Custom throttle bodies for L28

[luvemfast]

I am building a custom ITB injection system for my car, building upon Justin Olson and Ron Tyler's previous work.

 

I have a mechanical engineer at work helping me out on this, and will post progress as we go for others opinions.

 

What distance would people recommend from the valve to the throttle plate on an L28.

And runner length before and after throttle bodies.

 

Keeping in mind that I want to use the plenum still so that it can use the AFM and even be used for turbo applications, for anyone else who wants to.

 

Thanks, Simon

[Daeron]

Good question, I cannot say that I have seen a number ever bandied about.. I imagine it is probably very difficult to get much of a good idea, because it isnt exactly easy to vary in a controlled situation.. but I hope someone posts up a usable answer.

 

Unfortunately, if I am not mistaken, the best you are likely to get will be vague statements, at best anecdotal evidence (such-and-such an engine with X mm intake tract length ran this kind of power/torque band) rather than "this range is good for this, that range is good for that, this totally other range is utterly worthless, and this specific length will give you 124834 horsepower."

[luvemfast]

Anyone know how far the RB26DETT throttle bodies are from the head? I know they are a very different animal, but you gotta start somewhere.

 

I may need to look at the setup in the GTR's to get some idea's.

 

Anyone think that this is a waste of time? Would I be better to stick with the original single TB setup that Justin and Ron worked on?

 

I don't know what the USA spec E30 & E36 M3 and E34 M5 BMW's were. But we got the Euro specs which had ITB's. That's what gave me the iddea really.

[OlderThanMe]

The closer the throttle butterfly valve is to the back of the valve, the faster your response will be. So just as close as possible...

There are tings to consider though. Where are you going to put your fuel injectors? Stock location or maybe move them to the other side of the butterfly valve.

[Six_Shooter]

grapeaperacing.com

 

There are a few technical papers there, one of which deals with intake tract length.

 

At WOT the distance from the valve to the throttle plate is not important, since it's not really effecting anything (other than a very little restriction), the distacnce from the valve to the end of the intacke runner is the iportant part and will have an effect on the power band, along with runner diameter, and less so, any bends that are in that path.

 

As OTM said the distance from the valve to the throttle plate has an effect on throttle responce, but I'm sure how much of an effect this has will also be effected by the other runner dimentions as well, diameter, bends and such.

[luvemfast]

Here is a pic of the flange to bolt on to the head. (courtesy of Justin and Ron in another thread)

You can see where the injectors are going.

 

The head will need to be ported as the ID of the runners are 38.1mm (1.5inch).

 

Basically, I want to insert some TB's in between the two flanges in a straight line and run a plenum with piping to the front of the car.

Are the butterflies best higher or lower?

 

I understand I may sound a bit vague in what I'm after, but you've got to start somewhere.

 

I'll check www.grapeaperacing.com when I get home later. Thanks

[MONZTER]

Have you seen this company

 

http://www.jenvey.co.uk/

 

check out the advice and FaQ's section. It has some answers to your questions

 

Below is some info from that page

 

 

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.

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. 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.