ENGINE PORTING

Aside from adding a turbo or supercharger to a rotary engine, the only other way to increase performance is by porting the intake section of the engine. Porting a rotary engine is a way to extract more power from it, although often, the bigger the port the less drivable and practical the engine is. There are several different types of porting available for the rotary engine, ranging from mild all the way to the peripheral port. A turbo rotary engine can also receive a porting along the lines of a mild or extend port, bridge-porting a turbo motor is possible, but it is rare. While porting is still a popular engine modification, in recent years, the 13B Turbo motor has taken over as the main engine conversion for many older Mazdas, mainly due to their lower prices and increased reliability. But nothing can really beat the sound of a good ported rotary.

This is a guide to the different types of porting available and what their 'pro's' and con's' are.

Please use this as a guide only, for more information contact a specialist Rotary workshop.

Note: Information on this page is adapted from David Morris' (www.dmrh.com.au) article in Fast Fours and Rotaries magazine.

Standard Port

The standard port is common to Australian RX-2s, 3s and 4s. In 12A form, it is good for around 130hp. With intake and exhaust mods they can produce up to 180hp with excellent driveability and fuel efficiency with outright air-flow being the limiting factor. These standard housings form the basis for mild, extend and bridge porting.
 

PRO'S: Standard drivability and fuel efficiency
CON'S: Limited scope for power

 

Mild Port

A mild port is the first step to rotary porting. This version has been "shaped" slightly with a metal porting tool towards the upper area of the port, increasing air-flow and top-end power without creating any significant drawbacks. The port design is suitable for the standard induction with around 200hp being achievable. A mild port is a popular choice for those wanting slightly improved performance without changing the standard engine’s characteristics.

 

PRO'S: Smooth driving, good for daily street use
CON'S: Slight fuel consumption increase

 

Extend Port

The extend port is a larger version of the mild port and works best with improved intake and exhaust systems. Typically with these modifications, the engine will start to exhibit the rotary’s signature rough idle, although the extend port’s is very slight. There is also a slight but noticeable loss of low-down torque below 4,000rpm. Importantly the increase in air-flow ability translates to the potential for around 220hp providing the best compromise for regularly-used street cars. This is the popular choice for those who don’t want the numerous drawbacks of a bridge-port.

 

PRO'S: Good power increase while retaining drivability.
CON'S: Increase of fuel consumption and noise and the need for modified intake and exhaust systems to maximise power.

 

Bridge Port

The Bridge port uses an additional "eyebrow" opening alongside the original "modified" port and introduces the characteristic lumpy idling. The bridge that now exists between the 2 ports primary purpose is to ensure the rotors corner seals – which pass indirectly over the bridge – don’t fall out. The improved air-flow increases top-end power dramatically with a noticeable power-band peaking around 8,000rpm while drivability, smoothness and fuel efficiency is reduced. Maximum power of around 260-280hp is largely dependent on the choice of intake and exhaust systems. Unfortunately for street cars, faster normally means louder.

 

PRO'S: Very good potential for power, increased rev-range
CON'S: Poor drivability, fuel consumption and excessive noise

 

J or Monster Port

The J ported engine (also known as monster port) is as big as conventional side-plate porting can go. It is the same as the bridge port in design, but the bridged port is now fatter and extends past the face of the rotor and into the housing’s water seal / O-ring requiring the need for the seal to be cut back and filled with a metal type sealant such as "Devcon", plus, depending on the side plate used, the water gallery requires blocking off and filling on the side plate and the rotor matching.

Cutting into the rotor housing is also needed for port matching. The main problem with this design of port is a short life-span as water does and will seep through the seals. The main field where a J-port is likely to be seen is on the track where restrictions don't allow peripheral porting, or a slightly wider power band is desired. Typically around 300hp can be expected.

 

PRO'S: Slightly better power than a bridge port (5 to 10%) without the expense of a PP
CON'S: Short life-span, 6-12 months / 5,000-10,000km, narrow power band, need for free-flowing/loud exhaust system, poor drivability

 

Peripheral Port

The peripheral port is the ultimate form of porting for a rotary engine without turning to forced induction. Instead of conventional metal shaping with a grinder, the side ports are actually filled and completely new circular - peripheral - intake ports are fitted directly through the rotor housing and is easily identified on an engine by its tubular intake manifold. This modification has potential for over 300hp but as it is developed for racing, results in very poor drivability, fuel consumption and needs an exhaust system designed to produce excessive noise. Like the bridge port, the PP produces its maximum power past 8,000rpm creating increased stress and wear on components.

And instead of idling at around 1,000 – 1,500rpm for the previous porting methods, the PP engine will not idle much below 2,000rpm and is impractical and virtually unusable on the street.

PRO'S: The ultimate form of rotary porting for maximum power
CON'S: Excessive noise, extensive intake mods, very poor drivability and fuel consumption, relatively short engine life, very expensive, narrow power band