Back in my distant youth, before the invention of electronic fuel injection, the twin-choke carburettor was king. Replacing a single-choke constant depression unit or less complex single-choke carburettor – one feeding all the cylinders – an array of two or three of these twin-choke devices under the bonnet often impressed my mates. With one choke per cylinder, and feeding air directly from the atmosphere without a filter or airbox, the induction roar gave the impression of power even if the power actually produced was no more (or possibly even less) than that of the original single-choke installation.
These days though it’s not so easy to impress people, since opening the bonnet or hood to reveal the engine in all its glory displays only a mass of pipes and actuators. But for those in the know, the equivalent discussion to that of 50 years ago might be one of a single large throttle body against an array of port throttles.
Single large throttle plates are generally used in the original equipment installations of vehicle manufacturers. Comparatively cheap to manufacturer, and easy to set up on the bench away from the vehicle, they tend to be far more than just a throttle plate. With a couple of throttle angle potentiometers and an electric dc motor to open or shut the throttle in response to a signal from the engine’s ECU, these devices are no longer attached directly to the throttle pedal.
Used mainly where the vehicle needs to pass regulatory exhaust gas emission tests or to control the engine torque more precisely during transient conditions, in response to rapid opening (or closing) of the throttle, the response is relatively tardy as the manifold steadily fills up (or empties). Alongside this, the transient airflow has to be modelled so that the fuel demanded can be corrected to avoid rich or lean spikes in the exhaust gas, which could cause exhaust after-treatment issues downstream.
Port throttles on the other hand tend to be far more responsive to the throttle pedal, as the volume of air downstream of the throttle plate is rapidly consumed. This gives the driver a more urgent feel to the car during driving, particularly with the initial pull away from rest. The car may have the same overall power of the single throttle but the transient ‘feel’ of the vehicle is generally much better. The downside tends to be one of poorer control of exhaust emissions, despite the potential to improve fuel economy brought about by the reduced negative work on the piston at part-throttle.
Another advantage of port throttle applications is their ability to tolerate camshafts with a wider inlet and exhaust valve overlap at engine idle speeds. The close proximity of the closed throttle plate acts as a barrier to any residual exhaust gas finding its way back into the inlet port at intake valve opening, which in turn enables the engine to idle less erratically.
Perhaps the biggest advantage of using port throttle systems though is in the ability to design a suitable intake system to maximise outright engine performance. Using a number of simple intake runners (each with a throttle plate close to the intake valve) leading into a common plenum, while the length of the runner can be optimised for maximum performance, the plenum volume can be made as large as the space practically available, removing any performance-limiting design constraints away from the immediate vicinity of the engine and transferring it to the intake of the plenum. In the large single-throttle application, engine performance will inevitably be limited by the distance between the back of the intake valve and the throttle. Too large, and transient engine response will suffer; too small and the engine may suffer from inter-cylinder distribution problems.
But as an enthusiast appreciating the finer points of engine design I just like the throttle system detail under the bonnet.
Fig. 1 – Multi-throttle intake system – something to impress your friends?
Written by John Coxon