Machining the port windows for an open chamber 13B rotary

An open chamber rotary does not gain performance simply because every opening has been made larger. Useful porting follows the entire gas path—from the inlet tract through the housing and rotor windows, around the chamber, and back to the exhaust—while preserving enough chamber volume and rotor strength for the engine’s intended load.

Rotary porting terminology varies among builders. Street porting usually enlarges the windows while retaining more of the original bridge; bridgeport work opens the transition between the chamber and side port. An open chamber conversion should therefore be described by its finished geometry rather than by a label copied from another rotor.

This guide focuses on a 13B because port-window dimensions, seal locations and service documentation differ between early cast-iron 13B rotors, later high-chrome hardware and 20B conversions. The safest specification is a measured original casting combined with the relevant workshop manual, not a universal width or depth quoted online.

Australian builders also operate within a practical regulatory environment involving right-hand-drive cars, metric service data, state registration rules and a mix of imported and locally salvaged parts. A technically successful engine still has to pass inspection, remain quiet enough for its home state and run reliably on fuel available near home.

Porting layout	What changes	Typical character	Main consideration
Stock	Factory windows and chamber transitions remain	Predictable response and emissions	Limited flow at high engine speed
Street port	Port windows are enlarged while the original bridge remains	Better midrange breathing	Requires careful flow matching
Bridgeport	The bridge between chamber and side port is opened	Smoother crossover and stronger response	Not the same geometry as open-chamber work
Open chamber	Chamber transitions and matching windows are opened broadly	Greater high-rpm flow potential	More sensitive to tuning and rotor strength

Separate porting terms from the actual geometry

Porting terms describe intent, but they do not provide a cut specification. TurboRX7’s explanation of bridgeport and street-port layouts is useful before any metal is removed, particularly for distinguishing a street port from a genuine crossover or bridge opening.

Think of the engine as one continuous passage. Air enters through the inlet tract and housing, passes through the rotating rotor’s intake window, crosses the chamber and leaves through the exhaust window and side housing. Corners, sudden enlargements and mismatched openings disturb that path even when the calculated cross-sectional area looks generous.

Both rotor faces and side housings participate in the flow, so matching a rotor to an unmodified housing—or enlarging both without reference to the same flow path—can produce a disappointing result. The chamber walls, apex-seal area, rotor timing marks and nearby oil passages must remain undisturbed.

Establish a baseline before material leaves the casting

Record compression, leakdown, cam timing, injector pulse width, fuel-trim values and exhaust condition before disassembly. Use rotary plug readings as supporting evidence rather than as a substitute for mechanical tests, since heat, deposits and oil contamination can make a plug picture misleading.

Before cutting

• Photograph each rotor face and housing port.
• Measure the existing window and transition dimensions.
• Record compression and leakdown results.
• Mark rotor orientation, timing and sealing edges.

Keep the engine’s operating conditions consistent between tests. A comparison taken before and after machining is useful only if the fuel, ignition, boost, oil temperature and measuring equipment are comparable.

After machining

• Repeat compression and leakdown tests.
• Compare the port profile with the drawings.
• Check for burrs, cracks and witness marks.
• Record every changed dimension.

A useful decision is not whether the opening became larger, but whether the measured flow path improved without compromising chamber volume, rotor integrity or the sealing surfaces.

Prepare and inspect the rotor and housings

Strip the engine down completely, mark the rotor’s relationship to its housing and remove every carbon deposit before taking measurements. Isolate the battery and fuel system, wear eye and face protection around the grinder, and treat compressed air carefully because a dislodged rotary fragment can become a projectile.

Inspect the rotor for cracking, distortion and excessive wear around the seal track. Magnaflux or dye-penetrant checks can be appropriate when the casting history is unknown. Measure the chambers, windows and transition areas with suitable gauges rather than relying on photographs, and pressure-test a housing if cracking or previous repairs are suspected. For additional background, Narmadi rotary resources can help builders compare chamber and port surfaces, but dimensions must still come from the exact casting in hand.

Australian precision-machine shops can work in metric dimensions and Australian workshop-manual conventions, but workmanship still depends on rigid fixturing and repeatable tool paths. A hand-ground window can be effective, although CNC machining generally gives a more consistent profile and makes it easier to reproduce the same rotor and housing combination later.

Machine the windows as a matched flow path

Mark the rotor and housing windows against the same datum before cutting. Rough the material removal in stages, retain a uniform wall beside the chamber and avoid cutting into the apex-seal land or adjacent sealing edges. The aim is to remove restrictions without simply creating a larger opening with abrupt corners.

Templates and CNC programs are valuable because they preserve a known profile, but they should be checked against the actual casting rather than followed blindly. Rockom porting examples can provide useful visual references for the way rotor and housing surfaces are blended and finished.

Blend each chamber-to-window transition with a smooth sweep and radiused corners, then match the housing opening to the rotor’s swept path. Remove all burrs, abrasive particles and loose casting dust before reassembly. A clean flow passage, intact sealing edge and repeatable measurement record matter more than the most dramatic-looking scallop.

Balance airflow with fuel and operating conditions

Porting changes velocity, pressure and the engine’s usable torque curve; it does not add energy. A larger window may improve flow at high engine speed while reducing the velocity that supports strong low-speed response. On a turbocharged 13B, the mismatch can appear as delayed boost response, unstable idle or harshness under load.

Match chamber volume, compression ratio, intake tract, intercooler and turbocharger to the new airflow. When selecting a donor, RX-7 production figures can help narrow the likely year and specification of a complete car.

Local operating conditions matter. Australian fuel ranges from regular 91 through premium 95 and 98, with E10 widely available but not equally convenient in every regional area. Tune for the fuel actually available rather than an ideal blend. Hot summers, long distances between country towns, towing and slow traffic around Sydney or Melbourne also make cooling, fuel range and low-speed drivability more important than a narrow peak in high-rpm power.

Registration requirements are state-based. New South Wales uses the Service Advisory & Logbook process for many significant modifications, while other states apply their own certification, noise and emissions checks. Confirm the current rules for exhaust noise, catalyst removal and emissions-affecting changes with the authority where the RX-7 is registered.

Build, tune, and validate the finished engine

Reassemble with correct gaskets, O-rings and seals, then set eccentric-bearing preload, oil flow, injector balance and rotor timing to the workshop-manual specifications. Confirm that the oil galleries and squirters are clear and that no debris remains inside the housings.

Begin with conservative ignition and fuelling settings while gathering compression, leakdown, wideband oxygen, knock, oil-pressure and exhaust-temperature data. Check for oil starvation, seal leakage, elevated backpressure and any movement between the rotor and housing. Break in the engine and its new rotating assembly before repeated full-throttle or high-boost use.

Start with TurboRX7’s factory documentation, create a dated measurement sheet for the exact engine and use it to select and validate the port profile before the first sustained high-load run.