Fuel injection system

There is more information on the fuel injection system under the ‘Development’ tab. This section is intended to define what you will receive with your engine and why it is set-up the way it is.

Carburettors have been used since the dawn of the automotive industry. When coupled with a distributor or magneto they make for a reliable and cost effective way to control the fuel and sparks in your engine. Unfortunately they are always a compromise. People will tell you that you should never run your engine ‘lean’ and they are right. What do we mean by lean? For every 14.7 kilograms of air we put into an engine we are are trying to introduce 1 kilogram of petrol. If only 10 kg of air goes in with our 1 kg of fuel we would call that rich, if there is more than 14.7 kg of air, we would call that lean. When
there is not enough fuel in the fuel air mix bad things happen in the combustion chamber we get detonation. The fuel can burn at the wrong time, often way before the piston gets to the correct point for ignition so it works against the normal operation of the engine. In extreme cases the engine will get so hot that the piston reaches a point where it releases aluminium oxide. Aluminium oxide under the right circumstances will burn so you literally burn away your pistons, not good.

A carburettor is designed to make sure there is always at least 1kg of fuel going into your engine for every 14.7 kg of air. The issue is that through most of the engine’s range there will be far more than 1kg of fuel for every 14.7 kg of air, so most of the time it is running rich. If it is setup properly there will be one point where the fuel air ratio is exactly right, if you’re really lucky this will be where you use the engine the most, usually at cruising speed.

The function of a distributor is to ‘distribute’ a spark to the correct cylinder at the correct time. The distributor cap and rotor arm do the ‘distributing’ the timing is set by rotating weights and vacuum operated diaphragms. It’s all very clockwork for the 21st century. Inevitably moving parts in the distributor wear (especially if it is one of those Chinese ones that look like they are made of cheese) and the timing of your sparks can be all over the place. The distributor advance curve is set out by those aforementioned rotating weights and vacuum advance so the curve is fixed and must be very conservative to avoid firing the spark plugs too early, again leading to detonation.

Adding a modern fuel injection system to your engine gets rid of the compromises. We still use vacuum to help us decide when to fire the spark plugs, and engine speed but now we measure the vacuum and decide exactly when we want to fire the sparks. Spark advance can be controlled to within 0.1 of a degree. If I tell the ECU that I want the spark plugs to fire at 17.1 degrees they are fired at exactly 17.1 degrees cycle after cycle.


The angle of the crankshaft is measured by the crankshaft position sensor. In my installation this is mounted on a bracket that bolts onto the engine case where the fuel pump used to go. The bracket serves two functions, it holds the sensor in the right place and it blocks off a now redundant hole in the engine case. The sensor is what is called a Variable Reluctance device. Without going into too much detail it is a very simple part, there are no electronics in there just a magnet with a coil of wire wrapped around it. As a result they are extremely robust and can withstand just about anything including temperatures up to about 200 Celsius. The sensor points at some teeth cut into the surface of the crankshaft pulley, not something bolted on, so there is nothing to go wrong. To create this, 36 teeth at 10 degree intervals are machined and one of the teeth is removed to make 35. We call this a 36 minus 1 arrangement. The missing tooth allows the ECU to calculate when the engine is at top-dead-centre (no1. piston at the top of its compression stroke). This might sound a bit complicated but it is a system used in thousands of modern vehicles. The advantage is there are no moving parts to go wrong or wear out.

The second important sensor in the injection system is the MAP sensor (Manifold Absolute Pressure) not to be confused with fuel and timing ‘maps’ that are an entirely different thing. The MAP sensor simply measures the pressure in the inlet manifold. Its name is slightly confusing as in a normally aspirated engine (i.e. not turbocharged) it doesn’t really measure pressure, it measures vacuum. When your engine is idling the pistons are trying to pull air into the engine but the airflow is almost completely blocked by the throttle plate, the engine is sucking against a closed throttle. This creates a vacuum in the inlet manifold. In my engine I see a vacuum of between 40 and 50 percent at idle, that is to say, the inlet manifold contains about 45% of normal atmospheric pressure. When you fully open the throttle the manifold is open to the outside world and the pressure within it increases to normal atmospheric pressure. Partly opening the throttle will give a number somewhere in between. The nice thing about this is the amount of vacuum we measure can be easily related to the amount of air entering the engine so it makes it very easy to work out exactly how much fuel to introduce. In my engine installation the MAP sensor is mounted at the top right in the engine bay, next to the ECU. It is connected to the inlet manifold via a 3 mm rubber tube. The MAP sensor is a very robust unit, still used in most modern vehicles. I use MAP sensors manufactured by Hella or Magnetti Marelli.

The engine only requires the Crankshaft Position Sensor and MAP sensor to operate. All the other sensors improve the way the engine runs but are not absolutely necessary.


Hopefully the function of the Throttle Position Sensor (TPS) is obvious it measures the angle of the throttle shaft. My first prototype throttle body used the TPS from a Ford Mustang. The Mustang sensor is a carbon track potentiometer. They are cheap to make but contain as the name suggests a carbon track with a wiper sliding across it. The wiper is attached to the throttle arm. By passing a current through the wiper we can work out its position on the carbon track and hence the angle of the throttle arm. That is all fine except that eventually the wiper wears through the carbon track and the whole thing stops working properly. I had to replace one of mine when some fuel got onto it, not great for something attached to an engine. I’ve sourced a TPS sensor that uses a magnet and a hall effect sensor. The magnet is attached to the throttle shaft, the hall effect sensor can precisely work out the position of the magnet and hence the angle of the throttle arm. The advantage of this is that the hall effect element can be sealed so no liquids can come into contact with it, also there are no sliding parts to wear out. In this installation the TPS sensor is used to work out when the throttle is closed hence when the engine should be idling and to sense when the throttle is opened so that the ECU knows when to inject extra fuel to mimic the effect of the accelerator pump in a carburettor. Some injection systems use the TPS sensor as a primary sensor to replace the MAP sensor. This can be a good technique in a race engine where we only care about full throttle operation but I‘ve found that the use of a MAP sensor to be preferable for low speed running particularly where an idle control valve is used.

Temperature sensors are employed to monitor cylinder head temperature and inlet manifold temperature. Cylinder head temperature is used primarily to monitor when extra fuel should be injected into the engine for cold running. Inlet manifold temperature is particularly important in the single point throttle body setup as it tells the ECU when the inlet manifold is at the correct temperature for optimum operation. The temperature sensors are simple devices that contain a resistive element. The electrical resistance of that element changes with temperature. The ECU measures the resistance to calculate the temperature at the sensor. The engine will still run without these sensors, it will assume a default value that will work in most conditions but which might use slightly more fuel than normal.

I don’t currently monitor oil temperature as it is not a control input to the ECU. I once used oil temperature to try to work out when the engine was up to operating temperature but it takes far too long for the oil to warm up in an air-cooled engine. Much better to use cylinder head temperature after all combustion takes place in the cylinder heads and that is what we are interested in. An oil temperature sensor can be added to your engine for a modest amount but in reality oil pressure is much more useful.


The single point throttle body is of my own design and has so far gone through hundreds if not thousands of hours of development and testing. A throttle body is a much simpler device than a carburettor, its main function being somewhere to house a throttle plate and a Throttle Position Sensor. All carburettors used by VW use bronze bushes to support the throttle shaft. Eventually these wear allowing extra air to enter the inlet manifold causing poor running. All of my throttle bodies are fitted with sealed stainless-steel ball bearings for longevity. They should never wear out but if you find a way to damage one they can be easily replaced. The temptation with all throttle bodies and carburettors for that matter is to fit units with far too large a throttle for the engine. I started out with a 40mm throttle on my 1600 cc test engine which turned out to be too large. I now fit a 38 mm throttle plate to the single point injection throttle body.

The accelerator cable connects to left hand end of the throttle shaft via something I call the throttle pull. Unlike the original VW part the throttle pull is radiused so that the throttle cable stays in the same position in the tube that goes through the fan housing. This does necessitate the use of a stranded throttle cable (not a solid one as fitted as standard). A suitable throttle cable is supplied with every kit, spares available upon request. The throttle pull also incorporates the throttle stop screw. The throttle stop screw should never require adjustment. The opposite end of the throttle shaft goes into the Throttle Position Sensor, see previous chapter.

I’m an engineer and I like things that are well engineered. I am really pleased with the way the throttle body has turned out. It really is a thing to behold, maybe you have seen it on my stand at a VW show. Thanks to proper ball bearings operation of the throttle is much smoother than the standard carburettor. The injector is well supported with no potential for fuel leaks. It attaches to the inlet manifold via 4, M6 bolts to reduce any chance of air leaks at that interface. I made the first throttle body myself but it took way to long to be economically viable I have now found a local, UK based company to manufacture the throttle bodies. They use a state of the art 5 axis CNC machine to turn a 3.5 inch block of aluminium into a finished throttle body in a fraction of time it would take me to make. Production throttle bodies will be anodised to keep them looking good. If you want yours anodised in a particular colour please ask.

The injector is mounted around the back of the throttle body. Fuel is supplied by a pipe to the top of the injector, an electrical connector goes to the ECU. I have spent many hours trying to find the best injector for this application and have settled on a Bosch injector with a flow rate of around 370 cc/min at 3 Bar. See development section for further details. There is much more to specifying the right injector than just finding one with the right flow rate. The spray pattern of an injector is critical. Some injectors create a fine atomised mist of fuel, ideal for use in an application where the injector is remote from the cylinder heads. Other injectors spray a jet of fuel that would only work well if they are aimed directly at an engine’s inlet valve.

If an injector is too large it will be very difficult to get your engine to idle properly. Everyone talks about how much power their engine makes in their literature, very few talk about how well it idles or whether it has any flat spots in its mid range. Personally I spend very little time at maximum power – most of the time I’m cruising at part throttle or sat stationary in traffic or at a junction. As such, I’ve designed my injection system to work well under all driving conditions.

My engine idles with an injector pulse width about 1.1 mS, the dead-time of the injector is about 0.9 mS so it is only open for 200 uS each time a cylinder takes in some air, that’s 0.0002 Seconds. If I had chosen a larger injector this time would be even shorter and it might be impossible to get a good idle. The injector I’ve specified will flow enough fuel to produce 85 horsepower with a 3 Bar fuel pressure regulator. If you want more power than that I can supply a 3.5, 3.8 or 4 Bar fuel pressure regulator. In the single point injection system ultimate power is limited by the castings that connect the inlet manifold to the cylinder heads. These are the only used VW parts I use on the engine. They will flow enough air for around 100 hp. Beyond that we need a more complex solution. Aftermarket end castings are available from some suppliers, some claiming to be able to supply more air than the stock parts. I have yet to test them but as with most things the quality looks poor compared with the original VW parts.

Showing position of Injector and Throttle position sensor

CAD model of throttle body and inlet manifold

To the left of the throttle body is a device whose function might not be obvious, it is an Idle Speed Control Valve (ISCV) its function is to replace the notched plate on a carburettor that is part of the choke mechanism. It allows a certain amount of air to bypass the throttle plate in order to control the engine’s idle speed. Most aftermarket fuel injection systems don’t incorporate an ISCV but in my opinion it is a vital part of the system. When the engine is cold it lets extra air into the inlet manifold to allow the engine to run a little faster than normal. It is under the full control of the engine ECU, there is even a test mode so you can make sure it is operating correctly.

There is another trick we can do with the idle control valve. Many VW’s suffer from cutting out when they come to a stop, often after sitting at high speed for a period of time. There are a number of causes for this that I won’t go into here. I’ve setup the idle valve to open very slightly when the throttle is closed. So, as you come to a stop at a set of traffic lights at the end of a slip road the engine will sit at 1000 to 1100 RPM for a couple of seconds to absolutely guarantee the engine won’t cut out. This is completely seamless and you don’t notice it in normal driving.

The throttle body requires the use of a bespoke inlet manifold. On my test engines this has been made from aluminium tube welded together by me, I am no expert TIG welder as you will probably recognise. Production units will be cast and incorporate a boss for the manifold temperature sensor. The inlet manifold does not incorporate the ‘heat risers’ present on original VW manifolds. This is for several reasons, not just because they are horrible and always seem to be clogged up with carbon. They are also not compatible with some exhaust systems, and I’ve also heard that there is performance gain to be had by removing them. If you should ever need to remove the inlet manifold and throttle body assembly is is now so much easier. All that is required is to remove the two nuts from the left hand inlet casting, the bolt to the centre of the engine case and the clips on the inlet manifold boots and the whole manifold slides out from under the alternator stand. When heat risers are fitted you have to disassemble half the engine to get the inlet manifold out.

Air enters the throttle body from the Air Intake. This unit combines warm air from around the exhaust with cold air coming through the air filter. Warm air is required to prevent icing of the inlet manifold, see the development section for further information. Air filters are supplied by Apexi. Unlike most aftermarket air filters they bolt on using four M6 bolts. They are washable and should have a life in excess of 30k miles. Replacements are available from a number of online suppliers or directly from 1584 Engineering.

In my split screen installation I’ve fitted a firewall between the fuel tank and the engine. This keeps the worst of the engine’s heat away from the fuel tank and gives me somewhere to mount the various components of the fuel injection system. I’ve designed brackets that don’t require any holes to be drilled into your vehicle. I will develop a similar setup for other vehicles as demand requires it. Later bay window vans should only require a simple plate to be mounted to their existing firewall. All of the following parts will come pre assembled to the firewall / mounting plate.

MAP sensor – see previous section

ECU – as discussed in the development section. The ECU is a sealed unit containing a single printed circuit boar (PCB), the ECU is the heart of the fuel injection system. It contains no moving parts or sensors so there should never be anything to replace or repair. The ECU is designed by me so if you have any questions or queries I am the person to talk to. If you do find a way to break it most parts can be replaced at a sensible cost, unlike the ECU in your every day car. Communication with the ECU is via wireless BlueTooth this is great, no wires from the ECU to your laptop. A software licence is available for $15 that allows you to monitor all the functions of your ECU from the drivers seat of your car on a wireless, Android based tablet. Each ECU has a unique identity and passcode so only you can access it.

Relay and fuse box – the injection system requires two relays to function. One controls the low current power supply to the ECU, the other controls the high current supply to the injector and fuel pump. The fuel pump is controlled by an electronic switch inside the ECU. This means neither relay should ever wear out (they are rated for 100,000 cycles so they should last a while). Separate fuses are supplied for the Injector, Coil, Fuel Pump and ECU. The only connections required to the original vehicle wiring are to the wire that used to go to the coil (this is used to turn the ECU and relays on and off) and the positive and ground to the battery. New battery cables are supplied in the fuel injection kit. The wiring to the alternator and oil pressure switch remains unaffected by the new engine installation. When the ignition key is turned off there is absolutely no current draw from the vehicle battery. This is important for any vehicle that might be stored for long periods of time.

Coil – The system uses a ‘wasted spark coil’. In a four stroke engine each cylinder only fires for every two revolutions of the crankshaft. The spark plug only needs to fire at the end of the compression stroke. Each piston will be at the top of its stroke at the end of the exhaust stroke and at the end of the compression stroke (piston cycle goes: intake, compression, power, exhaust). So how does the engine know whether it is at the top of an exhaust stroke or a compression stroke? It doesn’t need to. A wasted spark coil fires a pair of spark plugs at the same time, one will be at the end of its compression stroke the other at the end of the exhaust stroke. The cylinder that’s at the end of its compression stroke will burn its fuel and power the engine, the spark at the cylinder that’s at the end of its exhaust stroke is wasted, it does nothing hence the term ‘wasted spark’. Why do we do this? If we only wanted to fire the spark plug of the cylinder that’s at the end of its compression stroke we would need an extra sensor to work out which cylinder that is. By using a wasted spark coil I can do without that sensor (sometimes called a cam sync). What is the disadvantage of doing this? Nothing really – we waste a tiny amount of energy in the wasted spark, and wasted spark systems only work up to about 10k RPM which is way over what we need.

Fuel pump – for an installation into a van I would recommend a fuel pump that’s located outside the fuel tank. The fuel pump is one of the few components with moving parts so it will wear out eventually. This shouldn’t happen for many thousands of miles. You can opt for an in-tank fuel pump but when it wears out you will need to remove the engine before you can remove the fuel tank to get to the pump. I wouldn’t want to do that at the side of the road. In many ways an in-tank gives a neater installation so I will leave that decision up to the customer. For an installation into a Beetle, Type 3 or Trekker where the fuel tank is easy to access I’d recommend an in tank fuel pump every time. In these vehicles there is less space in the engine compartment so the more components we can mount elsewhere the better. My preferred fuel pump is standard fitment to many Mercedes cars all of which have engines producing in excess of 180 hp. They are manufactured by either Magnetti Marelli, Pierburg or Bosch. Most aftermarket installations use a Bosch 044 pump these are larger than the pump I’ve specified and consume significantly more power. I will only use fuel pumps with a threaded fitting on their outlet, no push-on tubes with a jubilee clip.

Fuel filter – I only use fuel filters manufactured by Bosch or Mahle. The filter is fitted to the firewall above the fuel pump. Connections are via quick release fittings to enable quick tool-free replacement.

Fuel pressure regulator – Every fuel injection system requires a fuel pressure regulator. The injector requires a supply of fuel at constant pressure. The regulator itself is manufactured by Bosch. It is installed into a housing with threaded fitting on its inputs and outputs. Most systems use a fuel pressure of 3 Bar (43.5 PSI). I use a 3 Bar fuel pressure regulator in the single point injection system. This gives sufficient amount of fuel for my 85hp 1800cc engine. If you want to put this system on a more powerful engine you might want to use a 3.5, 3.8 or 4 Bar pressure regulator.

Fuel pipes – Fuel leaks are bad news in any engine, fuel is expensive so we shouldn’t be wasting it. Also fuel leaks can be very dangerous. I use PTFE fuel pipes in my fuel injection system. PTFE is compatible with all fuels including pure Ethanol. It handles E10 fuel with absolutely no issues. The PTFE tube has a stainless steel braided cover for abrasion resistance this is then coated in a black PVC coating. This makes it durable and easy to clean. I use the tube with threaded 7/16 steel fittings. These are designed for use with hydraulic fluid at very high pressure so are easily capable of carrying fuel at up to 4 Bar pressure. The fittings are permanently swaged onto the tube to create a very robust installation. I’ve steered clear of the anodised aluminium fittings you see so often on weekend race cars. These are fine on a car where they are being expected regularly but they have been known to crack, you will never see them in an OEM vehicle installation.

Fuel tank take-off – A fuel injection system requires a fuel supply and fuel return to the fuel tank. Fuel circulates through the fuel pump, fuel pressure regulator and back to the fuel tank continuously. To enable this we need to add a return to the fuel tank. In every VW fuel tank installation fuel is drawn off the tank at the bottom. Certainly this is the cheapest way to do it but it does have one obvious problem. If that 60 year old piece of pipe that’s connected to the bottom of your fuel tank should split, the contents of the tank are going to be all over the floor and there is very little you can do to stop it. This is especially hazardous in a van where the fuel tank is right next to the engine which may be running at the time. I’ve developed a dip pipe arrangement that overcomes this problem. The fuel tank already has a hole for fuel level sensor, a second hole is made in the tank and the dip pipe assembly is installed. You then have three pipes coming out of the top of your fuel tank. A fuel outlet that goes to a dip pipe that reaches the bottom of the tank, a fuel return that returns fuel to the top of the tank and a breather that allows air into the tank (you need to allow air in if fuel is going to come out). Now if any of the pipes split the only fuel you lose is what is in the pipe. A one way valve stops fuel from syphoning out of the tank. All full engine kits for installation into a van will be supplied with a brand new fuel tank with this assembly installed. If you have just fitted a new tank, send it to me and I will do the necessary modifications and knock the cost of the fuel tank off the bill.

If you just want to make your van a whole lot safer I’ll sell you a fuel tank with the outlet moved to the top on its own. The cost is probably less than the excess on your insurance policy.