Welcome to our guide to what’s in a performance fuel system for a car, with the best tips, tricks and advice, how they work, and why to upgrade.
Ask even the most mechanically illiterate person to name one thing an engine can’t run without, and they will say “fuel”. And when you increase the airflow in an engine, you need to increase the amount of fuel it receives to match, or you’ll be in for a very big bang at some point!
But it’s not just a case of remapping the ECU and fitting bigger and bigger fuel pumps and injectors, there’s more to consider, like the flow capacity of your fuel lines, rails, and even whether you need a fuel swirl pot. Then there’s the age-old discussion of twin pumps, twin injectors and so on. What you thought was a simple upgrade turns in to something quite involved.
To be honest though, compared to some engine components, it’s is relatively easy to understand once you know what’s going on and why. So it’s our mission in this guide to show you round a car’s fuel system so you know exactly when, why, and how to upgrade it.
Although these are often advertised, they’re is simply not needed in this country unless you’re doing some endurance racing. It works in exactly the same manner as a radiator or oil cooler, and generally cools the spare fuel as it returns to the fuel tank from the fuel rail. The usual engine bay and fuel pump heat can indeed raise fuel temperatures, which can decrease power and increase the chances of detonation. However, unless fuel pump flow is far too high, the increase in heat rarely justifies a fuel cooler on a road car.
The fuel rail is where the injectors mount to the rest of the system. It’s a rigid metal bar with inlet and outlet connections, along with push fittings for the injectors themselves. There are very few options for upgrades on fuel rails, aside from going to a larger bore for higher flow. On some straight-six engines, they have inlets on both ends and a central outlet to provide a more consistent fuel flow.
FUEL FEED LINE
The fuel feed line connects the main pump to the fuel rail and is made of reinforced rubber hose, sometimes with a steel overbraid for extra strength. You can’t use normal rubber hose or even a fuel line intended for carb-fed engines, because the pressures involved (usually 3-6bar) will burst a standard item.
FUEL RETURN LINE
Excess fuel that the regulator bleeds off is sent back to the fuel tank via the return line. Although the fuel isn’t generally under any pressure once it’s gone past the regulator, the return line is usually made of the same material as the feed line.
The fuel pump’s job is, strangely enough, to pump fuel from the tank to the injectors. Modern fuel pumps are electrically powered, though on older cars they’re mechanically driven from the engine. Most modern fuel pumps can produce a maximum of around 9bar (130psi) fuel pressure, but this is far too much for most systems. This is why the fuel pressure regulator is there – to keep the pressure at the desired level.
Most standard fuel pumps can cope with fairly small increases over the car’s factory power level, but an upgraded pump is usually needed to keep up when power is significantly upgraded. Upgraded pumps come in a huge variety of shapes and sizes, some capable of flowing over 1,000bhp worth of fuel, so getting the right unit is never a problem.
On more highly modified cars, fuel swirl pots are often installed between the fuel tank and the main fuel line to the engine. A swirl pot is a small baffled tank, usually around 2ltr in capacity. It’s used to prevent fuel surge when cornering hard, and to make sure that there’s a constant supply of fuel to the main pump. Swirl pots are fed from the fuel tank by a secondary pump, and the fuel that flows through the fuel pressure regulator goes back to the swirl pot. In turn, excess fuel that gets fed into the swirl pot then goes through an overflow pipe and back to the tank.
FUEL PRESSURE REGULATOR
The pressure regulator sits on the outlet side of the fuel rail and maintains a set pressure by relieving excess fuel pressure into a return line back to the tank. They’re simple, spring-loaded valves that open at a particular point, typically 3.5bar on most engines, but all after-market regulators are fully adjustable.
On most pressure regulators, you’ll see a small vacuum pipe connection on the top, which will be connected to the inlet manifold. This is so the regulator can sense manifold pressure. By doing this, the regulator can raise and lower the fuel pressure in parallel with the manifold pressure, which makes tuning the car far easier. The regulator will lower the fuel pressure by 1psi for every 1psi of manifold vacuum. Conversely, the regulator will increase fuel pressure by 1psi for every 1psi of manifold pressure.
Fuel injectors are small electronic solenoids that the ECU opens at precise moments and for exact periods of time to supply the correct amount of fuel to the engine. They’re mounted between the fuel rail and head, and are held in place by being clamped between the push fittings on both parts. There are countless different injectors in existence, all capable of flowing differing amounts of fuel, enabling you to choose the most suitable injector for your application.
On race cars, and some highly modified and tuned road cars the standard fuel tank is removed and replaced with a fuel cell, which is basically a racing fuel tank. There are safety reasons for using them, because they’re stronger and less likely to leak in the event of a crash or roll. They can also be much smaller, or indeed larger, than standard, so can be placed in a different location in the car. Finally, they offer some performance advantages in the fact that they’re often baffled, negating the need for a separate swirl pot.
SINGLE OR TWIN PUMP?
When going for big power, it’s quite common to add a second fuel pump to maintain the required flow, but there’s a potential risk you should be aware of. If one fuel pump fails, the single pump will not be able to keep up the required flow rates, so pressure will drop, the car will run lean, and the engine will fail. If you had a large, single fuel pump capable of the required flow, you’d be in a lot less danger of engine damage, because generally a pump will simply stop working if it fails. The car will then just stall with no engine damage occurring.
In reality, with new fuel pumps and good wiring, you’re unlikely to come across this problem. However, when you have an expensive engine, it’s good to have all the safety you can get.
As you’d expect, this removes foreign objects that may block the injectors or damage the engine. It may be mounted before or after the fuel pump, depending on application. Most standard fuel filters, while being relatively small, can usually cope with fairly large increases in flow, but larger racing ones are available, and necessary, for high-power vehicles. One final thing to note about the fuel filter is to remember to change it! People often forget it exists and so don’t maintain it. It becomes clogged and has the potential to destroy the engine by running it lean.
TWO INJECTROS PER CYLINDER?
You may have noticed that some cars run two (and in rare cases, three) injectors per cylinder, rather than the conventional single injector. There are various reasons for this, from older engine management systems being able to control large injectors accurately, to the fact that secondary injectors placed further away from the inlet port increases atomisation and lowers inlet temps, potentially increasing power and torque. The increase isn’t massive, but when you’re building everything to optimum levels, it’s a relatively common modification.
The danger of running multiple injectors is the same as when you use multiple fuel pumps – the risk of a single injector failing. If a single injector per cylinder setup has an injector stop working, the car will almost always just run on three cylinders with no damage caused. But if the failed injector was one of a pair, the cylinder will run lean and it’s highly likely that you’ll melt the piston or blow the head gasket. Again, it’s unlikely with new components and well-built electrics, but the risk is still there.
In a race car you are either taking an engine to its power limits, creating serious cornering G-forces, or both, and these things signal the main changes on a race cars fuel system. A fuel cell and/or swirl pot is used to minimise any fuel surge or starvation when cornering hard, and twin fuel injectors per cylinder is often used to improve fuel atomisation to increase maximum power.
On a road system you want the maximum performance with the minimum fuss, and that means a single injector per cylinder and no external fuel swirl pot.
See more car tuning guides