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Porting - Thanks to
autospeed.com for the use of this article
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. |
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Standard Ports
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. |
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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). Porting
Mods |
Porting Mods
Mild/Extended Port 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. |
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| 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. |
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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. |
Monster 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. |
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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. |
Take
Note!
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! |
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