DIAMETER
The diameter of the throttle bodies will be heavily influenced by a number of factors. Power, rpm, head design, cylinder capacity, the position of the throttle body in the inlet tract, and the position of the injector itself will all affect the size of the throttle body to be used. For example, fitting a set of motorbike throttle bodies which are designed to work on a small capacity, but very high-revving engine will not work effectively if you then decide to bolt them onto a V8 which will only rev to 6000 rpm.
The key behind a good set of throttle bodies is keeping the air speed correct for the application. Higher revving, smaller capacity engines will need a higher air speed but not necessarily need to flow great volumes of air in order to fill the cylinders.Therefore, a smaller diameter body is needed in order to keep the air speed up. Conversely on a larger capacity, lower-revving engine the air speed won’t need to be so high. The more critical factor here will be getting enough air to fill the cylinders, which is achieved with a larger diameter body.
There is a direct relationship between the diameter of the throttle body and the air speed. Although camshaft profiles and exhaust designs also play a part in this, the basic relationship is the smaller the diameter the higher the air speed, but the less the volume of air which can pass through in a given time.
However, in a performance tuned engine there will come a point when the physical size of the throttle body becomes the restriction to the air flow, regardless of air speed. As the engine’s power increases so will the amount of air it uses. At this point a throttle body which is too small will effectively choke the engine, so a larger one will be needed to allow the required amount of air through, while maintaining the same air speed.
As a general guide most tuners advise that a 40 mm throttle will support up to 50 bhp per cylinder, while 50 mm bodies are capable of supporting as much as 80 bhp per cylinder.That means, for a regular four-cylinder engine, a set of 40 mm units will give around 200 bhp, and for 300-320 bhp you’d need 50 mm throttle bodies.