’11 Subaru STI
2.5-liter EJ25 turbocharged 4-cylinder
The latest iteration of Subaru’s STI is back on track with its aggressive widebody stance and the fabled rear wing that was lost for a generation. There’s no doubting that the looks of the STI are on par with the best out there, but the new STI has lost a bit of its older sibling’s edge when it comes to performance. Its sluggish, lackluster power output needs a serious overhaul to deliver the type of performance we expect from a car of this nature.
The problem is with emissions and Subaru’s aging EJ25 engine having to meet strict requirements to pass them. The solution is simple, though — with a few mods and a proper tune, the beast within the STI is awakened.
First and foremost, the restrictive exhaust needs to be replaced with a quality-built, high-flow unit such as Milltek’s Sport turboback exhaust system from its exclusive North American Subaru dealer, Mann Engineering. Built and designed at home in England, the system is made from 100 percent 304 stainless steel and features a high-flow design utilizing a 3-inch bellmouth downpipe and a HJC Motorsports 100-cell count high-flow catalytic converter that’s built to withstand high heat and the abuse of a turbocharger. The rest of the system flows out to dual 2.325-inch mufflers that can be equipped with either 100mm titanium or polished stainless steel exhaust tips for a sleek OEM-like look.
On the intake side, the STI can also use a helping hand, and Takeda’s air intake is built to replace the front portion of the OEM intake setup. The anodized polished pipe places the dry filter element that’s surrounded by a heat shield into the STI’s front fenderwell, providing constant access to fresh air. Flow is said to increase 63 percent over its stock counterpart — a serious power boost.
Results: Milltek Exhaust
Plain and simple, the STI is a dog right off the showroom floor. It tries hard to deliver some type of excitement, but you can feel the ECU fighting the engine to ensure that any power it makes is done so in a clean, environmentally friendly way.
Time to change all that (and still be environmentally conscious). The Milltek exhaust that was installed on our test mule proved to be the ideal upgrade for this ’11 STI sedan. With such an intricate turboback exhaust, fitment issues can arise, but the Milltek is broken down into many small sections to ensure OEM-like tolerances and clearances. The factory O2 sensor is about the only thing that has to be moved from its original spot and placed further down the exhaust path, which requires some unclipping of plastic ties, but otherwise the system installs in just over an hour. The quad exhaust tips that are clamped onto the mufflers are adjustable, so you can set them to your desired length. If you have some extra cash, then we recommend splurging on the titanium tips because they add a unique look that you won’t find on other systems.
Sound-wise, the Milltek exhaust is an ideal blend of low rumble and aggressive, wide-open-throttle notes. Never does it annoy or drone during cruising — at times, you forget it’s even there (and we’re sure the police will, too).
Results: Takeda Intake
The Takeda air intake is well constructed and designed fitting the filter element into the fenderwell, where it’s closed off from the engine bay’s heat. It adds nice flair to the engine and is about as easy as installs get, taking roughly 30 to 45 minutes of your time from start to finish.
With a baseline number already in hand, we headed down to Church Automotive Testing to see exactly how much power these two items would add to the STI. Driving the car there, the STI felt like a new machine that delivered much better acceleration than stock — but just how much? With the stock air intake in place, the turboback Milltek Sport exhaust belted out 261.5 whp and 288.6 ft-lbs of torque, an improvement of 28.4 whp and 14 ft-lbs tq over the stock 233.1 whp and 274.6 ft-lbs tq. This was with little to no increase in boost pressure, proving that the stock system is holding back the EJ25 quite a bit.
The STI surprised everyone by producing a whopping 42.9-whp and 8.5–ft-lbs tq gain on top of the exhaust (304.4 whp and 2971 ft-lbs tq) with the Takeda air intake. Boost increased by 3 psi, and the air/fuel ratio was on the border of being too lean. Running the car untuned is very risky, as detonation can occur (and did in our case), which could quickly cost you a motor.
With a tune, not only can you expect to have your air/fuel ratio, timing, boost and fueling back to safe standards, but Church Automotive Testing can net another 47.2 ft-lbs tq and 1.7 whp all the while providing faster spool-up and better driveability. That’s a combo hardly worth passing on.
Milltek Sport Turboback Exhaust
- 100 percent stainless steel construction with mandrel-bent piping for optimal flow
- 3-inch bellmouth designed downpipe gets rid of restrictive stock downpipe
- High-flow, 100-cell catalytic converter increases power while filtering harmful emissions
- Exhaust doesn’t trigger check-engine light
- Quad 100mm exhaust tips in either stainless steel or titanium available
- Made in England with matching serial numbers on each component
Takeda Air Intake
- Anodized intake tube enhances engine bay appearance
- Improves throttle response throughout rpm range
- Increases airflow by 63 percent over stock intake
- Dry, non-oiled air filter is washable and reusable
With Milltek Exhaust
+ 28.4 whp
+ 14 ft-lbs tq
Before 233.1 whp & 274.6 ft-lbs tq
After 261.5 whp & 288.6 ft-lbs tq
With Takeda Intake
+ 42.9 whp
+ 8.5 ft-lbs tq
Before 261.5 whp & 288.6 ft-lbs tq
After 304.4 whp & 2971 ft-lbs tq
With Church Automotive Testing
+ 1.7 whp
+ 47.2 ft-lbs tq
Before 304.4 whp & 2971 ft-lbs tq
After 306.1 whp & 344.3 ft-lbs tq