Throttle Body Selection

NOTE: It is assumed that the advanced engine developer will have access to the usual experience / software / dyno time. The advice on this page is not intended to replace these.


Which Type of Throttle Body?

Twin bodies are the most straightforward solution for production engines, direct-to-head where available, or via a suitable manifold. 
Direct-to-head-bodies represent the simplest and neatest solution. They are harder to match to the inlet ports if this is required for the engine in question but have the advantage of being angled for best results, unlike a carburettor manifold. 
Single bodies represent the no compromise solution, particularly for competition use. The seperate manifold is easily matched to the inlet ports and the best mixture path is guaranteed. They are also available in fully tapered bore and twin injector types. Mounting, balance and maintenance are naturally more involved.


What Is The Best Throttle Body Diameter?


There are several factors to consider when selecting throttle body diameter, power output, RPM, cylinder head design, engine capacity, injector position and throttle body position along the inlet path.

As a general rule of thumb a large bore results in lower flow resistance, a small bore results in better throttle response and more precise air/fuel ration mix. There are a few guidelines to be followed when selecting bore size and assumed BHP/cylinder using an assumed rev range up to 9000rpm.

BHP Millimeters
35 40
40 42
45 45
55 48
65 50

There are exceptions; big v8s are one, they can still have a large bore and a low rev range.

What Is The Correct Overall System Length?


Induction length is one of the most important aspects of fuelling performance engines. 
In our experience an under-length system is the greatest cause of disapointment, with loss of up to 1/3 of power potential. There are a number of good books on the subject and the serious developer is referred to these and, in particular, dyno trials. A guide figure, from the face of the trumpet to the centre of the valve head is 350mm for a 9,000 RPM engine. Other RPM are proportional i.e. for 18,000 RPM the figure is approximately 175mm.

Any air feed system to an airbox or filter can have a large effect on the power curve and must be considered carefully particularly if the airbox is small. 
The induction system is part of a resonant whole from air inlet or trumpet to exhaust outlet and the ideal length is heavily influenced by the other components.

What Is The Best Position For The Butterfly?


The butterfly is an important aid to fuel mixing. When positioned too close to the valve this advantage will be lost whilst positioning far away may lead to a loss of response. 
As with the injector position (see below), higher RPM demands a larger butterfly to valve distance. A practical minimum figure for a 7 - 9,000 RPM engine is 200mm, whilst the maximum is dictated by the need to fit an air horn of reasonable length to achieve a good overall tract shape. One solution to this apparent compromise is the use of bodies with fully tapered bores which, in effect, extend the trumpet distance beyond the butterfly and into the manifold. For very high speeds above approximately 15,000 RPM, the ideal butterfly position is only just inside, or even outside the trumpet and a point is reached where a taper is no longer sufficient for good tract shape. For these circumstances we can supply bodies with the exponential trumpet shape machined into them as a special service, or barrel bodies which, by their nature, must be purpose designed in conjunction with the cylinder head.

What Is The Best Place For The Injectors?


Where one injector is to be used per cylinder the best compromise position is immediately downstream of the butterfly. This gains maximum advantage from local turbulence and gives results surprisingly close to the optimum at both ends of the rev-range. This is the recommended position for most applications. 
For performance at low RPM, economy and emissions the injector needs to be close to the valve and firing at the back of the valve head. This is the favoured position for production vehicles. 
For higher RPM (very approximately 8,000+) the injector needs to be near the intake end of the induction tract to give adequate mixing time and opportunity. The higher the RPM, the further upstream the injector needs to be. As a result, use of speeds above approximately 11,000 RPM may give best results with the injector mounted outside the inlet tract altogether (see our remote injector mounting). It is common to fit both lower and upper injectors in such a system to cover starting and low RPM, as well as high speeds.

What Is Required For A Complete Fuel Injection System?


Besides throttle bodies, linkage and manifold (if required) typical components are; A management system, wiring loom, fuel pump, fuel pressure regulator, fuel injectors, appropriate plumbing, air horns and a ducting / filtration system for the incoming air.

What Type Of Injector?


Dimensions: All Jenvey injector mountings and fuel rails will accept either the standard 'O' ring mounted injectors for 14mm bores as supplied by Bosch, Weber, Lucas, etc (64mm between 'O' ring centres) or the shorter 'Pico' style injectors (38mm between 'O' ring centres). 
There are a number of other injector types, using the same 'O' rings but with different lengths. These can be used on our twin throttle bodies with ease, but may require different fuel rail mountings on individual bodies. Please specify which you are using when ordering throttle bodies and fuel rails. 
Flow-rate: When fitting our throttle bodies to an otherwise standard engine bear in mind that increased power means increased fuel demand and the original equipment injectors are therefore usually inadequate.

What Manifold To Use?


When injecting into the throttle body, most of the mixing occurs within the manifold section. It is therefore important that the manifold is suitably proportioned to evenly accelerate gas speed and thus help fuel mixing and distribution. The straighter the run in to the ports the better. A manifold which curves in the same direction as the valve throats is preferred to one which causes the flow to pass through an "S" bend.

What Is The Best Air Horn ( Trumpet / Stack / Bellmouth ) ?


The air horn serves three main purposes;

1) To convert the pressure difference between bore and entrance into air velocity with the minimum of energy loss.

2) To act as the interface between the induction system and the atmosphere, i.e. the point at which pressure waves change sign and direction.

3) To complete the system to the required overall length.

For ease of description the air horn may be considered in two parts; the 'flare' and the 'tube'. 
The main job of the flare is to spread the low pressure zone over the largest possible area, to reduce local pressure reduction, whilst guiding incoming air into the tube with minimum disruption or induced vortices. The flare should be shaped to encourage air to enter from the sides but not from the rear of the mouth. This is achieved by either finishing the mouth with a sharp edge when the arc is a little beyond 90o from the air horn axis or by folding material back, parallel to the axis, when the arc is at, or just below, 90o to the axis. 
The main job of the tube is to accelerate the airflow smoothly and progressively. This is best achieved by an exponential shape, i.e. one where the radius of curvature is increasing constantly until the angle of the sides matches the next part of the system, usually the throttle body. 
It should be noted that the requirements for fuel injection and carburation do not always coincide and the best horns for one may not suit the other.