Large Single Throttle Bodies
Why not just use a large single throttle body?
The choice of thottle body size for the typical road car is a compromise between two opposing needs; to allow sufficient air flow for the engine to achieve its full power potential and to keep the butterfly small enough to allow a progressive throttle action at low openings.
The engine designer has a number of tricks available to help match these opposing needs. The no. 1 solution is to use a linkage which favourably varies the ratio between pedal movement and throttle action. For example, our SFG (big single) throttle body rotates the butterfly 1.4o for the first 10o of quadrant rotation (5.5mm of cable travel) and 20o for the last 10o of quadrant rotation. Other solutions include shaping the bore of the throttle body such that the gap on one side of the butterfly remains small for the first part of butterfly rotation or using two butterflies opening sequentially or using electronic throttle control.
The above techniques allow the designer to use a sufficiently large throttle body bore that rarely limits engine performance to a measurable degree. It follows that enlarging the original throttle body will only make a useful gain when other aspects of the engine have been changed to substantially increase the power output. The most likely change due to a larger throttle body, if any, on an otherwise standard engine is a car that goes slower round corners due to lack of fine throttle control for the driver. At best, there may be a slight improvement in response (see below).
It helps to understand the relationship between size and output to consider that a 2L Formula 3 engine produces 200+BHP through a 26mm (531 mm2) restrictor and a 75mm throttle body is eight times larger at 4,418 mm2! Similarly, in an experiment to curb the power of 270 BHP touring car engines, we reduced the size of the single throttle body to less than 44mm (1520 mm2) before there was any noticeable reduction. A 75mm body has three times the flow.
The single throttle body has a number of benefits for the mass producer. It requires no balancing or fine tuning since all cylinders are drawing from a common volume. It is cheaper to make than multiple bodies and, for everyday motoring, has the advantage that throttle response is gentle and delayed. This is because sudden opening of the throttle must first fill the entire induction system before the engine gets the full benefit. This effect is well suited to the needs of the inexpert driver since, whatever the throttle position, torque builds slowly and nothing happens suddenly.
Multiple individual throttle bodies are used in all naturally aspirated true race engines, most motorbikes and some hig performance road cars (e.g. BMW M series) i.e. where the driver prefers to be in full control. They have a number of advantages. The most obvious to the driver is virtually instantaneous throttle response whilst the individual inlet tracts allow true length tuning of the inlet system which has a large influence on torque and power output. In a well designed throttle body system the positioning of the injectors and butterflies aid fuel mixing, very important in higher RPM (i.e. above 5,000) engines.
In summary;
Single throttle bodies;
Are simple to set up.
Are cheap to make.
Make engine response gentle – will not frighten the average driver.
Are easily silenced.
Multiple throttle bodies;
Make engine response lively.
Increase power and torque by improving mixture quality – particularly at higher RPM.
Increase power and/or torque by resonance tuning – when correctly lengthed.
Give no-compromise performance.