TAPER/PARALLEL

There are two types of throttle body: tapered and parallel. Parallel are self-explanatory and both sides of the butterfly are the same size. Tapered bodies are tapered; the engine side of the body has a smaller diameter than the trumpet side. This effectively turns the whole inlet system into one big inlet trumpet. The advantage is a constant increase in air speed. As the diameter gets smaller, the air needs to accelerate to get through it. This gives a smooth progressive acceleration of the air speed. Thus making it better suited for higher revving applications.

TRUMPETS

Also known as air horns, stacks or bellmouths, trumpets play an important role in how air enters the engine. They are the place where air from the surrounding atmosphere enters your car’s induction system. There is a pressure difference between the induction system (which is under a vacuum caused by the intake stroke of the engine) and the surrounding atmosphere (which is at atmospheric pressure). Trumpets need to provide a smooth route for the air from the atmosphere to enter the induction system – and do so with as little energy loss as possible.

The way the trumpets taper from a larger diameter to a smaller one helps accelerate the air into the engine. It gives the required air speed, and provides a smooth progression to the next part of the induction system. This is usually the throttle body. The atmosphere end needs to be as large as possible to give the biggest area for air to enter the induction system. The narrower end needs to be exactly the same size as the throttle body to prevent any steps, causing turbulence and disruption to the airflow.

The trumpets complete the overall length of the induction system. These can be tailored to suit specific applications. As a rule, longer trumpets give increased torque and mid-range power, while shorter trumpets are better suited to higher-revving engines.

BUTTERFLY AND INJECTOR POSITIONS

Position of the throttle butterfly and injector(s) in the inlet tract is vital to the way air and fuel is mixed. If either is positioned too close to the inlet port the distance for the air and fuel to mix is shortened. This isn’t so noticeable at low engine speeds. However, at high rpms the air and fuel don’t have time or space to mix properly to get the best performance. You have to compromise because positioning the butterfly and injectors too far away from the inlet port will have a negative effect on throttle response. As with a single larger throttle body, the further away from the inlet port the more

space the air/fuel mixture needs to fill before reaching the cylinder. The increased time this takes gives the delayed response.

Most production vehicles have injectors positioned as close to the inlet port as possible for good performance at low rpms, good economy and lower emissions. It also reduces the time available for the air/ fuel to mix at high rpms. With regard to the position of the injector in relation to the butterfly, Jenvey found that the optimum place is immediately after the butterfly (engine side). Simon explains: “Turbulence created as air passes the butterfly helps the air and recently injected fuel to mix and gives close to optimum results at both ends of the rev range. We recommend this for most applications.”

By moving the injector towards the end of the trumpet you give the air and fuel more time to mix. This gives results at high rpms. However, the pay-off is drivability and throttle response at lower engine speeds.

MULTIPLE INJECTORS

You can run multiple injectors per throttle body. For most applications the two injectors will be in the same position. For some motorsport uses injectors are moved to each end of the inlet tract. By fitting one injector close to the inlet port and the other in the trumpet you can get a balance between lowdown throttle response and high rpm power. The injector closest to the port provides the almost instant throttle response. While the second in the trumpet allows the air and fuel sufficient time to mix to give optimum power at high rpms.

THROTTLE BODIES ON TURBOCHARGED ENGINES

Although usually associated with highperformance naturally-aspirated engines, throttle bodies will also work equally well in forced induction engines. Obviously they will need enclosing in a plenum chamber to hold the boost pressure. However, the same principles still apply. Rather than relying on the pressure difference between the vacuum of the engine on the intake stroke and the surrounding air, the surrounding air is boosted at a greater pressure and is forced into the throttle body. From there on the principles are the same.

DIAMETER

Power, rpm, head design, cylinder capacity, the position of the throttle body in the inlet tract and the position of the injector 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 if you then decide to bolt them onto a 5.0-litre V8 which will only rev to 6,000rpm. The key to a good set of throttle bodies is keeping the air speed correct for the application. Higher-revving, smaller-capacity engines will need a high 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 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 is filling the cylinders with air. This is achieved with a

The key to an effective and efficient set of individual throttle bodies is keeping the air speed correct for the specific application