Why Swap an LS3 into an RX-8?

The Mazda RX-8 is celebrated for its near-perfect 50:50 weight distribution, nimble chassis, and high-revving Renesis rotary engine. Yet the rotary’s inherent drawbacks—low torque, poor fuel economy, high oil consumption, and fragile apex seals—drive many enthusiasts to consider a powerhouse alternative. Dropping a GM LS3 V8 into the RX-8 transforms the car from a quirky sports coupe into a 600+ horsepower beast that retains its balanced handling. The LS3, with its 6.2 liters of displacement and cast-iron durability, responds beautifully to forced induction. Pairing it with an Edelbrock supercharger yields a linear, streetable torque curve that the rotary could only dream of offering.

Beyond raw power, an LS swap brings unmatched aftermarket support. Parts are readily available, labor is well-documented, and tuning is accessible via affordable standalone ECUs. Reliability skyrockets compared to a modified rotary, and the LS3’s ample torque makes daily driving a joy. For those aiming for 600+ wheel horsepower, the LS3-Edelbrock combination is a proven, relatively straightforward path.

Key Components for a 600+ HP Build

A successful RX-8 LS swap requires more than just an engine and supercharger. Each supporting component must be chosen to handle the increased power and fit the cramped RX-8 engine bay. Below are the essential parts you’ll need.

GM LS3 Long Block

Start with a genuine GM LS3 crate engine (PN 19370495 or later equivalent) for the best foundation. This engine features rectangular-port cylinder heads, a 6.2L bore, and a forged steel crankshaft. It can handle 600+ hp with a mild cam and supercharger, but internal upgrades (forged pistons and rods) are recommended if you plan to exceed 700 hp or run high boost.

Edelbrock E-Force Supercharger Kit

Edelbrock’s E-Force system for the LS3 (PN 15421) is a roots-type supercharger that sits low on the intake manifold, allowing clearance under the RX-8’s hood. It comes with an integrated intercooler, fuel rails, and injectors (52 lb/hr). Out of the box, it adds ~150 hp to a stock LS3, putting you near 575 crank horsepower. To crack 600 wheel horsepower, you’ll need a smaller pulley, higher-flow injectors (65 lb/hr or larger), and a custom tune.

Custom Engine Mounts and K-Swap Parts

The RX-8’s subframe requires either fabricated mounts or a bolt-in kit from specialists like American Autowerke or Longacre Racing. Expect to modify the sway bar, steering rack, and oil pan for clearance. Some kits include repositioned steering shafts to avoid interference with the LS exhaust manifolds.

Transmission Options

The stock RX-8 transmission won’t survive 600 hp. You have two main choices:

  • Tremec T-56 Magnum – The gold standard for LS swaps. Handles 700 ft-lb, with a close-ratio gearset perfect for the Edelbrock supercharger’s torque. Requires an adapter bellhousing or a GM-pattern bellhousing.
  • GM 4L80E Automatic – If you want a street-friendly auto, the 4L80E is robust but heavy. Expect to spend extra on a custom crossmember and a standalone controller.

Upgraded Fuel System

A stock RX-8 fuel system is woefully inadequate. Build a dedicated system with:

  • Aeromotive Stealth 340 in-tank pump (or dual 255s) for 600+ hp.
  • AN -6 or -8 feed lines from tank to rail.
  • High-flow injectors (80 lb/hr minimum for E85, 60 lb/hr for pump gas).
  • Fuel pressure regulator set to 58 psi for return-style systems.
  • Consider a dedicated surge tank and external pump for track use.

ECU and Wiring Harness

Ditch the Mazda ECU. Use a Holley Terminator X or HP EFI for plug-and-play GMPP compatibility, or go with the factory GM ECU (E38/E67) with a reflash from PSI Conversion. A standalone harness simplifies installation and allows precise control of timing, fuel, and boost. The Edelbrock supercharger requires a MAP sensor capable of reading 2-bar or higher—plan your ECU choice accordingly.

Installation Process: Step by Step

Converting an RX-8 to LS power is a major undertaking. Below is a condensed guide; expect 80–120 hours of labor if you’re experienced, or pay a shop $6,000–$10,000 for the swap.

1. Strip the Engine Bay

Remove the rotary engine, transmission, radiator, wiring harness, and accessories. The RX-8’s steering rack, brake booster, and ABS unit may need relocation or replacement with compact aftermarket units to clear the V8’s exhaust manifolds.

2. Modify the Subframe and Mounts

Bolt in or weld the LS engine mounts. You’ll likely need to notch the passenger side frame rail for the LS’s driver-side header clearance. Most kits require trimming the oil pan to clear the steering rack (a common solution is a Holley LS swap oil pan designed for F-bodies or C5 Corvettes).

3. Install the LS3 and Transmission

Drop the engine and transmission in as a unit. Connect the clutch hydraulic system (you’ll need a custom slave cylinder line). An RX-8’s transmission tunnel is spacious enough for the Tremec, but you may need to dent the tunnel for the shifter and shifter cables if using a remote mount.

4. Fit the Edelbrock Supercharger

Install the supercharger on the LS3 before dropping into the car. The E-Force uses the factory intake manifold location and sits low, but check hood clearance. Some builders cut the stock hood’s bracing or install a carbon fiber hood with a slight bulge. The supercharger’s intercooler pump and heat exchanger must be mounted in front of the radiator or in the bumper area.

5. Plumb the Cooling System

The RX-8 has a dual-zone cooling system (engine and oil coolers) that’s ill-suited for an LS. Use a Mishimoto dual-pass radiator or a custom unit with electric fans. The LS3’s water pump exits on the driver side—route hoses to a 160–180°F thermostat. Don’t forget an oil cooler (Setrab or Earl’s) to keep IATs in check under boost.

6. Wire and Refine

Run the standalone harness, GM pedal assembly (the RX-8 pedal is drive-by-wire but incompatible), and gauge cluster. You can retain the RX-8’s tachometer with a signal converter from Dakota Digital. Expect to spend a full weekend sorting wires, grounds, and sensors.

Cooling and Ancillary Upgrades

600 hp generates serious heat. The RX-8’s small engine bay leaves little room for airflow. Essential cooling mods include:

  • High-capacity aluminum radiator with dual 12-inch Spal fans.
  • Dewitts transmission cooler (if auto) or remote engine oil cooler.
  • Power steering cooler – the stock RX-8 pump can’t handle the extra heat; upgrade to a fluid cooler or electric power steering conversion.
  • Hood vents or louvered hood to extract hot air from the supercharger and intercooler.

Use a water-meth injection kit if you run high boost on pump gas—it lowers intake temps and prevents detonation.

Suspension, Brakes, and Drivetrain Tweaks

An added 300+ pounds over the front axle (the LS3 is heavier than the Renesis) changes the RX-8’s handling. Address these areas:

  • Coilovers – Stiffer springs (8–10 kg/mm front, 6–8 kg/mm rear) and adjustable dampers control body roll. Brands like Öhlins or KW offer RX-8-specific kits.
  • Reinforced rear differential – The stock RX-8 diff is weak. Install a Torsen T-2 or OS Giken LSD with upgraded axles from Drive Shaft Shop.
  • Big brake kit – StopTech ST-40 or Wilwood kits with 14-inch rotors and six-piston calipers are mandatory.
  • Subframe bushings – Replace with polyurethane or solid alumilite to handle the torque.

Tuning the Edelbrock Supercharged LS3

Getting to 600+ wheel horsepower requires meticulous tuning. Start with a base calibration from Edelbrock or a reputable tuner. Then:

  • Install a 2.5-inch or smaller supercharger pulley to increase boost to 9–10 psi.
  • Set the fuel system to E85 for cooler combustion and higher octane—this alone can gain 30–50 whp over pump gas.
  • Adjust the cam timing (if using a mild cam like a BTR Stage 2) to prevent reversion and keep the torque curve flat.
  • Use a wideband O2 sensor and log data on a Dynojet or Mustang dyno. Target 11.5:1 AFR at wide-open throttle.
  • Spark timing should be conservative: 10–12 degrees at peak boost, ramping up past 5,000 rpm to 18–20 degrees.

With an LS3, Edelbrock supercharger, E85, and a proper tune, you can expect 620–650 whp and 550 ft-lb at the wheels. That’s enough to surprise supercars without losing daily drivability.

An LS-swapped RX-8 will likely fail visual emissions inspections in states like California, New York, or Colorado. Expect to need a standalone ECU that supports CARB EO numbers (the Edelbrock E-Force for LS3 does have a CARB EO for certain applications, but not specifically for the RX-8). The only fully legal route is to move to a county without smog checks or register the car as a kit vehicle/off-road only. Always check local laws before starting the swap.

Cost Breakdown and Final Thoughts

A 600+ hp RX-8 LS swap isn’t cheap. Here’s a rough estimate:

  • LS3 crate engine: $8,000
  • Edelbrock supercharger kit: $7,500
  • Transmission (T-56 Magnum): $3,500
  • Engine mounts, headers, and misc: $2,500
  • Fuel system parts: $1,200
  • ECU and harness: $2,000
  • Cooling and suspension upgrades: $3,000
  • Labor and tuning: $5,000–$10,000

Total: $32,000–$38,000 for a professional build. A DIY build can save $5,000–$8,000 by doing your own wiring and mechanical work.

The reward? A 600+ hp RX-8 that handles like a dream, howls like an American V8, and leaves traction issues for the rear tires to solve. The LS3-Edelbrock combination offers near-instant throttle response and a powerband that makes every drive exhilarating. For those willing to tackle the complexities of the swap, the result is a truly unique, world-beating sports car.