Turbo Downpipe Guide Tuning Tips for Maximum Performance on the Nissan Gt-r R35

Introduction: Unleashing the Full Potential of Your R35 GT-R

The Nissan GT-R R35, often called "Godzilla," has dominated the performance car world since its debut. Its twin-turbocharged VR38DETT engine is a masterpiece of engineering, but like any production car, it comes with compromises to meet emissions, noise, and cost targets. One of the most impactful upgrades you can make to unleash its true power is replacing the factory turbo downpipes. A high-quality aftermarket downpipe reduces exhaust restriction, improves spool, and lays the foundation for significant horsepower gains. This expanded guide covers everything from understanding downpipe design to choosing the right part, installation best practices, and tuning for maximum safe performance.

Understanding Turbo Downpipes: The Gatekeepers of Exhaust Flow

The turbo downpipe is the section of exhaust that bolts directly to the turbine outlet of each turbocharger. In a twin-turbo layout like the R35, there are two downpipes – one for the passenger-side turbo and one for the driver-side turbo. They route exhaust gasses from the turbos to the main exhaust system, which then passes through the catalytic converters and mufflers.

The factory downpipes are heavily restricted. They incorporate a close-coupled catalytic converter designed to reduce cold-start emissions, but this creates significant backpressure. By replacing them with a less restrictive design, you allow exhaust gasses to exit the turbine more freely. This reduces exhaust manifold pressure before the turbine, allowing the turbocharger to spin with less resistance. The result is faster spool, higher peak boost potential, and a measurable increase in horsepower and torque throughout the rev range.

Benefits of Upgrading Your Downpipe: What the Numbers Say

Installing aftermarket downpipes on a GT-R R35 delivers results that are both dramatic and predictable. Independent dyno testing consistently shows peak gains of 30–50 horsepower on a tuned car, with torque increases of 40–60 lb-ft. The benefits go beyond peak numbers:

• Improved horsepower and torque – Especially in the mid-range where the factory downpipes choke flow.
• Enhanced throttle response – Reduced spool time means the turbos hit boost earlier.
• Better turbo spool time – The turbos can reach target boost 500–800 RPM sooner.
• Increased exhaust flow – Lowers the overall thermal load on engine components.
• Potential for higher boost levels – Less restriction means lower pre-turbine pressure, allowing a tune to push more boost safely.

These gains are even more pronounced when combined with an intake upgrade (such as upgraded intakes or a full CAI system) and a set of high-flow catalytic converters or a catless setup.

Choosing the Right Downpipe: Key Considerations for the R35

Not all downpipes are built the same. The R35 community has strong opinions on materials, sizing, and catalytic converter options. Here’s what you need to evaluate:

Material and Construction

Most performance downpipes are made from stainless steel (304 or 321 grade) for corrosion resistance and heat tolerance. Some budget options use aluminized steel, but stainless is strongly preferred for longevity. Look for mandrel-bent tubing to avoid kinks that create turbulence. The flange where the downpipe meets the turbo outlet should be CNC-machined flat to prevent exhaust leaks.

Catted vs. Catless

This is the biggest decision. Factory downpipes include a catalytic converter in each pipe. Aftermarket options offer:

• High-flow catted downpipes – Use a metallic substrate with fewer cells per inch (e.g., 200–300 CPSI) to improve flow while still passing emissions testing in many regions. Expect 20–30 whp gain.
• Catless downpipes – No catalytic converter at all. Maximum flow, highest gains (30–50 whp), but will trigger a check engine light (CEL) for catalyst efficiency unless tuned out. Not road-legal in many areas. Expect a louder, more aggressive exhaust note.

If you plan to tune the car and live in an area with emissions testing, high-flow catted downpipes are the practical choice. If you are building a track-only car, catless is the way to go.

Design and Fitment

Choose a downpipe design that minimizes restrictions. Look for a smooth, gradual transition from the oval turbo outlet to a round pipe. The best designs feature a smooth merge collector where the two downpipes join into the mid-pipe, and they maintain a full 3-inch inner diameter throughout. Avoid downpipes with sharp bends or crushed sections. Check that the downpipe includes a bung for the factory oxygen sensor, and ensure it is positioned correctly to avoid interference with the transmission, subframe, or chassis.

Heat Management

Aftermarket downpipes are often coated or wrapped to reduce under-hood temperatures. A ceramic coating (e.g., Jet-Hot) or a high-quality exhaust wrap helps keep heat inside the pipe, improving exhaust velocity and reducing the temperature of surrounding components like the intake tract and the engine block. This is especially important on the R35, which is sensitive to intake air temperatures.

Installation Tips: Getting It Right the First Time

Installing downpipes on the R35 is not a beginner-level job. It requires lifting the car, working under the vehicle, and dealing with tight spaces, especially around the passenger-side turbo. Here are essential steps and precautions:

• Work on a fully cold engine – The turbos and exhaust manifold retain heat for hours. Let the car sit overnight.
• Use a lift or sturdy jack stands – You need clearance to access the undercarriage. Never rely on a hydraulic jack alone.
• Soak all fasteners with penetrating oil – The factory exhaust nuts are notorious for seizing on the studs. Apply PB Blaster or similar several hours before starting, and again 15 minutes before turning wrenches.
• Replace all gaskets and lock nuts – Use new metal-with-graphite gaskets at the turbo-to-downpipe flange and new OEM-style nuts. Torque to 35–40 ft-lb.
• Disconnect the battery – You will be near oxygen sensor wiring; avoid short circuits.
• Remove necessary heat shields – The factory heat shields around the turbos must come off for access. Reinstall them if possible, or consider replacing with aftermarket heat shielding.
• Carefully route the oxygen sensor harness – The downstream O2 sensors must be reconnected without being pinched or stretched. On catless setups, you may need an O2 sensor spacer (defouler) to prevent CEL.
• Check all clearances – After bolting everything up, rotate the engine by hand (if possible) and ensure the downpipes don’t contact the transmission bellhousing, driveshaft, or subframe. Adjust if necessary.
• Test for leaks – Start the engine and use a soapy water spray at every joint. Tiny exhaust leaks can lean out the mixture and cause misfires under load.

Tuning After Installation: The Critical Next Step

Installing downpipes without recalibrating the engine electronics leaves performance on the table and risks running lean. The factory ECU is programmed to hit specific air-fuel ratios and boost targets. A less restrictive exhaust changes the mass air flow readings and alters the turbo’s behavior. Here’s what tuning after downpipe installation involves:

ECU Recalibration (Flash Tuning)

Your tuner will adjust the fuel maps, ignition timing, and boost control parameters. The table below shows typical changes:

Parameter	Pre-Tune	Post-Tune (Downpipes)
Target Boost (psi)	14–15	18–20 (with supporting mods)
Air-Fuel Ratio (WOT)	11.5:1	11.2–11.5:1 (richer for safety)
Ignition Timing	Conservative	Advanced 2–3° under load
Throttle Mapping	Linear	Faster tip-in

A good tune from a reputable shop like Cobb Tuning or ECUtek will deliver 30–50 whp over the base downpipe-only gain.

Wideband O2 Sensor Monitoring

Use a wideband oxygen sensor to get real-time AFR readings. The factory narrowband sensors are not accurate for tuning. Many tuners recommend installing a dedicated wideband bung in the downpipe or mid-pipe.

Boost Control Adjustments

With less backpressure, the wastegates may take slightly longer to open. Your tuner will adjust the boost control solenoid duty cycle to prevent boost spikes. On modified R35s, electronic boost controllers like the Cobb Accessport can be programmed for precise boost curves.

Safety Limits and Knock Detection

Higher boost and advanced timing increase the risk of detonation. A proper tune will include knock sensor monitoring and fuel trim adjustments. Always use high-octane fuel (91 or 93 RON minimum, or E85 for maximum gains).

Common Issues and Troubleshooting

After installing downpipes, some owners encounter problems. Here’s how to address them:

Check Engine Light (Code P0420/P0430)

Catless downpipes trigger a catalyst efficiency code. Solutions:

• Tune out the code – The cleanest fix, done by your tuner.
• O2 sensor spacers – Also called defoulers; they move the sensor out of the direct exhaust stream. Works for mild roughness but not always reliable.
• High-flow cats – Prevent the CEL altogether while still providing flow.

Exhaust Leaks

A leak at the turbo flange or joint can cause a whistling sound, loss of boost, and false lean AFRs. Re-torque all fasteners when hot (engine at operating temperature). If a gasket fails, replace immediately.

Increased Exhaust Drone

The R35’s cabin already has some resonance. Catless downpipes plus an aftermarket exhaust can create a drone at 1800–2200 rpm. Solutions include a resonated mid-pipe or sound-deadening material in the trunk area.

Heat Soak

Downpipes radiate more heat due to less shielding. Consider wrapping the pipes or adding turbo blankets. Monitor intake air temperatures (IATs) on your logging tool.

Maximizing Gains: Supporting Modifications

To get the most out of your new downpipes, consider these complementary upgrades:

• Intake system – Upgraded intakes (e.g., HKS, Stillen, AMS) reduce intake restriction and lower IATs.
• Intercooler – A larger intercooler core (upgraded or front-mount) prevents heat soak and maintains dense air charge.
• Fuel system – On high-boost setups, upgrade fuel injectors and fuel pump (e.g., DW 850cc, flex fuel sensor for E85).
• Cat-back exhaust – A 3-inch or larger cat-back completes the flow path, picking up another 5–10 whp.
• Transmission upgrade – The R35’s GR6 transmission can struggle with added torque. Consider a Dodge Performance or Shep stage upgrade if pushing over 600 whp.

Conclusion: Building a Reliable, High-Performance R35

Upgrading the turbo downpipes on your Nissan GT-R R35 is one of the most effective modifications you can make. It reduces backpressure, dramatically improves turbo response, and provides a solid foundation for higher power levels. However, the key to a successful upgrade lies in choosing the right downpipe for your goals (catted vs. catless, material, fitment), performing a meticulous installation, and following up with professional tuning. With the right approach, you can confidently add 30–50 wheel horsepower while maintaining drivability and reliability. Always monitor your engine parameters, use quality parts, and consult with experienced tuners who specialize in the R35 platform. For further reading, check out technical resources from AMS Performance and the comprehensive build guides on GT-R Life. Your Godzilla deserves nothing less than the best exhaust path possible.