How do I perform an S4 Turbo II engine swap on a Mazda RX-7 FB?

Difficulty: Expert

Swapping an S4 Turbo II (13B-T) engine into an FB chassis is a well-documented but extensive project. Because you are crossing generations, this isn't a simple drop-in swap; it requires custom fabrication, drivetrain upgrades, and significant wiring work.

Here is the breakdown of what it takes to get the S4 TII block mounted and running in your 1st-gen chassis.

Engine Mounting

The S4 engine uses side mounts from the factory, but the FB chassis is designed for a front mount. To physically bolt the block in, you need to swap the front cover to a GSL-SE front cover ⁹. You will also need a 13B GSL-SE oil pan to clear the subframe.

For the crossmember itself, you have three options: use a Racing Beat front mount bar, use a factory GSL-SE crossmember, or slot the mounting holes in your existing 12A front mount bar and crossmember exactly 20mm forward ⁵.

Note: If you decide to use a 12A front cover instead of a GSL-SE unit, you must retain the 12A’s 2-line oil metering pump (OMP) and route those lines directly to the rotor housings for proper lubrication ⁵.

Drivetrain & Transmission

Do not try to reuse your stock FB transmission or rear end. They will not survive Turbo II power output ¹³.

You need to step up to the S4 TII 5-speed transmission. Unlike naturally aspirated S4 transmissions—which can be bolted into an FB simply by swapping the tailshaft housing—the TII transmission is larger and requires a modified transmission crossmember ¹². You should also install a Stage 2 or better clutch and plan on upgrading the rear differential (such as swapping to an LSD) to handle the torque ¹³.

Because of the transmission change, you will need a custom driveshaft. I don't have a verified exact length measurement for this driveshaft, as the final number depends heavily on your specific engine placement tolerances and which rear end you go with. Don't pull a random number from the internet for this—the standard procedure is to bolt the engine and transmission into their final resting place, load the rear suspension (set the car on its wheels or put jack stands under the axle), and measure from the transmission output shaft seal to the differential input flange. Take that measurement to a local driveline shop and have them cut and balance a shaft for you.

For your gauges, the factory FC cluster requires heavy modification to fit the FB dash. Instead, you can run a mechanical speedometer cable straight from the TII transmission to your stock FB speedometer ⁹.

Fuel & Cooling

You are moving to a turbocharged, fuel-injected setup, so your fuel delivery and cooling systems need an overhaul:

• Fuel: Upgrade to a high-pressure EFI fuel pump (like a Walbro 255) and an adjustable fuel pressure regulator with return-style routing.
• Cooling: You will need a front-mount oil cooler setup (either a Racing Beat unit or a custom front-mount) ⁵. You will also need to fabricate intercooler piping and mount an intercooler behind the grille.

Engine Management

While it is technically possible to splice the S4 ECU and harness into the FB chassis, the factory computer is a massive limiting factor and lacks the tuning flexibility you need if you ever plan to turn up the boost or run a larger turbo. Standard practice for this swap is to ditch the factory ECU entirely and run a standalone management system like Haltech, Microtech, or Wolf 3D ¹³. This simplifies your chassis wiring significantly and keeps the engine safe once you start making power.