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Fuel Injectors
There are two common types of fuel injectors, pintle and disc (Lucas style). Pintle injectors have a superior spray pattern to disc actuated injectors, but disc injectors are less expensive and generally flow large amounts of fuel easily. If possible, always choose high flow pintle style injectors, as fuel atomization at anything other than full throttle (high velocity port flow) is superior, leading to better drivability and economy. Below is a picture illustrating what I mean about the spray patterns…

Over the years I have done a great deal of research into this matter, in the form of reading and experimentation. I am also privileged to have access to very thorough research on this subject done by major motor manufacturers and component suppliers (e.g. Bosch, Land Rover, etc). Although I cannot pass it on directly, I can at least give you the distilled essence.

The standard RV8 pump is indeed rated at 3.0 Bar, although it is only operated at 2.5 Bar so there is a useful margin available. There are a number of effects of increasing the fuel pressure which must be considered, and the results for the pump and injectors are quite different.

Firstly, increasing the fuel pressure will obviously put more strain on the pump. The flow of fuel through the pump provides it's cooling, and flow drops generally in a non-linear manner with increasing pressure (usually logarithmic).

In general, increasing pressure will improve the atomisation of the injectors for pintle and type II/III designs but not for the Lucas disc type. All fuel injectors are for a specific operating pressure, or pressure range. The effects of increasing the pressure are heavily dependant on the design.

For the Lucas disc design, increasing pressure will increase the flow up to a point. The injector has an upper and lower disc. The lower disc has a hole in the centre (the metering orifice), and the upper disc is lifted by the electro-magnetic solenoid to allow fuel to flow.

It is quite obvious that the higher the pressure, the harder it is to lift the upper disc to allow fuel to flow. This results in a much longer and more unstable opening time for the injector. After 3.3 Bar the fuel flow will actually start to decrease and become more unstable as the electro-magnet struggles to overcome the force of the fuel pressure closing it.

This also requires that the battery voltage compensations in the ECU are adjusted accordingly, since they are dependant on operating pressure.

Additionally the Lucas injector does not produce an atomised spray through the metering orifice. This function is provided by the plastic diffuser underneath, which is supposed to break up the jet into droplets. Quite often there is no atomisation at all.

To show you what really happens I have a couple of pictures for you. Here is a picture of Bosch (left) and Lucas spray patterns side-by-side in the injector cleaning machine at TVR Power:



Now here's another shot done with a faster exposure, using the machine at Shropshire Auto Service. You can clearly see the large droplets in the stream from both the Lucas injectors on the left.



In the simplest terms, fuel droplets burn from the outside in. It follows that the smaller they are the quicker they burn, hence releasing more of the energy contained in the fuel. Ultimately this gives the most efficient use of the fuel, and the cleanest exhaust emissions. There is a minimum size for fuel droplets, but it is not seen outside of Formula One where no engineering effort is spared.

Ever more stringent emissions legislation has driven advances in fuel injector design. Improvements in manufacturing technology have produced some far more effective fuel injectors, with the aim of producing more efficient combustion. Some of the best designs are the Bosch Type II and Type III versions.

Fuel enters the combustion process by two methods for this type of manifold injection system. Firstly there is fuel which is pulled from the manifold walls by the air stream entering the combustion chamber when the inlet valve is open. This fuel is deposited on the manifold walls by the injectors when air is not flowing in the inlet tract. During this time the fuel deposited will evaporate in a warm engine, so the smaller the droplets are the better and faster they evaporate. Some fuel droplets remain in suspension in the air in the inlet tract, and again the smaller they are the better.

During recent testing on standard TVR 5.0 Litre engines, the injectors picked up 8 lb/ft of torque everywhere over a hand picked flow-matched set of brand new Lucas originals. This has been accurately and scientifically tested dozens of times, and is always the case.

Interestingly the improved torque required 2-3 Degrees less ignition advance, denoting improved combustion speed and efficiency. If you simply put in an injector with better atomisation, you will indeed see a drop in power because the ignition timing is now too far advanced for the reasons given above.

When mapping a car and selecting injector sizes, the injectors should run no more than an 80% duty cycle at any point in the map. Obviously the time available for injection decreases with rising RPM. This limit is necessary in order to allow for transient fuelling (sudden throttle opening), and proper response when the engine is cold amongst other factors.

Whilst it is true that the standard Lucas injectors can be operated on motors producing 350BHP, it takes them to 100% opening at pressures well outside the operating envelope. I have asked this before and have never been given an answer - can anyone please tell me what the duty cycle was THEY MEASURED on one of these 350BHP cars? I think not.

A slight protrusion into the inlet air stream is permissible and can help mixture whilst air is flowing in the inlet tract, although injection during this time is undesirable since it can produce a non-homogeneous mixture. However the depth of protrusion can be adjusted, since the legs of the fuel rail are rather bendy!

Keeping the mixture atomized in the cylinder is one thing, but conditioning the mixture prior to delivery is another story. Our experience has been that as the velocity of the air traveling through the port increases, the spray pattern should become more confined.
Fan-type spray patterns (pintle) are best suited for low rpm operation, therefore giving excellent mileage and emissions, but the fan pattern can get lost in a hurry as port velocity increases.
While I'm not all that keen on the laser-beam style pattern provided by the Lucas (RCE style) injectors for low speed applications, they do work pretty well with these high rpm engines, as their stream penetrates and mixes with the high velocity flow with ease.

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.....The Old One....