Top-Feed Fuel Rail Conversion for the Mazda 20B Three-Rotor Engine
The Radium Engineering RAD20-1120 is a CNC machined billet aluminum fuel rail purpose-built for the primary injector location on the Mazda 20B three-rotor engine. The 20B runs side-feed injectors from the factory, a configuration that limits fuel delivery when you start pushing power on an already exotic platform. The RAD20-1120 converts the primary injection position to top-feed using the included injector seats, opening up compatibility with the full range of modern top-feed high-impedance injectors without hacking up the factory intake or plenum. Both feed ports are machined as 8AN ORB (O-ring Boss) to accept industry-standard AN fittings, and the billet construction handles the fuel pressures common in built 20B applications running alcohol, methanol, or high-boost pump gas setups.
Key Features
- CNC machined billet aluminum construction for the Mazda 20B three-rotor engine, primary injector rail position
- Converts factory side-feed injector configuration to top-feed using included injector seats
- 8AN ORB (O-ring Boss) feed and return ports for leak-free AN fitting connections
- Compatible with all standard top-feed high-impedance fuel injectors once seats are installed
- Supports gasoline, E85, and methanol fuel delivery systems
- Designed for use in conjunction with the Radium 20B secondary fuel rail (sold separately) for full engine coverage
- Precision machined in the USA by Radium Engineering
Fitment
| Year Range | Make | Model | Trim |
|---|---|---|---|
| 1990-1996 | Mazda | Cosmo | Type S, Type SX (20B-REW) |
Specifications
| Detail | Value |
|---|---|
| Part Number | RAD20-1120 |
| Brand | Radium Engineering |
| Rail Position | Primary (Front Rotor Bank) |
| Injector Configuration | Top-Feed (converts from side-feed using included seats) |
| Port Size | 8AN ORB (O-ring Boss) |
| Construction | CNC Machined Billet Aluminum |
| Compatible Fuels | Gasoline, E85, Methanol |
| Application | Mazda 20B-REW Three-Rotor Engine |
| Warranty | Radium Engineering Limited Warranty |
| UPC | N/A |
What’s Included
- Radium Engineering RAD20-1120 billet fuel rail (primary position)
- Top-feed injector seats (quantity matches primary rotor bank injector count)
- Mounting hardware
Frequently Asked Questions
What engine does the RAD20-1120 fit?
The RAD20-1120 is built for the Mazda 20B-REW three-rotor engine. This engine was found factory-installed in the Mazda Cosmo (Type S and Type SX) from 1990 to 1996. It is also commonly swapped into Mazda RX-7 FC and FD chassis for high-power three-rotor builds, and this fuel rail fits the 20B regardless of the chassis it is installed in, as it attaches to the engine itself rather than any chassis-specific mounting location.
Why does the Mazda 20B need a top-feed fuel rail conversion?
The factory 20B-REW uses side-feed injectors, which are limited in flow rate compared to the top-feed injectors available in the aftermarket. Side-feed options max out well below what a built 20B running E85 or methanol requires at high boost. Converting to top-feed using this rail and included injector seats opens up injector selection to the full range of modern high-impedance injectors from brands like ID, DeatschWerks, Injector Dynamics, and Fuel Injector Clinic.
What injectors are compatible with this fuel rail?
Any standard top-feed high-impedance (saturated) fuel injector with the correct body length and O-ring dimensions will be compatible once the included injector seats are installed. Commonly paired injectors for high-power 20B builds include the Injector Dynamics ID2000, ID2600, DeatschWerks DW2200, and Fuel Injector Clinic 2150cc and 2650cc units. Confirm injector clip orientation and body diameter against the injector seat dimensions before ordering injectors.
Do I need to purchase the secondary fuel rail separately?
Yes. This rail covers the primary injector location on the 20B three-rotor. The secondary injector rail is a separate Radium part. For a complete fuel system overhaul on the 20B, both the primary and secondary rails are required. Radium sells each rail separately so builders targeting the primary bank only, such as staged injection strategies, can purchase accordingly.
What AN fitting size do the 8AN ORB ports accept?
The feed and return ports are machined as 8AN ORB, which means they use an O-ring face seal on a standard -8 AN threaded port. You will need 8AN ORB to 8AN flare adapters or direct 8AN ORB hose end fittings to connect your fuel feed and return lines. These fittings are widely available from brands like Fragola, Russell, and Aeroquip and are not included with the rail.
Is this fuel rail compatible with methanol and E85?
Yes. The billet aluminum construction and O-ring sealing surfaces on this rail are compatible with gasoline, E85, and methanol. The injector seats included with the rail are likewise alcohol-safe. For full alcohol compatibility, ensure that your fuel lines, pump, and filter are also rated for the fuel type being used.
Does installation require machining or modification to the intake manifold?
This depends on your specific 20B intake configuration. This rail is designed to bolt into the 20B engine’s primary injector location, replacing the side-feed rail. Depending on whether you are running a factory manifold or an aftermarket intake, minor fitment adjustments may be needed. Consulting a machinist or rotary specialist familiar with 20B top-feed conversions is recommended before installation.
Is this rail designed for a specific injector spacing or pattern?
Yes. This rail is machined specifically for the primary rotor bank injector spacing of the 20B engine. It is not a generic universal rail and cannot be used on other rotary engines such as the 13B without a different Radium rail designed for that engine. The included injector seats are also 20B primary-specific in their positioning and angle.
Installation Overview
- Depressurize the fuel system and disconnect the battery before beginning work on the 20B fuel rail.
- Remove the factory side-feed fuel rail from the primary injector position, carefully extracting all factory injectors without damaging the intake sealing surfaces.
- Press or slide the included top-feed injector seats into position at each injector bore in the manifold, ensuring full seating and correct O-ring placement.
- Install the selected top-feed injectors into the rail, lubricating the O-rings with clean fuel or O-ring lubricant before pressing in.
- Position the rail assembly over the injector seats and press the injectors into the seats, then secure the rail using the included mounting hardware.
- Connect 8AN ORB fittings to the feed and return ports, routing to your fuel system plumbing.
- Reconnect the battery, cycle the ignition to prime the fuel system, and inspect all connections for leaks before starting the engine.
- After initial startup, verify fuel pressure at idle and confirm no leaks are present before returning the vehicle to service.
WARNING: This product can expose you to chemicals including lead and phthalates, which are known to the State of California to cause cancer and reproductive harm. For more information go to www.P65Warnings.ca.gov.
