Classic Inlines
603 W Pecos Ave
Mesa, AZ 85210

Throttle Body Q&A

Note: It is assumed that the advanced engine builder and/or developer will have access to the usual experience, software, and dyno time required to determine the correct throttle body for a specific engine. Hence, the advise on this page is only intended as a rule of thumb.

Most Commonly Asked Questions
What is the best throttle body diameter?
What is the correct overall system length?
What is the best position for the butterfly?
Which type of throttle body is best?
Where is the best place for the injectors?
Which type of injector should be used?
What is required for a complete fuel injection system?
Which type of manifold design is best?
Which throttle potentiometers fit Jenvey bodies?
Which is the best air horn design?
Can Jenvey bodies be pressurized (turbo-charged)?
Can Jenvey bodies be connected to an Air Bypass valve?
Butterflies, barrels or slides?
Why not just use a big single throttle body?

What is the best throttle body diameter?
Factors influencing size are; power output, rpm, cylinder head design, cylinder capacity, position of the throttle body in the inlet tract, and position of the injector. Choice of bore size, or diameter, is a balanced compromise resulting from the following:

1) A larger bore leads to lower flow resistance, but obeys the laws of diminishing returns.
2) A smaller bore leads to better throttle control and response and improved fuel mixing.
3) The system should be considered, in total, from the trumpet flange to valve head.
4) Proportioned accordingly, but never undersized.

Basic references for BHP per cylinder assuming a 120mm (4.725") length, from the butterfly to valve head, and a max of 9,000 rpm.


These power figures may be increased by up to 10% in a purpose designed and well proportioned system. As butterfly to valve distance increases, butterfly size will need to increase in proportion to system taper and vice versa. Lower revving engines, and those with injectors placed before the butterfly, will generally accept a larger body.

What is the correct overall system length?
Induction length is one of the most important aspects of fueling performance engines.
Experiences shows an under-length system is the greatest cause of disappointment, with losses 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 center of the valve head, is 350mm (13.78") for a 9,000 RPM engine. Other RPM's are proportional (for 18,000 RPM the figure is 175mm). Any air feed system to an air box or filter can have a large effect on the power curve and must be considered carefully, particularly if the air box 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, while positioning far away may lead to a loss of response. As with the injector position (see below), higher RPM's demand a larger butterfly to valve distance. A practical minimum figure for a 7-9,000 RPM engine is 200mm (7.875"), while 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.

Which type of throttle body?
Twin bodies are the most straight forward 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 carburetor manifold.
Single bodies represent the no-compromise solution, particularly for competition use. The separate 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.

Where 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 low emissions the injector needs to be close to the valve and firing at the back of the valve head. This is the favored position for production vehicles.

For higher RPM (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.

Which type of injector should be used?
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 centers) or the shorter 'Pico' style injectors (38mm between 'O' ring centers).

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, therefore the original equipment injectors are usually inadequate.

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.

Which type of manifold design is best?
When injecting into the throttle body (e.g. our types TB, TH, TF, TA, Direct-to-head and SF, SS or ST//1), 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.

Which throttle potentiometers fit Jenvey bodies?
We use a relatively popular mechanical interface for the throttle potentiometer. Popular types are; Colvern CP17 series (as supplied by Classic Inlines), Spectrol, and Weber (via an adapter). A number of production car throttle pots (e.g. Rover K series) will also fit directly to the bodies. The Colvern CP17 throttle potentiometer may be mounted to either end of most installations and spindle rotation is typically 82 degrees.

Which is the best air horn design - 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 90 degrees from the air horn axis or by folding material back parallel to the axis when the arc is at, or just below, 90 degrees 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. At the intake end this should blend smoothly with the flare.
It should be noted that the requirements for fuel injection and carburetion do not always coincide and the best horns for one may not suit the other.

Can Jenvey bodies be pressurized (turbo-charged)?
Jenvey bodies can generally be used with boosts up to 80psi, although we recommend that you contact our technical department if boost of more than 35psi or temperatures above 150ºC are expected since some models require special treatment for high pressures and/or temperatures.

Can Jenvey bodies be connected to an Air Bypass valve?
Components and complete kits are available to connect the output from an ABV to throttle bodies. For more information, see "Using an Air Bypass Valve".

Butterflies, barrels or slides?
20 years ago, carburetors and mechanical fuel injection were the only choice to fuel a racing car engine. Selection of air valve type was simple and apparently carved in stone; carburetors used butterflies and injection used slide throttles. Motor Bikes and their history are a special case but what follows is true for any performance 4-stroke petrol engine.

With the advent of electronic fuel injection and a more adventurous (better funded) approach from the leading engine designers, it was discovered that butterflies, whilst sometimes (but not always) giving slightly less power than slides, inevitably gave better lap times. The explanation was simple; butterflies give more progressive throttle control, improved transient conditions, and aid mixture quality throughout the RPM range.

As a result of these discoveries most (possibly all) of the leading car race engine manufacturers switched to using butterflies. Lap times continued to tumble, but there was a problem brewing for the future.
As peak RPM increased year by year, the required induction system length reduced. At the same time, the ideal butterfly to valve distance increased. Over about 15,000 rpm, the ideal butterfly position falls outside the induction system - clearly useless. Enter the barrel.

The barrel has some, but not all of the attributes of a butterfly. Opening is reasonable progressive and, like the butterfly, it is easily packaged. The great advantage is that it can be made as a continuation of the port shape, regardless of profile (slides would overlap), and thus be placed near or even in the cylinder head, allowing for a very short system to suit the 18,000+ RPM which is now common. Any compromises (poor idle control, tendency to stick, poor flow vector control, etc.) were once believed to be offset by the sheer power available at these RPM, although once again most of the leading car race engine manufacturers (e.g. F1) had switched back to using butterflies by 2006. It follows that barrels on a sub 15,000 RPM engine will suffer from the compromises without gaining the possible benefits.

The main advantage of the barrel - maintenance of port profile - can be obtained by using fully profiled butterflies. These are made to precisely fit the port profile and are shaped in cross-section to achieve the required characteristics; minimum drag, controlled turbulence or whatever else best suits the application. This is now the preferred solution for top-end engines used in Formula 1, World Super Bikes and some sports-racing engines. Jenvey Dynamics supply these for specific applications since they must be designed to suit the engine and cylinder head used.

In summary; Butterflies are best wherever they can be used. Jenvey Dynamics have a design and make service for engine specific profiled butterfly bodies. Barrels are suitable only for engines running at over 15,000 RPM and must be designed to suit a particular engine type. Jenvey Dynamics have a design and make service for engine - specific barrel bodies. Slide throttles are best reserved for classics, if the rules prohibit a change.

In a back-to-back comparison, using a Rover K series engine in race trim, Jenvey road going butterfly bodies were found to give significantly more power at all RPM when tested against barrels. Full race butterfly bodies would further increase the margin. Whilst the power improvements are unlikely to be found in all engine types, the performance gains almost certainly will.

Why not just use a big single throttle body?
The choice of throttle 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 favorably varies the ratio between pedal movement and throttle action. For example, our SFG (big single) throttle body rotates the butterfly 1.4 degrees for the first 10 degrees of quadrant rotation (5.5mm of cable travel) and 20 degrees for the last 10 degrees 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, using two butterflies opening sequentially and 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 sq mm) restrictor, whilst, say, a 75mm throttle body is eight times larger at 4,418 sq mm! 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 sq mm) 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 also cheaper to make than multiple bodies and, for everyday motoring, has the advantage that throttle response is gentle and delayed since 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 throttle bodies are used in all naturally-aspirated true race engines, most motorbikes and some top-end 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 – 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.

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 lengthened.
Give no-compromise performance.

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