Conversion of an existing carburettor or plenum-based installation to throttle body injection is relatively straightforward provided that you fully understand what is required for the installation.  If you are replacing carbs then you will need the following parts:

  • An EMS

  • A baffled fuel tank

  • A high-pressure injection fuel pump

  • A fuel pressure regulator

  • Some injectors of the right capacity

  • The appropriate ‘snap-on’ connectors for the injectors' wiring

  • A configuration of throttle bodies (optionally with manifold)

  • A throttle linkage

  • A throttle position sensor (usually supplied with the EMS)

  • A coolant temperature sensor (usually supplied with the EMS)

  • An air temperature sensor (usually supplied with the EMS)

  • A fuel rail (often included with the throttle bodies)

  • Air horns and air filter

  • Plenty of high-pressure rubber fuel hose and clips

  • Some 8mm fuel pipe

  • Patience and a sense of humour.

If you are converting from an existing plenum-based injection system, then you may not need to convert your fuel tank and can usually retain the fuel pump, injectors, fuel rail and pressure regulator.  Quite often the throttle pot and coolant sensor are also re-usable especially with plug-compatible EMS replacements.

Fuel Tank

The main factor to consider when converting from carburettors to injection is the fuel delivery system.  The fuel tank is the first link in the fuel delivery chain.  A normal un-baffled fuel tank is not suitable for an injected engine since under the influence of the various ‘G’ forces encountered in a moving vehicle, the fuel can move away from the tank pick-up and cause the fuel pump to suck air.  With a carburettor-based system the carb has a float chamber from which the fuel can be drawn if the pump supply dries up. An injection system on the other hand has no such reservoir.  If the supply of fuel to the pump dries up then the engine will cut out due to lack of fuel.  This is exacerbated by the fact that the fuel pump runs all the time with an injection system, with surplus fuel being diverted back to the tank via the pressure regulator.

There are two ways of counteracting this fuel starvation.  One way is to compartmentalise the tank, i.e. build a compartment around the pump's inlet which is fluid-tight and use one-way valves that allow fuel in to the compartment but not out again.  This keeps the fuel in the area of the pump inlet, which also can be supplemented by fitting a small conventional auxiliary pump that can shunt fuel from the opposite end of the tank to counteract the effects of fuel-surge.  The other way is to use a fuel reservoir or surge-pot that holds a litre or so of fuel, and supplies the pump regardless of the fuel situation in the tank.  This is fed by a small pump from the tank or by gravity, and is sufficient for several seconds of engine activity.  Ensuring that the fuel returned from the pressure regulator is directed at the pump outlet can also minimise the effects of surge in the fuel tank.  You cannot convert to injection and not pay attention to your fuel tank - it absolutely must be baffled and compartmentalised, or fitted with a surge-pot.

Fuel Pump, lines and regulator

An injection fuel pump is very different to a conventional fuel pump used to supply carburettors.  Firstly it runs all the time and does not ‘stall’ as a conventional pump does when the float chambers are full.  It also supplies fuel at a much higher pressure than a normal pump at around 80-100psi compared with 5-6psi.  It is also essential that the pump be fed by gravity, since an injection pump is designed as a ‘blow’ pump rather than a ‘suck’ pump.  The requirement to gravity feed the pump normally means that it has to be mounted underneath and adjacent to the fuel tank, so a fused power supply is required to be run into that area.  Since the fuel is continuously delivered and returned to the tank, two fuel pipes are required - a supply pipe and a return pipe.  Normally the existing fuel line can be used as the return pipe with a new line laid in for the supply.  When plumbing in the pump it is absolutely essential that high-pressure fuel pipe is used.  Normal rubber hose will not do.  It will burst and is a fire hazard.  Ensure that you only use properly rated hose capable of withstanding in excess of 60psi.  The inlet to the pump is normally 12mm internal size, so the spur from the tank must be this size also.  The remainder of the fuel pipe can be 8mm copper or steel tubing.  Ensure that the ends of the tube are ‘flared’ to help the integrity of any joins.

Injection pumps are noisy so make sure that you mount your pump in a cradle of some kind suspended by rubber cotton reels or wrap it in some sound deadening material before mounting.  Don’t take chances with the pump - it must be properly insulated and leak free.

Injection pumps require that the fuel be filtered before it reaches the pump.  In some cases this is not easy to arrange.  However any dirt or rubbish entering the pump can and will cause it to lock solid and render it permanently inoperable or damaged.  Where space is limited, a fine wire-mesh screen can be used in the inlet to the pump provided that it is fitted in such a way that it cannot enter the pump.  This will screen any reasonably sized particles.  If you are using this method ensure you clean/change the screen regularly and fit a proper fuel filter following the pump.

There are plenty of injection pumps to be found in the scrapyards - most vehicles post-1989 are fitted with injection systems and are a good source of pumps and injectors.  If you select a vehicle with a suitably sized engine then the pump should be up to the job.  It's likely that the injectors won't be far out either.  It is quite possible that the fuel pressure regulator might be suitable assuming that it is not integrated with the fuel rail.  My pump injectors and pressure regulator came from a broken Sierra Cosworth.  Alternatively you can source the pump from a motor factor or specialised supplier.

Induction system

If you already have twin Webers or Dellortos fitted to your engine then the obvious choice of induction system is a flange compatible throttle body kit such as the TB throttle bodies from Jenvey.  These will bolt on directly in place of the similar styled DCOEs or DHLAs.  If you have IDA or IDF Webers then the TF bodies are flange compatible.  If your engine is not already equipped with dual sidedraught/downdraught carbs then you must make the appropriate selection of either dual or single throttle bodies with an appropriate manifold and air horns/filters.  I have had some success having back-plates made to take the dual ITG filter on the end of a set of air horns attached to either dual or single throttle bodies.  This makes a nice neat installation.  If you are using the parts retained from a carburettor set-up then you can re-use the filters and back-plates.  If you cannot obtain a suitable manifold for your engine then it is possible to fabricate one.  Some throttle bodies will bolt directly to the cylinder head notably some of the range produced by Jenvey.

If you are upgrading from a plenum based system then you may find that you can re-use the fuel rail, injectors, pressure regulator and throttle position sensor.  This will save money and aggravation.  Some ingenuity may be required in the fabrication of brackets to attach the OEM components to the new throttle bodies but it is not a difficult task.

When buying the throttle bodies you must also purchase a throttle linkage since the type used on twin sidedraught carburettors is not suitable and cannot be used.  Generally throttle body kits come complete with fuel rails that are designed to take the standard Bosch type of injector.

Air horns are generally necessary and the main limiting factor for length is the space available on the inlet side of the engine.  Measure carefully here to ensure that what you are buying will fit.

The throttle potentiometer is normally fitted to the end of the spindle on one of the throttle bodies, ensure that it is fitted so that it is opening and not closing, e.g. against the spring tension.

Plumbing in

After running the fuel line as close as possible to the end of the fuel rail, the plumbing in is a simple task.  If you are retaining an existing fuel rail arrangement then it should simply be a matter of bolting on the rail and connecting as before.  When fitting a new rail it is important to ensure that the injectors are properly clipped to the rail and that the rail when fitted holds the injectors firmly into their position in the inlet manifold or throttle body pockets.  The fuel supply should be connected to one end of the fuel rail with the pressure regulator connected to the other.  The outlet of the pressure regulator is then connected to the fuel tank return pipe.  The return pipe should dump its fuel as close as possible to the pump outlet in the tank.


Generally the only things to connect are the fuel pump which requires a fused supply that is switched by the ignition, the throttle potentiometer which is connected to the EMS, the coolant and air temperature senders that are again connected to the EMS and the injectors themselves.  Finding a place for the coolant temperature sender is not always easy but often it is possible to drill and tap an existing boss somewhere on the engine which must be the engine side of the thermostat, preferably in the head.  The air temperature sender should be mounted as near to the inlet trumpets as possible.

Depending on the type of injection - batched, grouped or sequential, the injectors may be wired in parallel or in series.  Follow the instructions which come with the EMS to make sure that you do this correctly.  It is a good idea to bolt the throttle bodies to a dummy manifold (a piece of angle iron suitably drilled with a few correctly spaced holes will do) in order to make the injector loom and fit and adapt the throttle linkage and other ancillaries.  Doing this while the bodies are not attached to the car is much more convenient as it makes the set-up more accessible.  Any problems that arise with linkages, air horns, wiring etc. can be much more easily solved.  Depending on resistance some injectors will need a resistor in series in order for the EMS to fire them correctly.  Ensure that this is mounted and connected correctly.  When this has all been fitted satisfactorily, all that remains is to power on the pump and ensure that it is circulating fuel before starting the mapping process.