Single point throttle body

I wanted to build an engine that would last at least as long as an original VW built engine (60,000 miles minimum) but that should be better in every respect. It should drive beautifully, be efficient to run, require as little maintenance as possible, be quieter and more powerful, be able to keep up with modern traffic, never cut-out at traffic lights as my other VW’s do occasionally and take advantage of as many modern automotive advancements as possible.

It would be easy to build the same dual throttle body engine as so many others do but I had no plans to race my camper and the idea of miles of high pressure fuel pipes running back and fourth across my engine bay scared me a little bit. I also remembered the issues I’d experienced with the throttle linkage setup with the motorbike throttles earlier.

I came up with the idea of a single throttle body to keep things as simple as I could. It would be mounted in the centre of the engine bay like the original Solex carburettor. This would enable it to line-up with the original accelerator cable, doing away with the need for a linkage between throttle bodies. Most manufacturers have used them at some point including VW. They went out of favour a few years ago when ever more stringent emissions regulations made them impractical. I liked the idea of a system with as few moving parts as possible and a single high pressure fuel line (in reality you need a few high pressure lines but the others are very short).

I started out in a slightly unusual way of building myself a CNC milling machine. My reasoning being that if I had my own CNC machine I wouldn’t be reliant on others to produce the parts I needed, hopefully this would allow me to develop my design more quickly than if I used external suppliers. It also gives me the flexibility to tailor my designs to a particular application.

I wanted to create parts of real quality, that fitted together perfectly. Everything should be manufactured to at least the standard of modern vehicles’. The best way to achieve this is to machine them from solid aluminium billets using state of the art CNC machining equipment.

I set about designing a throttle body I could manufacture in the way I wanted, using the best materials I could source. There are a few steps to go through to create any part

  • Create a 3D model of the part you want to create. This necessitates the use of a computer running 3D modelling software. It took about two weeks work to design the single throttle body.
  • Convert the model into a format recognised by the next stage of the process. Common file types are .step .stl or .iges
  • Import the model file into a software package that can convert it into a program that the CNC machine can understand. My CNC uses programs based around lines of G-code. It is possible to write G-code manually but the first file I created was 32,000 lines long, that would’ve taken a while.
  • Import the G-code file onto the computer that runs the CNC machine. Insert a suitable cutter, load up your block of material and let the machine go to work on it (there’s a bit more to it than that but I’m trying to keep things short here).

I had to go through this process for each of the 6 sides of my aluminium block.

This is the first single point throttle body emerging from a 3 inch block of aluminium. 

Here the external machining is complete

And this is the finished part

Fitted with fuel injector, throttle position sensor (TPS) and idle control valve.

Mounted to mocked-up test engine

With the the throttle body connected to my new ECU I quickly had a running engine. But I could never get it to idle as well as I wanted. Even though it was the middle of summer and 25 degrees outside ice was forming on the outside of the inlet manifold (at this stage the manifold was stainless steel). I had a heat gun to hand so an an experiment I poked the warm end into the air intake. Instantly the engine changed speed and the ice melted. I turned down the heat gun to find out how much heat was required and found I could run it on its lowest setting with the same result.

Whether you are introducing fuel using a carburettor or a fuel injector the cause of icing is the same. We are trying to turn petrol into a vapour so it can be burned efficiently. When a substance changes from a liquid to a gas it it goes through a change of state and absorbs energy. The fuel is drawing in energy from the atmosphere, it is this that causes the icing. The problem is once the manifold drops below about 20 degrees Celsius the fuel doesn’t form a vapour, it re-condenses back into a liquid which is then difficult to burn and causes a pool of fuel to form in the bottom of the manifold. If you are running a single centre mounted carburettor even a stock one this could well be an issue for you.

This came as a bit of a surprise as I had found that to get the engine to idle I needed a tiny amount of fuel each injector pulse was supplying fuel for about 300 micro seconds, that is 0.0003 seconds. Even this tiny amount of fuel was enough to cause icing on my manifold. The solution was something similar to the arrangement VW used on almost all of its air filters. A tube draws warm air up from around the exhaust into the air intake. A flap in the air intake means that at idle only warm air is drawn in, as engine speed increases the flap opens and the engine draws in mostly cold air.

Any engine tuner will tell you that an engine should be fed with cool air. Cool air is denser than warm air so it contains more oxygen and we need the oxygen for combustion. The trick is to introduce the right amount of warm air for optimum power and perfect running.