This article was first published in 1999.
Porting is one of the most tried and tested methods of extracting more power from Mazda's highly potent rotary engine. This process involves enlarging/reshaping the front, middle and rear end plates and the exhaust ports of the engine. It's essentially the same principle as porting a conventional piston engine - but in effect it also changes the cam timing and lift at the same time! In addition to modifying the port area though, it is also important to take a look at the runners that feed them.
To get the run-down on today's porting technology, we spoke to Greg Mildren of Adelaide's Mildren Race Engineering. Greg's been building rotaries for over ten years and has pieced together around 150 modified ones - ranging right up to 369hp at the wheels tear-away 13Bs.
Most rotary tuners have their own special port designs that they've developed, but each of these will fall into one of the categories that we've listed below.
Standard ports are best described as a compromise between fuel economy, power and emissions. The earlier rotaries were freer in their dedication to making power, but at the same time there was less knowledge around to make the most of it. As a general rule, it is possible to get another 10% power though the standard port of an atmo inducted rotary engine. If you're modifying on a budget, one important point to remember is that all pre-1986 rotary plates and housings are interchangeable - so you can mix and match for good results.
12A - The 12A engine is most commonly found in early-mid RX7s and there are two distinctly different versions - that fitted to the Series 1 and the Series 2/3. The Series 1 engine has smaller runners and ports compared to the later versions. These entry-level motors were good for about 130 tame hp in stock form - but it's easy to improve upon this.
12A turbo - As fitted to the Japanese market RX7 and Cosmo, this engine was able to produce 165hp. Being equipped with a turbocharger, the standard porting of this engine is quite different to the atmo version.
13B 4 Port - This is an early-model engine that was discontinued in 1986 to make way for the 13B 6 port. It made up to around 140hp in its ultimate guise. But because it's such a solidly constructed engine, it's the ideal platform to build up a high power race engine - there's lots of material that can be removed for increased flow.
13B 6 Port - Released around '86, the Series 4 RX7 6 Port engine made 160hp and used an extra set of staged ports to aid breathing. Triggered by exhaust backpressure, a rotary sleeve opens the secondary ports up to allow more gas to pass. This system served to deliver good bottom end torque combined with top end power. However, there isn't much more that can be done to these port designs to gain more power.
13B turbo - Another 4 port design, this force inducted engine came with a factory air-to-air intercooler to make up to 200hp. It used larger ports and runners than the atmo 13B 4 port and Greg tells us that removing its stock EFI hardware and replacing it with a carb makes an effective and easy upgrade over a 12A or early 13B. With this conversion, around 134 rear wheel horse power is possible (with a conventional street exhaust). Of course, if you keep the EFI set up you would probably gain more power on top of that figure. The latest twin-turbo Series 6 RX7 versions have even larger ports and runners again, which are built up from all-new end plates (that are non-interchangeable).
Mild and extended ports are very similar in specification. Here, the standard port shape is enlarged slightly while maintaining the same basic shape. Extended ports though (by definition), have more of an emphasis on elongating the shape of the port. An extended port is usually slightly more powerful and a tad louder.
Greg says that mild/extend ports are "street type" ports, which offer similar drivability, economy and emission properties to standard. The advantage is anywhere up to 10% more power can be reached, with the torque range extending higher up the rev scale. Up to approx 200hp is attainable with other mods. These are Greg's preferred ports for a car that's 100% street driven, due to their value for money and blend of drivability and power. You'll pay around A$250 for a basic mild or extended port job.
Bridge Port/J Port
In both of these examples, an additional port is cut into the plate alongside the original port - which is usually slightly modified as well. A bridge port is simply a diluted version of a J port and makes roughly 10% less power - but with slightly better bottom end torque. Greg feels it's a better option to go for a J-port though, because it takes him a similar amount of time to create as a bridgey and there's a more significant power increase waiting at the end of it. Minor rotor housing mods are also needed when performing this type of porting.
Both bridge and J port modified engines rely on a highly efficient intake and exhaust - without which they can actually drop power. However, in some cases peak power can be up to 50% improved - with a maximum of around 240hp (with a street exhaust/intake) to 260hp (with a race exhaust/intake). A J ported rotary can rev to 8000-9000 with few problems, but torque does start to come in higher at around 3500. The trade-offs are more severe than a mild port - both economy and drivability are poor, and flames can be expected belching from the exhaust too (especially with a carby induction set up). A bridge port is also the first stage where brap-brapping exhaust noise starts to become an issue. Expect to hand over around A$350 for a bridge port or A$380 for a J port.
A monster port is an even bigger and badder version of a J port. In this mutha, an additional huge port that extends into the face of the rotor is ground into the plate and mods to the rotor housings are also required.
A monster makes good torque from 4000-9000 rpm and a 100% power increase can be achieved. A maximum of around 280hp can be extracted from an engine equipped with a race exhaust and intake. Drivability is all but gone - but it's still it's marginally better than the next form of porting - PPs. Monster ports check in with a bill of A$480.
Peripheral Port (aka PP)
Peripheral porting is the most extreme form of rotary breathing enhancement - and it's (by far!) the loudest. Low rpm torque, drivability and economy are completely lost but it's just the ticket for peak power - often, over 100% improvements can be gained over standard. (Note that with really good engine management, a PP can be driven on the street quite successfully - but you'll need injector end-point setting facilities and other such management features.) The effective torque band is also moved way up the tacho - beginning from around 5000 and building to a theoretical 10,000 rpm! (And building an engine to rev this high is another matter!) A PP won't idle much below 1800 rpm either - so by all accounts, it's a high revving and highly stressed engine that ain't for the street. However, in excess of 300hp can be found at the flywheel when combined with a race intake and exhaust. In order to construct a PP, the standard side ports must be filled or blocked off somewhere upstream in the intake.
Then, relatively large diameter ports are machined through the rotor housing (yes, on its periphery!) and into these are inserted aluminium sleeves that are shaped for best results with a die grinder. A sealant is then used to form a seal between the housing and the sleeve. There's a lot of work involved - hence the A$1000 approximate price.
- Greg recommends that when you pass the extended/mild port stage, it's a good idea to go for a modified engine rebuild while it's apart. That's because with the engine making significantly more power and eager to rev higher, its internal stresses are greatly escalated. An appropriate rebuild should include different clearances and tolerances, bearings and a revised apex seal material. A beefier driveline (ie clutch, gearbox and diff) will probably be needed too.
- Another question is whether to go for programmable EFI or the simple carb. Greg says EFI is always going to be preferable - but, if cost is prohibitive, a good carb (such as a Weber) still performs quite well. A major porting job doesn't usually deliver very good drivability or economy when fed by a carb though.
- Lastly, don't try to perform a backyard porting job. It's very easy to go backwards in power with a bad job. It really is an art getting the shape and size of the port spot-on and making sure they're consistent throughout the engine.
- Here's an example of someone else's work that Greg pulled from his bin. Notice how the edges of the port have been randomly ground out and the shape is irregular (and in some places sharp-edged). These shapes also varied from port to port to really make sure the engine ran like a dog. Yucko!
Mildren Race Engineering
+61 8 8443 8299