Project FD RX7 Restomod: Part 35 – DBW Pedal and Bosch 82mm Throttle Body

We ditch the cable and open up a whole world of drivability and tuning possibilities with a drive-by-wire pedal and Bosch 82mm throttle body setup on our FD.

Back before the engine fire in Part-10 that sent us down a very long and expensive rabbit hole, our 13B-REW engine was fairly stock with the stock cable-actuated throttle.

The throttle cable comes out of the firewall and does a big loop around the upper intake manifold before connecting to the throttle body.

The FD RX-7’s throttle body is quite intricate and unique, featuring 3 butterflies.  The single lower 45mm throttle body feeds the smaller primary intake ports for the engine, while the upper two 50mm butterflies feed the larger secondary ports for the engine with an equivalent air flow of a 71mm throttle body.  The upper intake manifold splits the airflow after the butterflies between the primary and secondary ports.

When cracking the throttle at tip-in, only the lower primary butterfly initially starts to open, sending low volume but higher velocity air to the smaller primary ports of the engine to improve response.  Shortly after, the twin secondary throttles start to open and air is fed to all 4 ports of the 13B REW engine.  All of this has been very well thought out and engineered to improve transient throttle response.

The throttle body has a thermowax idle speed stop that raises the idle RPM until the wax inside the device melts from the coolant that is routed through the throttle body, allowing the throttle body return spring to compress the internal spring inside the wax housing to close the throttle plates a little more to lower the idle RPM at operating temperature.

The stock throttle body works surprisingly well even in high power applications.  The total surface area of the 3 butterflies is equivalent to a single 84mm throttle body, which is much larger than the cross section of a typical 2.5” charge pipe (63.5mm diameter).