The 4B11T engine has gained a reputation for exceptional performance potential, especially when modified with carefully selected aftermarket parts. In this article, we examine real-world results from a 2015 Mitsubishi Lancer Evolution X equipped with a Cosworth downpipe and race exhaust system. Dyno testing confirmed a peak output of 370 horsepower at the wheels, demonstrating how targeted exhaust upgrades can unlock significant power without major internal engine work.

The 4B11T Engine: Background and Potential

First introduced in 2007, the Mitsubishi 4B11T is a 2.0-liter turbocharged inline-four engine that powers the Lancer Evolution X and a handful of other Mitsubishi models. It features an aluminum cylinder block and head, dual overhead camshafts, and Mitsubishi’s signature MIVEC variable valve timing system. The engine’s closed-deck design and robust internals—forged connecting rods, cast pistons, and a forged steel crankshaft—allow it to handle substantial power increases over its factory rating of 291 horsepower in North American specification.

Stock, the 4B11T leaves room for improvement. The factory turbocharger (a Mitsubishi TD05HR-16G8 twin-scroll unit) is capable, but the restrictive exhaust system and downpipe limit flow and response. Many enthusiasts find that upgrading the exhaust path is one of the most effective first steps toward extracting more power. The 4B11T community has documented numerous builds, but the 370 hp result we cover here is particularly notable because it was achieved with minimal supporting modifications—just a downpipe and cat-back exhaust, without a turbo upgrade or internal engine work.

Key Modifications: Cosworth Downpipe and Race Exhaust

To reach the 370 hp mark, two critical components were added: the Cosworth downpipe and a race exhaust system from the same manufacturer. Both parts are designed to improve exhaust gas flow, reduce backpressure, and increase engine efficiency. Together, they form a high-flow exhaust line that allows the turbocharger to breathe more freely.

Cosworth Downpipe

The Cosworth downpipe replaces the restrictive factory downpipe that connects the turbocharger outlet to the exhaust system. The factory downpipe features a narrow diameter and a catalytic converter with a dense substrate that creates significant backpressure. Cosworth’s version uses a larger 3-inch diameter stainless steel tube and, in this build, a high-flow catalytic converter or a straight test pipe option (the 370 hp result was achieved with the high-flow cat). The downpipe also features a smooth mandrel bend and a merged collector at the turbo flange to minimize turbulence. A second oxygen sensor bung is provided for proper wideband tuning. These changes reduce exhaust gas temperature and pressure, allowing the turbo to spool more quickly and maintain boost pressure across the rev range.

Race Exhaust System

The Cosworth race exhaust system completes the flow path. It is a cat-back system with 3-inch tubing and a straight-through resonator and muffler design that minimizes restriction while still meeting reasonable noise limits (though it’s louder than stock). The exhaust uses T304 stainless steel and is TIG-welded for durability. It replaces the heavy, restrictive factory exhaust with a lightweight, free-flowing system that saves approximately 10 pounds. The exhaust note becomes much more aggressive, with a deep, crisp tone that many Evo X owners find satisfying. The system is designed to bolt directly to the Cosworth downpipe using supplied hardware and gaskets, simplifying installation.

Real-World Dyno Results: 370 HP and Beyond

After installing the downpipe and exhaust, the vehicle was dyno-tested on a Mustang MD-500 AWD dynamometer in a controlled environment. The car was running a standard 93-octane pump gas tune (via EcuTek software) but no other engine modifications. The peak reading was 370 horsepower at the wheels (whp) and 340 lb-ft of torque at the wheels. This represents an increase of approximately 80–100 whp over the stock power level, depending on the baseline dyno run.

For reference, a stock Evo X typically makes 270–285 whp on a Mustang dyno. The 370 whp figure translates to roughly 440–460 crank horsepower accounting for drivetrain losses (around 20%). The boost level was increased from the stock 18–20 psi to 24 psi, which the stock turbo and engine internals handled safely with the improved exhaust flow and proper tuning.

Power Curve Analysis

The dyno chart shows a broad powerband. Peak horsepower occurs at approximately 6,200 rpm, while peak torque arrives earlier at 4,500 rpm. The curve is notably flatter than stock, with over 300 lb-ft of torque available from 3,800 to 6,000 rpm. This is due to the reduced backpressure allowing the twin-scroll turbo to deliver boost more consistently. Throttle response improved, with the turbo reaching full boost about 300–500 rpm sooner than on the factory exhaust system. The driver experiences a smooth, sustained pull that makes the car feel significantly faster than the numbers suggest.

Installation Considerations

Installing a Cosworth downpipe and race exhaust is a moderately involved job best suited to someone with basic mechanical skills and proper tools. Here is a step-by-step overview, along with tips to avoid common pitfalls.

  • Preparation: Lift the vehicle securely on jack stands or a lift. Allow the exhaust system to cool completely. Disconnect the battery negative terminal to avoid accidental sensor issues.
  • Removing factory parts: Unbolt the factory downpipe from the turbo. You will need a set of weighted sockets and a penetrating lubricant like PB Blaster—the bolts are often rusted. Next, remove the factory cat-back exhaust, which is in multiple sections with rubber hangers.
  • Installing the Cosworth downpipe: Fit the downpipe’s gasket (supplied) onto the turbo outlet. Lift the downpipe into place and loosely thread the bolts. Ensure the oxygen sensor wiring is not pinched. Torque bolts to 30–35 ft-lb. Reinstall the sensor(s) if you are using the original ones; the Cosworth downpipe has bungs in convenient locations.
  • Installing the race exhaust: The Cosworth system is designed as a direct bolt-on using factory hanger locations. Slide the exhaust sections together (with gaskets) and attach the hangers to the rubber mounts. Tighten all band clamps evenly. Be careful not to overtighten and crush the pipes.
  • Final checks: Lower the vehicle. Start the engine and inspect for leaks, particularly at the downpipe flange. Some systems require a slight adjustment to avoid contact with the underbody brace or heat shields—aligning the exhaust while it is lightly bolted helps.

Most installations take about 2–4 hours. If you are not comfortable working under a car, a professional shop can complete the work in less than two hours. The total parts cost (downpipe and race exhaust) is approximately $1,200–$1,500.

Driving Impressions and Sound

The most immediate change after installation is the sound. The exhaust note becomes much louder and more aggressive, with a pronounced turbo spool sound audible inside the cabin. At idle, it has a deep rumble; at wide-open throttle, it announces the car’s presence. The volume is manageable for daily driving if the valve timing is not overly aggressive (the Cosworth system is not unbearably loud).

Throttle response sharpens noticeably. A light tap on the pedal brings an immediate forward surge, and boost builds with less lag. On the road, the car feels significantly stronger in the mid-range, making highway passing effortless. The improved exhaust flow also reduces heat soak under prolonged hard driving, which helps maintain consistent performance on track days or canyon runs.

Alternative Modifications to Complement the Setup

The 370 hp result was achieved with only the downpipe and exhaust plus a professional ECU tune. However, several supporting upgrades can further enhance the package while maintaining reliability:

  • Intercooler upgrade: A larger front-mount intercooler (like those from Garrett or AMS) reduces intake air temperatures on hot days, allowing more aggressive timing and boost.
  • Fuel system: The stock fuel pump and injectors are adequate for 370 whp, but if you plan to go higher (e.g., 400+ whp), larger injectors and a fuel pump (such as a Walbro 450) are recommended.
  • Intake system: A high-flow intake kit (like Cosworth or HKS) can further reduce restriction on the inlet side and improve throttle response.
  • ECU tuning: The custom tune is essential. Without it, the engine would run lean and potentially knock. For those who do not have access to a dyno tune, a well-known off-the-shelf file like those from Brentune or EcuTek can provide similar results.

It is worth noting that the stock TD05 turbocharger is nearing its efficiency limit at 24 psi. Going beyond 400 whp typically requires a larger turbocharger, upgraded intercooler, and supporting fuel modifications.

Value for Enthusiasts

The Cosworth downpipe and race exhaust combination offers excellent value for Evo X owners looking for a substantial horsepower gain without entering the engine bay. The cost per horsepower (roughly $15–20 per whp) is competitive, and the installation is straightforward. Moreover, these parts are fully reversible, meaning the car can be returned to stock configuration if needed.

For those who enjoy a more visceral driving experience, the sound and responsiveness improvements alone may justify the investment. The 370 hp result is a proven, repeatable outcome that aligns with what other owners have achieved in forums and dyno threads. When combined with a proper tune, this exhaust setup becomes one of the most impactful single modifications for the 4B11T.

Conclusion

The Cosworth downpipe and race exhaust system elevated a stock 4B11T engine to a genuine 370 horsepower at the wheels, demonstrating that effective exhaust flow improvement can unlock the engine’s latent potential. The combination reduces backpressure, improves spool, and delivers a broad powerband that transforms the driving experience. For any Mitsubishi Lancer Evolution X owner seeking a proven, reliable power increase, this modification package is a well-considered starting point. As with any performance upgrade, proper tuning and careful installation are essential to maximize results and ensure long-term engine health.