K20 RX8 Engine Swap
(Complete)
(Complete)
The Honda K20 Engine Swap Project was undertaken to integrate a high-performance, turbocharged Honda K20 powertrain into a Mazda RX-8 chassis, creating a functional cross-platform engine system. The project focuses on applying mechanical design and fabrication principles to overcome spatial, structural, and drivetrain challenges while preparing the platform for boosted performance. Extensive modifications were required, including firewall and cross-member reconstruction, custom engine and transmission mounts, and drivetrain realignment to accommodate the new powertrain. The build also includes the integration of a turbo system, emphasizing engine power, cooling efficiency, and drivability under increased load and pressure. This project demonstrates hands-on experience in powertrain engineering, fabrication, and systems integration, combining performance tuning with practical mechanical design.
Goals
Select a compatible high-performance engine (Honda K20) suitable for integration into the Mazda RX-8 chassis.
Achieve precise drivetrain alignment and proper torque transfer between the engine, transmission, and driveshaft.
Identify and reinforce structural modification points (firewall, cross-member, subframe) for long-term reliability and to maintain safety and structural integrity in all modifications (weld quality, mount strength, vibration isolation).
Achieve integration of fuel, cooling, exhaust, electrical systems, sensors, and ECU communication for safe and efficient operation.
Ensure final assembly maintains clearance, accessibility, and serviceability for future maintenance.
Document measurements, design modifications, and fabrication methods.
LEARN
Chassis
The chassis stage focuses on preparing the RX-8 platform for integration with the Honda K20 powertrain. This process began with disassembly, engine bay cleaning, and removal of the stock rotary engine and components. Structural modification work, including firewall trimming, cross-member adjustments, and subframe clearance checks, will ensure proper engine fitment and drivetrain alignment. These early steps establish the foundation for mount fabrication, turbo integration, and future wiring and ECU installation.
The RX-8 chassis leaving the shop i bought it from for teardown and preparation prior to engine bay modification.
Engine bay stripped, prepped, painted and ready for the K20 engine assembly to be dropped in.
Power Train
The powertrain stage focuses on integrating the Honda K20 engine and transmission assembly into the Mazda RX-8 chassis. This process involves precise mechanical adaptation, including modifying the firewall, subframe, and cross-member to achieve correct drivetrain geometry and mounting alignment. The goal is to ensure the engine sits securely and transmits power efficiently while maintaining balance and serviceability. Future development will include turbocharger integration, drivetrain reinforcement, and ECU calibration to support boosted operation.
The Honda K20 engine undergoing preparation for fitment, with compression numbers verified before installation. I bought it used so I had to make sure the engine had good compression (even though the owner said he compression checked it).
I purchased a used 2005 manual transmission, test-fitted and aligned to match the 2008 RX-8’s driveshaft and tunnel geometry.
Electronics
The electronic control system is central to the RX-8 K20 swap, managing engine communication, sensor input, and system diagnostics. This stage focuses on integrating the K-Pro 4 ECU and CANverter module to establish reliable data transmission between the Honda powertrain and Mazda chassis electronics. The design process includes configuring input/output channels for sensors, calibrating CAN communication, and verifying signal integrity under automotive electrical conditions.
The Honda K-Pro4, a modified stock Honda ecu for special tuning capabilities.
CANverter communication module used to bridge and translate data between the Honda ECU and RX-8’s CAN bus network.
Parts List
This section outlines the major components required for the K20 engine swap, including powertrain hardware, wiring, and supporting systems. Each part was selected based on compatibility, reliability, and performance under automotive operating conditions. The list covers essential elements such as the engine assembly, transmission, ECU, sensors, fuel system components, and turbo hardware, as well as materials for fabrication and wiring integration. Together, these parts form the foundation for a complete, functional powertrain conversion.
The RX-8 K20 swap involved a combination of mechanical fabrication, electrical integration, and iterative prototyping to merge two completely different platforms, Mazda’s chassis and Honda’s K-series engine. The process required significant modification of structural components, adaptation of drivetrain geometry, and custom wiring solutions to ensure reliable communication between systems. Each major subsystem, chassis, powertrain, and ECU, was developed through detailed planning, hands-on fabrication, and collaboration, emphasizing engineering precision and safety.
Chassis
Major structural modifications were required to fit the Honda K20 engine within the RX-8’s tight engine bay. This included cutting and reshaping sections of the firewall and subframe, followed by reinforcement welding with assistance from a friend. To gain access and route necessary wiring and mounts, the entire dashboard assembly was removed, allowing a full view of the cabin-side structure. These steps were essential to establish proper engine alignment, turbo clearance, and long-term rigidity for the swapped setup.
Powertrain
The powertrain integration focused on adapting the K20 engine to the RX8’s drivetrain. This required installing a custom adapter plate to mate the Honda engine with the Mazda transmission and upgrading the clutch system to handle increased power. A turbocharger setup was also added, introducing new requirements for oil feed/return routing, exhaust clearance, and intercooler plumbing. Mount fabrication, driveline alignment, and load distribution were optimized through iterative fitting to achieve stable geometry and reliable torque transfer.
Electronics
Electronic integration was one of the most complex phases of the build. A CAN bus converter module was implemented to allow communication between the Honda K-Pro ECU and the RX-8’s native electrical systems. Custom wiring harnesses were fabricated using high-quality automotive connectors, ensuring clean signal routing for sensors, ignition, and auxiliary controls. The ECU setup was calibrated to synchronize fuel delivery, ignition timing, and turbo management, with CAN data streams verified through testing to confirm stable cross-platform communication.
First time starting the swapped car.
First time driving the swapped car on the street.
Mechanical & Design
Designed and modified structural components including firewall, subframe, and engine mounts for cross-platform engine integration.
Fabrication through precision cutting, fitment adjustments, and reinforcement welding.
Powertrain & Fabrication
Adapted Honda K20 engine to RX-8 drivetrain using custom adapter plates and clutch system modifications.
Installed turbocharger assembly, routed oil feed/return lines, and designed charge piping for optimal clearance.
Calibrated engine-to-transmission alignment for driveline balance and load distribution.
Integrated supporting systems including new fuel lines, cooling connections, and exhaust routing.
Electrical & Electronics
Integrated Honda K-Pro ECU into Mazda RX-8 wiring architecture using a CAN bus converter interface.
Designed and assembled custom wiring harnesses using sealed automotive connectors for reliability.
Configured and validated CAN communication between ECU, sensors, and auxiliary control systems.
Testing & Troubleshoointg
Performed mechanical and electrical validation including starter engagement, fuel priming, and communication checks.
Diagnosed and resolved issues with grounding, CAN signal noise, and sensor data mismatches.
Project Management & Collaboration
Coordinated fabrication assistance for welding and machining tasks.
Managed timeline for parts sourcing, installation phases, and system testing.
Documented progress, design updates, and wiring diagrams for long-term maintenance and reference.
This entire swap project is documented on my YouTube channel, where I walk through every major phase — from teardown and fabrication to wiring, ECU integration, and first startup. The series captures the hands-on challenges, design decisions, and engineering process behind bringing a turbocharged Honda K20 engine to life inside a Mazda RX-8 chassis. There is an entire playlist that you can see here K20 RX8 Swap Playlist.