Introduction: The LS-Swapped RX-8 Beyond the Engine Swap

The Mazda RX-8, with its low curb weight, near 50/50 weight distribution, and stiff chassis, is a natural candidate for a high-performance engine transplant. While the factory Renesis rotary engine offers a unique character, the reliability and torque of a GM LS series V8 have made LS swaps one of the most popular conversions in the platform's history. However, dropping an LS3, LS2, or LQ9 into the RX-8’s engine bay is only half the battle. The success of the build hinges on the supporting modifications that allow the engine to breathe, stay cool, and put power to the ground without breaking components. This article focuses on the three most critical categories of supporting mods: cooling, exhaust, and drivetrain. Neglecting these areas results in a car that is either unreliable, down on power, or simply unsafe to drive.

Cooling System Upgrades: Managing LS Heat in a Tight Bay

The RX-8 engine bay was designed around a compact rotary engine that produces less overall thermal energy than a pushrod V8. An LS engine, particularly when modified, generates significant heat that must be managed to prevent overheating, detonation, and premature component failure. A stock RX-8 cooling system is wholly inadequate for this task. The following upgrades are essential for maintaining consistent operating temperatures under hard driving.

Radiator Core Size and Core Material

The most impactful cooling upgrade is a larger, more efficient radiator. A standard copper or stock replacement aluminum radiator simply does not have the surface area or heat rejection capacity required by an LS engine in a tight RX-8 engine bay. Look for a dual-pass or triple-pass aluminum radiator specifically designed for LS swapped RX-8s. Many aftermarket vendors offer radiators that are wider and thicker than the factory unit while still fitting between the chassis rails. A core thickness of 2 to 3 inches is common, but ensure the radiator is paired with proper ducting to direct airflow through the core rather than around it. Sikky and Hinson Motorsports offer bolt-in radiator packages designed specifically for LS swapped RX-8 chassis.

Electric Fan Systems and Shroud Integration

Mechanical fans are rarely used in LS swapped RX-8s due to clearance issues and parasitic losses. High-output electric fans are the standard solution, but they must be controlled properly. A contour fan unit or a pair of high-CFM Spal fans combined with a custom aluminum shroud provides the most effective airflow. The shroud is critical: it prevents air from recirculating around the fan edges and draws air uniformly through the entire radiator core. Use a variable speed controller or a programmable thermostatic switch to cycle the fans based on coolant temperature. Set the primary fan to activate around 185-195°F and a secondary fan to activate around 200-210°F. Wiring the fans to a manual override switch is a good practice for track use.

Thermostat Selection and Coolant Flow Path

A factory LS thermostat is typically rated at 195-197°F, which is fine for a stock application but may be too hot for a high-performance swap in a tight engine bay. A 160°F or 180°F high-flow thermostat is a common upgrade. The lower temperature rating helps keep peak coolant temperatures in check, especially during sustained high-load operation. Ensure the thermostat housing has a bleed port to purge air from the system, as trapped air is a leading cause of hot spots and overheating in swapped cars. Use a water pump with a billet impeller (such as a Stewart or high-flow mechanical pump) to increase coolant flow rate at low RPM.

Expansion Tank and Silicone Hoses

A properly sized coolant expansion tank (also called a recovery tank or overflow tank) is often overlooked but is critical for system stability. The tank must be mounted higher than the highest point in the cooling system to allow air to purge naturally. Choose a tank with a pressure cap rated at 16-18 PSI to raise the boiling point of the coolant. Replace all factory rubber hoses with silicone hoses rated for higher temperatures and pressures. Silicone hoses resist swelling and cracking better than rubber, especially when exposed to the higher underhood temperatures of an LS engine. Heater hose routing should also be considered; many LS swaps simplify the system by capping the heater core ports, but if retaining heat, use a high-quality Y-connector to integrate the heater core into the main coolant circuit.

Oil Cooling Considerations

While coolant handles engine jacket temperatures, the LS engine's oil system also requires attention. The stock oil cooler (if equipped) on some LS engines is adequate for street use, but a larger, air-to-oil cooler with a thermostatic sandwich plate is recommended for track or high-boost applications. The RX-8’s front bumper area is tight, but a 16-row or 19-row oil cooler can be mounted in front of the radiator or behind the lower bumper opening with proper ducting. Use -10 AN lines for oil cooler routing to minimize pressure drop.

Exhaust System Design: Flow, Sound, and Clearance

The exhaust system is responsible for evacuating spent gases from the combustion chambers and plays a direct role in power output, sound quality, and chassis clearance. A poorly designed exhaust system can choke the LS engine by 20-30 horsepower and create clearance issues that lead to rattling or heat damage. Every LS swapped RX-8 needs a custom exhaust system, but the choices made in component selection determine the final result.

Header Selection: Long-Tube vs. Shorty

Long-tube headers are the gold standard for LS engine performance in a swap. They provide superior scavenging at mid-range and high RPM, which directly translates to more torque and horsepower. However, long-tube headers require more space. In an RX-8, clearance around the steering shaft, frame rails, and subframe is tight. Many swap-specific headers from Sikky, Hinson, or V8 Roadsters are designed with custom bends and flange angles to clear these obstacles. Shorty headers are easier to install and may fit without modifying the firewall or steering components, but they typically sacrifice 10-15 HP compared to a good long-tube design. For a street-driven car, a quality 1-3/4 inch primary tube long-tube header with 3-inch collectors is a strong starting point. Coat the headers with a high-temperature ceramic coating (inside and out) to reduce underhood temperatures and prevent thermal fatigue.

Mid-Pipe and Catalytic Converter Strategy

From the header collectors, the exhaust merges into a mid-pipe. An X-pipe or H-pipe crossover should be placed after the collectors to balance exhaust pulses and improve low-end torque. X-pipes generally offer slightly better high-RPM performance, while H-pipes produce a deeper exhaust note. For street legality, high-flow catalytic converters from MagnaFlow or G-Sport are necessary. Use a 3-inch diameter system throughout for naturally aspirated engines making up to 500 HP. For boosted applications or larger displacement engines, 3.5-inch or 4-inch piping may be required. Ensure the mid-pipe is routed carefully around the RX-8’s transmission tunnel and fuel tank. The tunnel gets very hot; using DEI heat wrap or a heat shield on the tunnel is recommended.

Cat-Back Exhaust and Muffler Selection

The cat-back section affects both power and sound. A straight-through, chambered muffler design offers the best flow and a deep, aggressive note without being overly raspy. Avoid cheap glass-pack mufflers that blow out or drone excessively. A 3-inch cat-back exhaust with a high-quality muffler like a MagnaFlow 14-inch oval or a Borla Pro XS provides excellent flow and a controlled sound. Consider a single exhaust exit versus dual exits. The RX-8 chassis has cutouts for dual exhaust tips, but a single 4-inch tip on one side is simpler and often sounds cleaner. If retaining dual tips, ensure the Y-pipe split is optimized to avoid turbulence.

Heat Management and Clearance

The exhaust system runs very close to the RX-8’s aluminum driveshaft, fuel lines, and floor pan. Use header wrap on the primary tubes and a DEI titanium exhaust wrap on the mid-pipe and cat-back sections near sensitive components. This reduces radiant heat, lowers interior cabin temperatures, and prevents fuel vaporization issues. Check clearance at full suspension droop and under load, as the exhaust can shift and contact the chassis under hard cornering.

Drivetrain and Powertrain Mounting: Handling the Torque

The RX-8’s factory drivetrain was designed for a 200 HP rotary engine. When you install an LS engine making 400+ HP and 400+ lb-ft of torque, the transmission, driveshaft, differential, axles, and clutch all become potential failure points. A comprehensive drivetrain upgrade is non-negotiable for any LS swapped RX-8 that will be driven hard.

Transmission Selection and Mounting

The most popular transmission choices for an LS swapped RX-8 are the T56 6-speed manual, the Tremec TR-6060, and the CD009 (from the Nissan 350Z/370Z). Each has its own set of considerations. The T56 is the most common choice due to its availability and aftermarket support. It handles up to 600 lb-ft of torque in stock form and offers a wide gear ratio spread. The CD009 is growing in popularity due to its lower cost and compact size, but it requires a conversion bell housing and adapter plate. Regardless of which transmission is chosen, a heavy-duty clutch with a sprung hub is essential. A single-disc clutch from McLeod, ACT, or Centerforce rated for at least 600 lb-ft of torque is a baseline for street/strip use. For track use, a dual-disc clutch provides better heat capacity and pedal feel. The transmission must be mounted securely to the RX-8’s tunnel using a custom crossmember or a pre-fabricated mount kit from a swap vendor. Use polyurethane or solid transmission mounts to minimize driveline movement during hard shifts.

Clutch Actuation and Hydraulics

The RX-8’s hydraulic clutch system must be adapted to the LS engine’s slave cylinder. Many swaps use a hydraulic throwout bearing from McLeod or Tilton, which eliminates the need for a bulky slave cylinder fork. This simplifies installation and improves clutch pedal feel. The master cylinder bore size must match the slave cylinder to achieve proper pedal travel and engagement. A 7/8-inch or 1-inch bore master cylinder is typically used with a T56 hydraulic throwout bearing. Bleeding the system is critical; use a vacuum bleeder or a pressure bleeder to remove all air.

Driveshaft and Axle Upgrades

A stock RX-8 driveshaft will not handle LS torque. The shaft is too thin and the U-joints are too weak. Have a custom one-piece aluminum or steel driveshaft made with 1330 or 1350 series U-joints. A 3-inch diameter driveshaft with a wall thickness of 0.083 inches is a good starting point for a street car. Ensure the driveshaft is balanced to prevent vibrations at highway speeds. The rear axles must also be upgraded. The RX-8’s factory half shafts are prone to failure under high torque. Upgraded axles with thicker shafts and stronger CV joints from DSS (Drive Shaft Shop) or aftermarket sources are essential. For cars exceeding 500 HP, consider a 8.8-inch Ford rear axle swap from a Mustang or Explorer, which provides a larger pinion bearing and more gear ratio options. This is a significant metalwork project but is the ultimate solution for reliability.

Differential and Final Drive Ratio

The stock RX-8 differential is a 4.44:1 ratio. This ratio is too steep for an LS engine, which has plenty of low-end torque. The engine will rev very high on the highway, generating excessive heat and noise. A 3.55:1 or 3.73:1 final drive ratio is much more appropriate for an LS swapped RX-8. This allows the engine to cruise at lower RPM, reduces driveline stress, and improves fuel economy. The differential must also be a limited-slip differential (LSD). The RX-8’s stock LSD is weak and not designed for the torque of an LS. A Tom's Stage 2 or OS Giken clutch-type LSD or a Quaife ATB heliсal limited-slip differential are both excellent upgrades. These differentials provide predictable locking behavior and can handle 500+ HP without failure. The differential housing should be upgraded to a PPF (Power Plant Frame) delete kit that uses solid or polyurethane bushings to eliminate driveline slop.

Engine and Transmission Mounts

Proper engine mounts are the foundation of a successful swap. Factory rubber mounts will compress and allow the engine to shift under load, causing alignment issues with the cooling fan, exhaust, and driveshaft. Use solid or polyurethane engine mounts from a reputable swap vendor. These mounts are CNC-machined to position the engine in the correct location relative to the subframe and firewall. The transmission mount should also be upgraded to a polyurethane or solid mount. This reduces driveline movement, prevents the exhaust from contacting the chassis, and improves throttle response by reducing engine rock.

Supporting Systems and Final Considerations

Fuel System Integration

The RX-8’s factory fuel pump is mounted in the fuel tank and is not designed for the higher flow demands of an LS engine. A fuel pump upgrade is essential. An in-tank pump from AEM, Walbro 525/535, or a DeatschWerks DW400 is a direct fit with minor wiring modifications. For naturally aspirated LS builds up to 500 HP, a single 340 LPH pump is typically sufficient. For forced induction or larger builds, consider a dual pump setup or a surge tank with an external pump. The fuel lines should be upgraded to -6 AN or -8 AN PTFE-lined hose to handle the higher pressure and flow. Install a fuel pressure regulator and a pressure gauge at the fuel rail to ensure consistent fuel delivery.

Wiring and Electronics

The RX-8’s factory ECU and wiring harness are not compatible with an LS engine. A standalone ECU system such as a Holley Terminator X Max or a GM Gen 4 ECM with a custom harness is necessary. These systems control the engine, transmission (if using a 4L60E or 4L80E automatic), cooling fans, and gauges. The wiring harness must be routed carefully to avoid interference with moving parts and heat sources. Use DEI loom wrap and heat shrink to protect the wiring. Consider upgrading the alternator to a higher-output unit (140-160 amps) to power the electric fans, fuel pump, and aftermarket electronics.

Suspension and Brakes

An LS swap adds significant weight to the front of the RX-8. The factory suspension and brakes may be overwhelmed. Upgrade the front and rear sway bars to stiffer units (such as Racing Beat or Progress) to reduce body roll. Install stiffer springs and adjustable dampers to control the added weight. A coilover system from BC Racing, KW, or Ohlins allows for corner weighting and ride height adjustment. The brakes should be upgraded to a Big Brake Kit (BBK) with 4- or 6-piston calipers and larger rotors. The stock RX-8 brakes are marginal for a 3000-lb car with 400+ HP. A StopTech or Wilwood BBK with 14-inch rotors provides the stopping power needed.

Conclusion: Building a Complete Package

An RX-8 LS swap is a rewarding project that transforms a lightweight, balanced chassis into a reliable, high-performance machine. However, the engine swap itself is just the starting point. The cooling system must be completely redesigned with a larger radiator, high-output fans, and proper ducting to manage the heat of a V8. The exhaust system must be carefully chosen and routed to maximize power while maintaining clearance and controlling sound. The drivetrain, including the transmission, clutch, driveshaft, differential, axles, and mounts, must be upgraded to handle the torque without failure.

By investing in these supporting modifications, you not only ensure the car is reliable and safe but also unlock the full potential of the LS engine. A properly executed RX-8 LS swap with excellent cooling, free-flowing exhaust, and a bulletproof drivetrain is one of the most engaging and capable sports cars you can build. Take the time to research each component, source quality parts from reputable vendors, and do the installation correctly the first time. The result will be a car that performs far beyond anything Mazda originally imagined. For more detailed build guides and component recommendations, consult resources from the RX-8 Club forum and LSX Magazine.