Pay attention to this as all you people with
side 4 port RX8 engines are going to need something
like this to get more than about 150 HP.
Make a folder on your hard disk and save it.

The plenum should be made real strong as when the
throttle is closed there is high vacuum in the plenum
and it will collapse. My suggestion is 1/8 thick door skin
mahogany ply covered on both sides with one or two layers
of carbon fiber.

Of course this will work with a carb or fuel injection.
The mass airflow sensor goes up stream of the butterfly
throttle. A 3" (76 mm) inch ID throttle and mass airflow
sensor is shown. My guess is you will get about 200 HP
at 7500 RPM with this configuration. A 2.75" (70 mm) version
may also work. You can get more HP with a side port engine
but you must take it apart and port it. If you are going to
do that you may as well convert it to a p-port.

The difficult part is the transition from the two oval ports
on the center housing to the 1.5 inch round tubes.
The ports are only 40 mm high and 20 mm wide. They are separated
from each other by only 5 mm. This transition needs
to be hand made with a die grinder out of a block of aluminum
an NC mill or a casting. Squashing the round tubes to ovals
in a press does not get it as the area changes. You want to
keep the runner area  constant. Besides that approach is very
difficult to weld. The devil is in the details. The front
and back RX8 housings have round holes unlike the 13B with
ovals in those locations. In that regard the RX8 is a bit
easier. At least the low power RX8 version is. The six port high
power RX8 engine is another matter. The side 6 port manifold becomes
a real nightmare.

It is not practical to make a slide throttle with a side port
engine. Taking your engine apart and installing p-ports and
a slide throttle would result in about 250 HP.


Paul Lamar