Individual Throttle Body Sizing/Design - Tuner-Junction

njn63 · #1 (**permalink**) March 14th, 2006, 02:44 AM

Quote:

A. 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.(ca.= approximately) 120mm from butterfly to valve head and a max of 9,000 rpm are:

Up to 30 BHP/CYLINDER - 30mm
up to 33 BHP/CYLINDER - 32mm
up to 39 BHP/CYLINDER - 35mm
up to 46 BHP/CYLINDER - 38mm
up to 51 BHP/CYLINDER - 40mm
up to 56 BHP/CYLINDER - 42mm
Up to 65 BHP/CYLINDER - 45mm
up to 74 BHP/CYLINDER - 48mm
up to 80 BHP/CYLINDER - 50mm
up to 87 BHP/CYLINDER - 52mm
up to 93 BHP/CYLINDER - 54mm.

These power figures may be increased by up to 10% in a purpose - designed, 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.

B. 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 can be heavily influenced by the other components.

C. Which type of throttle body?

Twin bodies are the most straightforward solution for tuning 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. 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.

D. 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 , 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.

E. Where is the best place for the injectors?

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.

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.

F. 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.

G. What type of injector?

I. Injector Dimensions :

All Jenvey injector mountings and fuel rails will accept the standard 'O' ring mounted injectors for 14mm bores as supplied by Bosch, Weber, Lucas, etc. These are typically 63mm between 'O' ring centres.

All except the diecast aluminium fuel rails will also accept the shorter 'Pico' style injectors. Please specify which you are using when ordering throttle bodies and fuel rails.

There are a number of other injector types, using the same 'O' rings but with a different length. These can be used on our twin throttle bodies with ease, but may require special fuel rail mountings on individual bodies.

II. Injector 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.

H. 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 gasses to undergo an "S" bend.

Blake · #2 (**permalink**) March 14th, 2006, 08:48 AM

Thats a good Faq article. Read that a few times while i was searching for which ITB's to run.

hcarcrx · #3 (**permalink**) March 14th, 2006, 09:39 PM

very informative faq!!