The term ‘fuel injection’ was once a major marketing tagline, bandied about by car manufacturers with great promises of performance and efficiency. A marvel of modern technology that would not only make our cars faster but would make them more reliable and cheaper to run. We think of fuel injection as a modern development but it can be found on some production road cars as early as the 1950s. It wasn’t until after the oil crisis of the early 1970s that fuel injection would start to become commonplace and by the 1980s the humble carburetor was well on its way out.

If you own a classic car from the 1970s onwards, there is a good chance it has fuel injection, particularly if it is German. Mercedes, BMW, and Porsche were all at the forefront of its evolution. And if you own a modern classic from the 80s or 90s, it will almost certainly be fuel injected. In this installment of Let’s Get Specific, we’ll cover some basics of fuel injection: How it works, the main types, and how to get the best out of it. Just to be clear, this article refers to gasoline engines only, not diesel. Let’s dive in.

What is fuel injection?

First things first, you need to understand that all engines require a certain ratio of air to fuel, and that this ratio of air to fuel needs to be maintained under all driving conditions whether you’re idling at a light, cruising on the highway, or wide open with the gas pedal buried in the carpet. The ideal ratio is 14.7 parts air to one part fuel. Too much fuel and you’ll be running rich, not enough and you’ll be running lean. Now we’ve covered that bit, I'll explain the benefits of fuel injection by first taking a quick look at carburetors.

Before fuel injection rose to prominence, our fueling concerns were served by carburetors. On the one hand, a carburetor is a finely-tuned instrument, but it is also a rather crude and simple device. It relies upon negative air pressure passing through a venturi to draw fuel out through a measured “jet.” In simple terms, the size of the jet, or in some cases the size of a metering needle, determines the amount of fuel allowed to enter the airstream. Most carburetors only have two fuel circuits with which to maintain an optimum air to fuel ratio under all driving conditions. The problem is that they can’t do this perfectly. They might get close, but a carburetor will always be compromised; there will always be a point in the RPM range when the engine is running a little lean or a little rich.

Images courtesy of Daniel Lackey

Fuel injection was conceived to address carburetors’ shortcomings. The basic principle is that instead of the fuel being passively drawn through a fixed jet, it is injected into the airstream in a precisely measured dose. That way, the correct amount of fuel can always be delivered and the air to fuel ratio precisely maintained. If the engine needs more fuel, the injector delivers more fuel; if it needs less, the injector delivers less. This is precisely why fuel injection provides greater efficiency and performance. The injectors on your classic engine are controlled by one of two systems, mechanical and electronic. Let’s take a look at the differences.

Mechanical vs Electronic

Mechanical fuel injection (MFI) is an older system and can be found on cars from the 50s, 60s and 70s. MFI uses passive injectors and a mechanically driven metering unit to control the quantity of fuel being injected. Both Mercedes and Chevrolet were early adopters of MFI back in the 1950s. By the mid-60s race teams around the world were running it and by the late 60s mechanical injection was becoming commonplace on sports and luxury cars around the world. In Britain you’d find the Lucas injection system on your Triumph TR5 for example, in Germany a Kugelfischer system on your BMW and in the USA, you might find a Rochester system on your Corvette. Although MFI delivers fuel more accurately than a carburetor, it does have one major limitation. It has no intelligence. The metering unit is a fixed mechanical device and, much like a carburetor, is unable to alter its fuel delivery in changing conditions.

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Images courtesy of Daniel Lackey

By the mid-70s, mechanical injection was being phased out in favour of electronic fuel injection (EFI). The EFI system has active injectors that are triggered electronically, doing away with the mechanical metering unit and instead using a computer or ECU and a range of sensors to determine how much fuel is required.

EFI began to emerge in the late 60s and early 70s from the German manufacturer Bosch. Early systems used a mechanical airflow meter and air pressure and temperature sensors to determine the amount of fuel required. In 1979, Bosch introduced the Motronic system, the first to have a digital engine control unit (ECU). The great advantage of EFI is that the fuel delivery can be consistently altered according to information from various sensors in and around the engine. Sensors measuring air flow, temperature, manifold pressure, and even the mixture of burnt gases in the exhaust (lambda). An ECU can process a great deal of information and adjust the fuel delivery accordingly.

How to maintain a healthy fuel injection system

Most injection systems are very reliable, even the old ones. The biggest cause of problems is not the injection system itself but the general health of the engine and the rest of the vehicle. Owners can be quick to blame the fuel system when things go wrong but very often the problems lie elsewhere. Always ensure your ignition system is in perfect order. Fresh spark plugs, HT leads, coil, contact breakers, and condenser will go a long way to improving your engine’s performance. This should go without saying but only fit the best parts from reputable manufacturers. Avoid buying cheap ignition components.

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Images courtesy of Daniel Lackey

Consider the condition of your fuel tank and fuel pump. These cars are getting old now and a lot of fuel tanks have a build up of rust and other fine particles just waiting to block your injectors. Fuel injection systems require high fuel pressure, 60 to 100 psi for our classics. If your old pump can’t deliver the pressure then your engine's performance will suffer. Ensure that all flexible fuel hoses are in good condition and replace any that look at all dry or cracked. Lastly, make sure that all gaskets and o-rings are doing their job. A small air leak from a bad gasket, o-ring, or worn out throttle body can cause problems, as can dirty air flow meters. It is worth mentioning here that modern fuels have higher levels of ethanol that can also be detrimental to older rubber components. It may become necessary to upgrade hoses and o-rings with items that are compatible with modern fuels.

If everything above is in tip top condition and you still have an issue, then it’s time to consult a professional. Generally speaking, fuel injection systems fall within the remit of specialists, so your local mechanic may not be able to help. For MFI systems, new parts are rarely available but there are specialists who can restore worn out metering units or test and rebuild your injectors. Electronic systems are more reliable but you must ensure that every sensor is doing its job. There are injection specialists who can test the spray pattern of your injectors and either rebuild or replace them if they are faulty. Similarly, old ECUs can be repaired or replaced, but you must ensure the replacement is correct for your model of car.

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Images courtesy of Daniel Lackey

Getting the best out of your fuel injection: tuning and upgrades

All fuel injection systems will run at their best when they, and the rest of the car, are well maintained. But what do you do if you want more performance? One downside to MFI systems is that they are not easily tunable. Most will have a very simple mixture adjustment to bring the entire fuel curve from lean to rich. Its intended use is to compensate for ambient air pressure, for example, if you live in the mountains or at the beach, this mixture adjustment may need to be optimised. If you want to make performance upgrades, such as hotter camshafts, then the fueling has to be altered. With MFI, the only way to achieve this is to alter the fuel curve by modifying the metering unit, a task that is highly specialised. Although tuning is more straightforward with EFI, it will still require an expert who can access the ECU and make the necessary changes. Alternatively, it may also be possible to replace a factory ECU with a modern stand-alone unit that can be properly mapped to suit your engine.

Fuel injection is a huge subject but hopefully this introduction will shed some light on what's going on under the hood. A better understanding of your classic will only help to empower you as an owner and enrich your experience.

Want to keep reading? Check out:

• Let's Get Specific: Suspension

Meet our contributors

Daniel Lackey is a long-time photographer, automotive writer, and Porsche Carrera Cup GB Team Manager for CCK Motorsport. Needless to say, he keeps pretty busy when not contributing to Marqued!

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