Why Upgrade to a Borla Turbo Downpipe on Your Honda Civic Si

The downpipe is one of the most restrictive components in the factory exhaust system of a turbocharged vehicle like the Honda Civic Si. By replacing the stock downpipe with a high-flow Borla Turbo Downpipe, you reduce backpressure, allowing exhaust gases to exit the turbocharger more efficiently. This results in quicker spool times, increased horsepower and torque, and a noticeably more aggressive exhaust note. Borla is known for its precision engineering, high-quality T304 stainless steel construction, and robust sound tuning, making it a top choice for enthusiasts who want both performance gains and durability.

Installing a Borla downpipe is a rewarding project that can be completed in a home garage with basic mechanical skills and the right tools. This step-by-step guide provides detailed instructions, pro tips, and important safety and tuning considerations to help you get the most out of your upgrade.

Benefits of Upgrading to a Borla Turbo Downpipe

Before diving into the installation, it helps to understand exactly what you gain by swapping the factory downpipe for a Borla unit.

  • Increased Horsepower and Torque: A less restrictive downpipe allows the turbo to spool faster and more efficiently, typically yielding gains of 15–25 wheel horsepower on a stock or lightly tuned Civic Si, depending on other modifications.
  • Improved Turbo Response: Reduced backpressure means the turbo reaches boost threshold sooner, giving you stronger acceleration from lower RPMs.
  • More Aggressive Exhaust Sound: Borla downpipes are engineered to enhance the exhaust note without introducing drone. Expect a deeper, louder tone under load.
  • High-Quality Materials: Borla uses T304 stainless steel and mandrel-bent tubing for durability and long life, resisting corrosion and heat cycling better than factory components.
  • Bolts Directly to Factory Mounting Points: The Borla downpipe is designed as a direct bolt-on replacement, meaning no custom fabrication or welding is required for a proper fit.

Tools and Materials Needed

Having everything ready before you start will save time and frustration. Below is a complete list of what you need for a smooth installation.

  • Borla Turbo Downpipe – Ensure you have the correct part number for your specific Honda Civic Si model year (10th or 11th gen, depending on your vehicle).
  • Socket set – Metric sockets from 8mm to 19mm, including deep-well sockets for hard-to-reach nuts. A 3/8-inch drive set is sufficient for most fasteners, but a 1/2-inch drive may be needed for stubborn bolts.
  • Wrench set – Combination wrenches (open-end and box-end) in metric sizes, particularly 12mm, 14mm, and 17mm.
  • Ratchet extensions and universal joint – A 6-inch and 12-inch extension plus a swivel socket or universal joint will help access awkwardly positioned bolts.
  • Torque wrench – Essential for tightening fasteners to the manufacturer’s specification, preventing both under-tightening and over-tightening.
  • Jack and jack stands – A hydraulic floor jack and two sturdy jack stands rated for at least 3 tons. Never rely on a jack alone to support the vehicle.
  • Safety goggles and gloves – Protect your eyes from debris and your hands from sharp edges and hot surfaces.
  • Penetrating oil – A product like WD-40 Specialist or PB Blaster will loosen rusted or seized bolts on the factory downpipe.
  • Exhaust gasket (if needed) – Borla downpipes often include new gaskets, but it is wise to have a spare turbo-to-downpipe gasket on hand, as the factory gasket may not seal properly when reused.
  • Shop rags and a drip pan – Expect some oil or coolant residue near the turbo area; rags and a pan keep your workspace clean.
  • Breaker bar – A 1/2-inch drive breaker bar (18–24 inches) can help break loose stubborn bolts without damaging your ratchet.
  • Anti-seize compound – Apply to downpipe bolts during reinstallation to prevent future corrosion and make removal easier.

Preparation Steps

Proper preparation sets the stage for a successful installation. Rushing this phase can lead to mistakes or safety hazards.

Park your Honda Civic Si on a level, solid surface such as a concrete garage floor. Allow the engine and exhaust system to cool completely before working underneath the vehicle. A hot exhaust can cause severe burns and is more difficult to work with due to thermal expansion.

Disconnect the negative terminal of the battery to prevent any accidental electrical shorts while working near sensors or wiring. This is especially important because the downpipe is close to the oxygen sensor connectors.

Gather all tools and lay them out within reach. Open the Borla downpipe box and verify that all components are present, including gaskets, bolts, and any hardware specific to your kit.

Apply penetrating oil to all bolts you will remove, especially the turbo-to-downpipe flange nuts and the downpipe-to-exhaust connection. Let the oil soak for at least 10–15 minutes to break up rust and corrosion.

Step 1: Lift the Vehicle and Secure on Jack Stands

Position the hydraulic jack under the front center jacking point (located behind the front bumper, near the oil pan). Raise the front of the vehicle until the wheels are several inches off the ground. Place a jack stand under each front pinch weld or designated lift point, ensuring both stands are at the same height and that the vehicle is stable. Gently lower the vehicle onto the stands and verify it does not wobble. Give the car a gentle push to confirm stability before crawling underneath.

Never work under a vehicle supported only by a jack. This is a critical safety rule. Jack stands are mandatory.

Step 2: Remove the Stock Downpipe

Locate the factory downpipe. It connects the turbocharger outlet to the rest of the exhaust system, typically via a flange near the bottom of the engine bay. On the Civic Si, the downpipe is tucked between the engine block and the firewall, so you may need to work from both above and below.

2.1 Disconnect the Oxygen Sensors

The downpipe houses one or two oxygen sensors (the upstream wideband sensor and, on some models, a downstream narrowband sensor). Carefully unplug each sensor connector from the wiring harness. Use an oxygen sensor socket or a 22mm wrench to unscrew each sensor from the downpipe. Set them aside in a clean location, as they will be reinstalled into the Borla downpipe.

2.2 Remove the Turbo-to-Downpipe Bolts

Using a socket and extension, access the bolts or nuts securing the downpipe flange to the turbocharger outlet. These are often 12mm or 14mm fasteners. Spray them with penetrating oil if you have not already. A breaker bar may be needed to break them loose if they are seized. Remove all fasteners and set them aside. You may need a universal joint to reach the rear bolts that face the firewall.

2.3 Detach the Downpipe from the Exhaust System

Work underneath the vehicle to locate the connection between the downpipe and the mid-pipe or front pipe. Loosen and remove the bolts at this flange. Some vehicles use a spring-loaded joint with two bolts; others use a standard flange. Carefully separate the downpipe from the exhaust system. You may need to wiggle or tap the pipe with a rubber mallet to break it free if it is stuck.

2.4 Remove the Downpipe from the Vehicle

Once all fasteners are removed, slide the downpipe out from underneath the car. On the Civic Si, this usually means lowering it toward the ground, angling it past the subframe and steering components. Work slowly to avoid damaging surrounding parts. If the downpipe is stuck, check for any missed bolts or brackets.

Step 3: Prepare for Borla Downpipe Installation

Before installing the new downpipe, clean the mating surfaces thoroughly. Use a wire brush or a scraper to remove any old gasket material, carbon deposits, or debris from the turbocharger outlet flange. Wipe the area clean with a shop rag. A clean surface ensures a proper seal and prevents exhaust leaks.

Apply a thin layer of anti-seize compound to the threads of the turbo-to-downpipe bolts and the oxygen sensor threads. This will make future removal much easier.

If your Borla kit includes new gaskets, position them on the turbo flange and the exhaust flange now. Some kits use a metal ring gasket that sits inside the flange; ensure it is seated evenly.

Step 4: Install the Borla Turbo Downpipe

4.1 Position the Downpipe

Lift the Borla downpipe into place from underneath the vehicle. Align the flange at the top of the downpipe with the turbocharger outlet. Make sure the gasket stays in position as you raise the pipe. The downpipe should seat flush against the turbo flange without any force. If it does not align, check that the pipe is not hitting any suspension or chassis components and adjust the angle as needed.

4.2 Secure the Turbo-to-Downpipe Connection

Hand-thread the bolts or nuts into the turbo flange. Tighten them in a crisscross pattern to ensure even pressure on the gasket. Once hand-tight, use a torque wrench to tighten each fastener to the manufacturer’s specification. For most Honda Civic Si applications, this is around 30–40 foot-pounds, but always verify the spec in your Borla instructions. Do not overtighten, as this can strip threads or warp the flange.

4.3 Connect the Downpipe to the Exhaust System

Slide the lower end of the downpipe into the mid-pipe or front pipe flange. Align the bolt holes and insert the bolts. If the kit uses spring-loaded hardware, compress the springs and install the nuts. Tighten to the specified torque. Ensure the connection is snug but not overly tight, as some systems require a slight amount of flex for vibration damping.

4.4 Reinstall the Oxygen Sensors

Thread each oxygen sensor back into the bungs on the Borla downpipe. Apply anti-seize to the threads, but avoid getting any on the sensor tip. Tighten each sensor to about 30 foot-pounds using the oxygen sensor socket. Reconnect the sensor wiring harnesses, ensuring the clips lock securely.

Step 5: Check for Exhaust Leaks

With all mechanical connections tightened and sensors reconnected, it is time to test your work. Reconnect the negative battery terminal. Start the engine and let it idle. Listen carefully for any hissing, ticking, or whooshing sounds that indicate an exhaust leak. Common leak points are the turbo flange, the exhaust connection flange, and around the oxygen sensor bungs.

If you hear a leak, shut off the engine, allow it to cool, and tighten the offending bolts slightly. For minor leaks, a small adjustment may suffice. For persistent leaks, you may need to replace the gasket or inspect the flanges for damage.

Check for any rattles or contact between the downpipe and the vehicle’s frame, subframe, or heat shields. If the pipe is touching metal, you may need to adjust its position or add a heat shield spacer.

Step 6: Lower the Vehicle and Perform Final Checks

Once you confirm there are no exhaust leaks and no contact issues, carefully raise the vehicle using the jack, remove the jack stands, and lower the car to the ground. Tighten the lug nuts on the front wheels to the manufacturer’s specification if you removed them.

Double-check all bolts from above and below, ensuring everything is secure. Recheck the torque on the turbo flange bolts, as thermal cycling can sometimes loosen them slightly after the first heat cycle. It is a good practice to re-torque them after a short drive once the system has gone through one heat and cool cycle.

Take the vehicle for a short test drive (5–10 minutes). Listen for leaks, rattles, or unusual noises. Monitor boost pressure (if you have a gauge) to ensure the turbo spools normally. A properly installed Borla downpipe will deliver smooth, responsive power with a deeper exhaust note.

ECU Tuning Considerations

Installing a high-flow downpipe on a turbocharged engine changes the exhaust flow characteristics, which in turn affects air-fuel ratios and boost control. While the Honda Civic Si’s ECU can compensate to some degree, a downpipe upgrade benefits greatly from a custom tune or a quality off-the-shelf (OTS) calibration.

Why you should consider tuning: The factory ECU is calibrated for the stock exhaust restriction. A freer-flowing downpipe can cause the engine to run leaner in certain conditions, potentially leading to knock or reduced performance. A tune optimizes fuel and timing maps to take full advantage of the increased flow, yielding safer and more consistent power gains.

Many aftermarket tuners offer pre-loaded maps for downpipe upgrades. Flash tuning tools like Hondata or KTuner are popular choices for the Civic Si. If you are not comfortable tuning yourself, consult a professional dyno tuner who specializes in Honda vehicles.

For more information on tuning options, check out Hondata’s official site for Civic Si tuning solutions.

Post-Installation Break-In and Driving Tips

Your new Borla downpipe will perform best after a short break-in period. During the first 100–200 miles, avoid sustained full-throttle runs and high RPMs. This allows the gaskets to seat properly and the components to settle into their final positions.

Check the downpipe flange bolts for tightness after the first heat cycle. It is common for bolts to loosen slightly as the metal expands and contracts. Re-torque them to spec for peace of mind.

Be aware that a high-flow downpipe will increase exhaust volume, especially under hard acceleration. This may be noticeable to passengers and pedestrians. Ensure your vehicle complies with local noise ordinances. Some areas have strict regulations about exhaust modifications, including emissions compliance. A high-flow downpipe may trigger a check engine light due to reduced backpressure altering the oxygen sensor readings, particularly if you do not have a tune that disables or adjusts the sensor monitoring.

Maintenance Tips for Longevity

Borla uses T304 stainless steel, which is highly resistant to corrosion. However, the downpipe is exposed to extreme heat and road debris. Periodically inspect the pipe for dents, cracks, or rust spots, especially if you drive in areas where roads are salted in winter. Clean the exterior with a mild soap and water solution; avoid abrasive cleaners that can scratch the finish.

Re-torque the flange bolts once a year to maintain a leak-free seal. If you remove the downpipe for any reason, replace the gaskets to ensure a proper seal.

For a deeper understanding of exhaust system principles and performance upgrades, the Borla official website offers technical resources and product specifications. Additionally, enthusiast communities such as CivicX Forum provide real-world installation tips and shared experiences from other Civic Si owners who have performed this exact upgrade.

Conclusion

Installing a Borla Turbo Downpipe on your Honda Civic Si is one of the most effective and rewarding upgrades you can make. The installation process is straightforward for a DIY mechanic with basic tools, and the performance benefits are immediate and noticeable. Reduced turbo lag, increased horsepower, and a more aggressive exhaust note transform the driving experience without compromising daily drivability.

By following this guide, you can complete the installation with confidence. Remember to prioritize safety, double-check all connections, and consider ECU tuning to fully unlock the potential of your new downpipe. Whether you are building a track-day weapon or simply enhancing your daily driver, the Borla downpipe delivers the quality and performance that Honda enthusiasts have trusted for decades.

Drive responsibly, respect local regulations, and enjoy the sound and performance of your upgraded Civic Si.