Throttle Body FAQs
NOTE: It is assumed that
the advanced engine developer will have access to the usual experience /
software / dyno time. The advise on this page is not intended to replace
these.
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 is a balanced compromise resulting from the
following;
1) A larger bore leads to lower flow resistance, but obeying the laws
of diminishing returns.
2) A smaller bore leads to better throttle control and response (never
underestimate) and improved fuel mixing.
3) The system should be considered in total - from (at least) trumpet
flange to cylinder and proportioned accordingly.
Basic references for BHP per cylinder, assuming ca 120mm from
butterfly to valve head and a max of 9,000 rpm are;
Up to 30 - 30mm, up to 33 - 32mm, up to 39 - 35mm, up to 46 - 38mm, up
to 51 - 40mm, up to 56 - 42mm
Up to 65 - 45mm, up to 74 - 48mm, up to 80 - 50mm, up to 87 - 52mm, up
to 93 - 54mm.
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 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 ca 175mm.
The induction system is part of a resonant whole - from trumpet to
exhaust outlet - and the ideal length is heavily influenced by the
other components.
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 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.
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 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 our 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 (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.
What
throttle potentiometers will fit these bodies?
We use a relatively popular mechanical interface
for the throttle potentiometer. See the layout drawing for dimension
details. Popular types are; Colvern CP17 series (as supplied by Jenvey),
Spectrol and (via an adapter) Weber. A number of production car throttle
pots (e.g. Rover K series) will also fit directly to the bodies.
The throttle potentiometer may be mounted to either end of most
installations and rotation is typically 82 degrees.
Can
our bodies be pressure charged?
Jenvey bodies can generally be used with boosts up
to 6 bar, although we recommend that you contact our technical
department if boost of more than 2.5 bar or temperatures above 150ºC
are expected since some models require special treatment for high
pressures and/or temperatures.
Can
our bodies be connected to an Air Bypass valve?
Components and complete kits are available to
connect the output from an ABV to throttle bodies. More information is
available on a specific instruction sheet.
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 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 carburation
do not always coincide and the best horns for one may not suit the
other.
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