

Not a huge gain but the more significant thing was that we did it by reducing overall ignition timing through the whole map and lowering boost. These two indicators show that the compression of this engine is too high for the boost and timing that was requested from the COBB 91 octane Stage 1 map.

This made sense considering the vehicle's ignition advanced multiplier (IAM) had dropped while driving the car on the COBB map.


When you look at the overlay of the PREracing custom map and the COBB S1 map you will notice one thing. The COBB map makes more power above 6000rpm than the PREracing map. The reason for this is that while the ECU is seeing knock and actively pulling timing during this area however the total timing is still higher than our tune.

This area almost always knocks the IAM down on the COBB map if it hadn’t already dropped. In order to keep the IAM up, we decided to reduce timing in that area on the PREracing tune so the ECU doesn’t see any knock and therefore doesn’t drop the IAM down. This effectively reduces where you should shift the car from a stock redline of 6500 down to about 6100.

We felt that this was better for the longevity of the engine to not knock at all instead of knocking and having the ECU pull timing. We focus our tunes on raising peek power and power under the curve which is achieved well before 6500 rpm on this motor and for the sake of engine safety there is really no reason to rev the engine past 6100 rpm.


Unfortunately once driving even with our lower timing and boost targets the WRX’s computer still wants to pull timing in places. While this automatic reduction wasn’t as severe as the COBB Stage1 map it was still more than we like to see.

The easiest solution here is to add octane but unfortunately, the pumps here in Portland only deliver 92 octane fuel not the 93 that is available in many parts of the country.


Since the car is completely stock still we wanted back to back to back comparisons of stock to COBB Stage1 to PREracing Custom Stage1 so off to the track we went. The local Portland International Raceway dragstrip hosts quarter-mile staged and timed drag races just about every Friday and Saturday night from April to September and is perfect for our testing needs.

While our racetrack is essentially at sea level on the day of testing the effective density altitude of the air was 1512ft. This means that under standard atmospheric and temperature conditions we were effectively breathing as if we were racing at 1512ft, not sea level. The exact weight of our stock 2015 Subaru WRX was 3,307 lb and with the driver, we’re at an even 3,600 lb.

First pass we loaded the stock tune back onto the WRX, drove around for a few min to allow for ECU adjustments, and staged the car to race. Launching a Subaru WRX can be a bit challenging as it is very easy to bog the engine on takeoff which will cause horrible times.

Fortunately, we practiced a few launches in the shop parking lot and figured out a technique that provided for the most part ~1.8 seconds 60ft times.


While some reviews put the car at 13.8 seconds at 100-101 mph our car on this day clearly was making less horsepower or weighs more than the cars tested, however, these are baseline numbers for showing gains and were consistent enough for that purpose.

  • 60ft = 1.899

  • 1/8 ET = 9.067

  • 1/8 MPH = 76.62

  • 1/4 ET = 14.208

  • 1/4 MPH = 94.63 – clean pass


After we felt we had a good cross-section of baselines for stock we loaded COBB OTS Stage 1 tune onto the WRX and gave that a few passes.

Immediately the difference was noticeable as we gained 3.27 mph for a total of 13.950 seconds at 97.9 mph. As on the street, the IAM was dropping during the pulls and in 4th gear, the car felt pretty flat.

  • 60ft = 1.886

  • 1/8 ET = 8.864

  • 1/8 MPH = 77.63

  • 1/4 ET = 13.950

  • 1/4 MPH = 97.90 – Clean pass, IAM dropped slightly around 2nd gear


As before with the OTS map, we loaded the PREracing Custom Stage 1 Tune and gave it a few passes. The WRX responded well to the additional torque from the PREracing tune posting faster 1/8th ET and 1/8 mph. 3rd gear didn’t feel nearly as flat as the COBB tune and most importantly the IAM did not drop during the run.

  • 60ft = 1.864

  • 1/8 ET = 8.801

  • 1/8 MPH = 78.18

  • 1/4 ET = 13.869

  • 1/4 MPH = 97.75 – Clean pass

The difference in ET with the PREracing map was primarily from changes to torque delivery which allows more power “under the curve” than the COBB OTS map. Since peek-for-peek horsepower was very close the final MPH was also very similar but the elapsed time was a noticed difference. For the day both the highest MPH and fastest Elapsed Time were from the PREracing custom map.


After all this testing two things are very apparent. One, the car needs better fuel. The complete lack of ignition timing really seems to be holding this engine back. Two, back pressure seems really high. Boost literally falls on its face as RPM increases and while it gives a decent amount of torque for a 2.0-liter engine it severely limits the overall power it can produce.

Combine that with the requirement of the engine to run extremely low ignition timing before seeing knock and you end up with a car that comes in pretty strong but doesn’t finish particularly well, the exact opposite of 2.0 Liter WRX’s in the past.

Our next set of tests is going to address one of the issues identified as we show the gains and benefits of converting to ethanol-based fuels. Stay tuned!

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