Fuel Injection: Introduction
So, what do you need to do to make the conversion?
First up, several changes are required in the fuel system for the conversion from carbs to EFI.
1) Addition of a fuel return line.
This is due to conventional EFI return-style fuel pressure regulators that regulate pressure by sending excess fuel back to the tank in a continuous cycle rather than a dead-head style regulator, often used with carburetors, that simply starts and stops flow. A return-style regulator offers benefits (over dead-head regulators) including a quieter and longer lasting fuel pump (less stressed), cooler fuel (recirculates), and more stable fuel pressure (consistent fuel volume delivery).
2) Addition of a fuel collector or accumulator
With the high volume of fuel circulating in a return-style fuel system it is important that the fuel pump inlet in the bottom of the tank doesn't become uncovered during cornering or acceleration when the tank is part full. If it does, air is introduced into the fuel system resulting in potentially catastrophic lean running of the engine (less fuel than expected is injected as mixed with air….). Production EFI cars typically have a collector unit inside the fuel tank with flap valves that prevent fuel escaping and which directly receive any returned fuel.
3) High pressure pump
Whilst carbs only require sufficient pressure to feed their internal fuel bowls (and not so much pressure as to prevent the float closing the needle valve when the bowl is full) fuel injection requires high pressure, as the injector itself is nothing more than an electronically controlled valve with a nozzle designed to atomize pressurized fuel into a fine mist.
4) Fuel regulator
Whilst with carbs the actual fuel pressure isn't overly important (other than ensuring that there is enough to get fuel into the bowls and not too much that the needle valve can't close when the bowl is full) a consistent pressure is essential for fuel injection. Any fluctuation in fuel pressure will directly affect the amount of fuel injected into the engine for a given pulse width at the injector (as will the pressure in the manifold, but this, unlike fuel pressure, will be measured by the ECU).
What I am planning:
The plan for my car is to replace my low pressure pump and filter with high pressure equivalents and to replace the dead-head regulator in the engine bay with a return-style unit. This will necessitate an additional fuel line the length of the vehicle (return line) which I'll fit alongside my existing one on the side of the centre tunnel (both are 3/8” aluminium hard-lines with AN-6 fittings to flexible hoses).
I'll solve the problems of fuel collector and return line at the tank by using an external fuel collector pot (sourced from Burton Performance). This collector pot has three connections, includes an integral fuel filter (I'll use another filter post-fuel-pump on the high-pressure side of the fuel system), and is designed to be gravity fed from the fuel tank. The upper two connections are to the fuel tank and the return line from the fuel pressure regulator. The third connection, at the bottom of the pot, is the supply to the high-pressure pump. As the pot is only emptied by the fuel consumed by the engine, rather than the fuel circulated (the return line feeds the pot rather than going back to the fuel tank), it gives a large reservoir of fuel for when the acceleration or cornering moves fuel in the tank away from the tank's outlet. A slight downside of this recirculation approach is that any heating of fuel (from the pump and engine bay) is localized to the volume of fuel in the pot rather than the entire tank, but I don't believe this will be significant. The upside is simplicity and easy retro-fitting.