Introduction to the HKS Legamax Premium Turbo-Back Exhaust for the Nissan 370Z

The HKS Legamax Premium series stands as a benchmark in aftermarket exhaust engineering, offering a blend of weight reduction, increased flow capacity, and a refined exhaust note. For Nissan 370Z owners seeking to unlock additional horsepower and torque while maintaining a civilised cabin experience, this turbo-back system provides a direct path to improved performance. The system replaces the restrictive factory exhaust from the turbocharger outlet all the way to the tailpipe, reducing backpressure and allowing the engine to breathe more freely. This guide presents best practices for installing the HKS Legamax Premium turbo-back exhaust on your 370Z, covering preparation, detailed steps, torque specifications, common pitfalls, and post-installation care.

Preparation and Workspace Setup

Gather All Necessary Tools and Equipment

Before beginning the installation, ensure every tool is within reach. A typical install requires a metric socket set (10mm, 12mm, 14mm, 17mm), a ratchet with long and short extensions, combination wrenches (12mm, 14mm, 17mm), a torque wrench capable of reading up to 80 ft-lb, a breaker bar, penetrating oil (such as PB Blaster), a flathead screwdriver or trim tool, a jack with jack stands or a lift, safety goggles, and mechanic gloves. Having a cutting tool (reciprocating saw or angle grinder) on standby may be necessary if bolts are corroded or if the stock exhaust needs to be cut for removal. A torque wrench is especially important for proper flange bolt tightening to avoid leaks or broken studs.

Inspect the HKS Legamax System and Vehicle Compatibility

Unbox the new HKS Legamax Premium turbo-back exhaust and lay out all components. Verify that the system matches your Nissan 370Z model year (typically 2009-2020). The kit should include the downpipe, front pipe, mid-pipe, rear section, gaskets, bolts, and springs. Check for any shipping damage, particularly on the polished stainless steel tips and the flex pipes. Also confirm that you have the required replacement gaskets; reusing old gaskets is not recommended because compression sealing surfaces degrade over time. The HKS system uses high-quality multi-layer steel (MLS) gaskets that should be replaced with every exhaust removal.

Prepare the Workspace and Vehicle

Work on a level surface in a well-ventilated area. Raise the rear of the vehicle with a jack and secure it on jack stands placed under the reinforced frame rails. For added stability, also lift the front slightly if you need access to downpipe bolts. Allow the engine and exhaust to cool completely – hot exhaust components can cause severe burns and may warp flange surfaces if loosened while hot. Apply penetrating oil to all exhaust flange nuts and studs 30 minutes before starting, especially on the downpipe connection to the turbo and the mid-pipe flange.

Removing the Stock Exhaust System

Remove the Rear Section and Mid-Pipe

Begin at the rear of the vehicle. Using a 12mm or 14mm socket, remove the bolts connecting the stock muffler to the mid-pipe. Support the muffler with a jack or have an assistant hold it to prevent stress on the hangers. Slide the rubber hangers off the metal posts – use a flathead screwdriver or hanger removal tool to avoid tearing the rubber. Lower the stock muffler and set it aside. Next, loosen the mid-pipe bolts at the front and rear flanges. The stock mid-pipe often has a bracket that bolts to the transmission; remove that bracket. Carefully maneuver the mid-pipe out from under the vehicle. If the exhaust is seized, use a reciprocating saw to cut the pipe near the flange and remove pieces separately.

Remove the Front Pipe and Downpipe

For the front pipe and downpipe, access may be easier from above or below. Disconnect the oxygen sensors (O2 sensors) using an O2 sensor socket or a 22mm wrench. Be gentle with the wiring harness clips to avoid breaking them. Remove the bolts securing the downpipe to the turbine housing – these are often 14mm and can be extremely tight. A breaker bar may be necessary. On Nissan 370Zs, the front pipe connects to the downpipe with a spring-loaded V-band or three-bolt flange depending on model year. Unbolt the front pipe from the downpipe and remove both pieces. Take care not to damage the turbo studs; if a stud breaks, extraction will be time-consuming. Inspect the turbo flange for any carbon buildup or debris and clean it with a brass brush.

Installing the HKS Legamax Premium Turbo-Back System

Install the Downpipe and Front Pipe

Fit the HKS downpipe onto the turbo outlet studs using new gaskets. Apply a small amount of anti-seize to the stud threads. Tighten the downpipe nuts in a cross pattern to 35 ft-lb (refer to HKS manual for exact torque). Attach the front pipe to the downpipe using the supplied bolts and springs if applicable. The springs allow for thermal expansion and reduce vibration. Torque these fasteners to the manufacturer specification, typically 30-40 ft-lb. Ensure the front pipe is oriented correctly to clear the subframe and transmission; HKS designs these pipes to fit without modification, but slight adjustments may be needed to avoid contact with the steering shaft or chassis. Use a flashlight to verify there is at least 5mm clearance around all moving components.

Reattach the Oxygen Sensors

Transfer the factory O2 sensors from the stock front pipe to the HKS front pipe. The Legamax system includes bungs in the correct locations. Clean the threads of the sensors and apply anti-seize (do not get anti-seize on the sensor tip). Torque the sensors to 30 ft-lb. Reconnect the wiring harnesses and secure them with zip ties to prevent contact with the exhaust or driveshaft.

Install the Mid-Pipe and Rear Section

With the front section in place, install the mid-pipe. The mid-pipe typically connects to the front pipe with a slip joint or flange. Use the supplied clamp and gasket. Do not fully tighten the clamps until the entire system is aligned. Slide the mid-pipe hangers into the rubber mounts. Then attach the rear section (muffler) to the mid-pipe. The HKS Legamax Premium muffler features a polished stainless tip and a compact design. Position the muffler loosely and align the tips with the cutout in the rear bumper. Adjust the hangers and slide the tip in or out to achieve perfect centering. Once satisfied, tighten all clamp bolts and flange bolts starting from the front and working backward. Torque the mid-pipe to rear section bolts to 25-30 ft-lb, and the rear section clamps to 20 ft-lb. Recheck all bolts after a brief heat cycle.

Torque Specifications and Final Fastener Check

Creating a torque reference table helps ensure consistent clamping force. Below are typical values (always verify with HKS instructions):

  1. Downpipe to turbo: 35 ft-lb
  2. Front pipe to downpipe: 30-40 ft-lb (use springs if supplied)
  3. Mid-pipe flanges: 25-30 ft-lb
  4. Rear section bolts: 25-30 ft-lb
  5. O2 sensors: 30 ft-lb
  6. Bracket bolts (transmission mount): 20 ft-lb

After tightening all fasteners, go back and double-check each one. Vibrations during the first drive can loosen connections that were not fully torqued.

Post-Installation Checks and Heat Cycling

Inspect for Exhaust Leaks

Start the engine and let it idle. Place your hand near each joint (be careful of heat) to feel for escaping gas. A soapy water spray can help identify small leaks – bubbles will form at leak points. Listen for unusual hissing or tapping sounds. If you detect a leak, shut the engine off and tighten the corresponding flange or clamp. On spring-loaded joints, ensure the springs are properly compressed and not binding.

Heat Cycle Procedure

A proper heat cycle allows the gaskets to seat and the metal to expand uniformly. Drive the car gently for 10-15 minutes, varying RPMs but avoiding full throttle. Then allow the vehicle to cool completely. After cooling, retorque all accessible exhaust bolts and clamps. This step is critical because thermal expansion and contraction can cause fasteners to loosen. Many installers skip retorquing, which leads to leaks and annoying rattles later.

Check Clearance and Driveline Interference

With the car on a lift or jack stands again, turn the steering wheel fully left and right while observing clearance to the exhaust. Listen for any contact with the subframe, steering rack, or driveshaft. Also check that the exhaust does not rest against the body under the heat shields. If there is contact, loosen the affected section, reposition, and retighten. The HKS Legamax is engineered to fit precisely, but slight chassis variations may require minor adjustment.

Performance Expectations and Sound Character

The HKS Legamax Premium turbo-back exhaust is designed to add significant horsepower and torque across the rev range. On a stock Nissan 370Z, gains of 20-30 wheel horsepower are typical, with the largest improvement coming from the downpipe and front pipe upgrade. The exhaust note deepens and becomes more aggressive under acceleration while remaining relatively quiet during cruising – a hallmark of the Legamax Premium. You can expect a clean exhaust tone without excessive drone at highway speeds. The system also reduces weight by over 10 kg compared to the stock exhaust, contributing to better handling and quicker spool-up of the turbocharger.

For those seeking even more performance, pairing this exhaust with a tune from a reputable Nissan specialist will unlock the full potential of the increased flow. A custom ECU calibration can optimize fuel and ignition timing for the lower backpressure, resulting in smoother power delivery and improved fuel efficiency under light load.

Common Issues and Troubleshooting

Exhaust Drone

While the HKS Legamax is known for minimal drone, some users report a resonant frequency around 1800-2200 RPM. This can often be mitigated by adjusting the exhaust hangers to relieve tension, or by ensuring the system is not touching the transmission tunnel. If drone persists, consider adding a helmholtz resonator or swapping to a resonated mid-pipe offered by HKS as an optional part.

Check Engine Light (CEL) – Catalyst Efficiency Code

The HKS Legamax Premium turbo-back system may delete the secondary catalytic converters if it includes a test pipe or straight front pipe. On OBDII vehicles, this can trigger a catalyst efficiency code (P0420). To avoid a CEL, retain a high-flow catalytic converter or use a spark plug defouler on the rear O2 sensor. Many 370Z owners install an O2 sensor spacer (angled mini-cat) to trick the ECU. Alternatively, have the rear O2 sensors tuned out in the ECU software.

Fitment Issues with Aftermarket Differentials or Sway Bars

On vehicles with upgraded rear sway bars or differential mounts, the mid-pipe may come into contact with the bar or the differential housing. Check clearance and, if necessary, use a polyurethane hanger that positions the pipe slightly lower. HKS provides rubber hangers that offer enough flexibility, but aftermarket adjustable hangers are available if needed.

Maintenance and Long-Term Care

Stainless steel exhausts like the HKS Legamax are durable but require occasional attention. Wash the polished tips with a non-abrasive metal polish to maintain their shine, especially in regions where road salt is used. Inspect the flex joints for cracking every 12 months. Retorque all bolts annually, particularly the downpipe-to-turbo nuts, which can back out due to thermal cycling. Applying a nickel-based anti-seize on all threads during installation will ease future removal.

If you live in an area with strict emissions testing, retain the original catalytic converters or ensure the HKS system is equipped with high-flow cats. The Legamax Premium version offered by HKS typically includes a high-flow catalyst in the downpipe, making it street-legal in many jurisdictions – but always verify local laws.

Conclusion

Installing the HKS Legamax Premium turbo-back exhaust on your Nissan 370Z is a rewarding upgrade that enhances both the driving experience and the performance envelope. By following the best practices outlined above – thorough preparation, careful removal of the stock system, correct torque specifications, heat cycling, and post-installation checks – you can achieve a leak-free, drone-free installation that delivers the full benefits of HKS engineering. Whether you are a seasoned mechanic or a dedicated enthusiast, taking the time to perform each step methodically will ensure years of trouble-free enjoyment. For more detailed product information and official specifications, visit the HKS Japan website or consult the Nissan 370Z Forum for community feedback and installation photos.