Fuel systems

"Improving fuelling is a major part of tuning."

Cars need a precise mix of oxygen and fuel to burn effectively.

Much of the engine tuning done by TorqueCars members involves increasing the cars capacity to burn the air and fuel mix, which must be supplied in an exact ratio.

Much attention is generally given to the air intake and we have discussed the various aspects of forced induction.

We should not overlook the fuelling as it is essential to keep the mix right.

Power is lost when the engine adjusts itself to stop it running too rich or lean.

Many power losses, which manifest themselves as flat spots, at certain points in the rev band can often be traced to a fuelling issue.

Adding bigger fuel injectors will not make more power. That would be like adding a kilo of salt to your meal expecting it to taste better.

Fuel has to be matched to the air flow in a very precise ratio, for example the petrol ratio is 14.7 to 1. (Every 14.7 grams of air will require 1 gram of fuel.)

  • Petrol 14.7:1
  • Diesel 14.5:1
  • Ethanol 9:1
  • Hydrogen 34.3:1

A car used a lambda sensor to detect oxygen in the exhaust, and this indicates whether an engine is ok or needs trimming because it's running too lean or too rich.

SAFETY NOTE: Manufacturers have started to introduce direct fuel injection in engines. This is where the fuel goes straight into the cylinder rather than being sprayed into the incoming air stream. Diesel engines first used this technology but many car makers are using direct injection on petrol engines in a bid to meet tight emissions standards. These direct injection systems operate at very high pressures and should only be maintained by a qualified mechanic.

When pushing more air into an engine you should also match it with more fuel. Otherwise you will be running too lean and will miss out on the potential performance.

Components of a fuel system

  • Fuel pump - This is the essential and sucks the fuel from the tank piping it to the injectors.
  • regulator valve - This adjusts the amount of pressure in the fuel system and directly relates to the throttle (accelerator) position. More throttle requires more fuel.
  • Injectors - these squirt the fuel as a fine mist allowing it to mix with the intake air charge.
  • Intake manifold - This is where the fuel mixes with the air before entering the cylinders. (Although manufacturers are now perfecting direct injection where fuel is sprayed directly into the cylinders.
  • Return pipes - Excess fuel is returned to the tank. This process is carefully controlled and ensures that irregular pressure fluctuations do not happen.
  • Fuel lines - these carry the fuel from the tank to the engine and back again if not used. Typical OEM fuel lines are only rated to 55psi, direct injection runs higher than that.

Fuel Pump improvements

If the fuel pump is not man enough for the job the engine will lose fuel pressure at high revs. Fitting additional fuel pumps or larger fuel pumps will help to ensure a good delivery of fuel at the engine.

Some cars experience starvation on long bends where the fuel pick up is on one side of the tank.

If the tank is running low the fuel will flow to the other side of the tank causing the pickup to run dry.

This was noted on some early Subaru Imprezas although most will now have  an uprated fuel pick up to address this issue.

Fuel pressure valves

Regulator valves are not known for their speed or smooth operation, especially when they get old.

On earlier fuel injected engines, fitting an uprated fuel pressure valve will greatly improve throttle response. Owners report a much snappier response when they put their foot down and problems such as surging and hesitancy in traffic are also improved.

Some drivers also report more top end power although theoretically the amount of fuel delivered is the same. This could be attributable to the improvement over a standard sticking hesitant fuel valve though.

Modern direct injection engines are fairly efficient but improvements can be achieved by altering the ECU mapping.


Over time the injectors will clog up. This will reduce their ability to create a fine mist or fuel which mixes easily with the air. Instead a dribble of fuel is delivered which is then only partially burnt.

A fuel injector cleaner used regularly can really help to keep them operating at peak efficiency and TorqueCars fully endorse the BG44k product.

Choosing fuel injector upgrades

Injectors are rated by cc - the amount of fuel they deliver. When substantially increasing engine power it becomes imperative to uprate the injectors.

Look to other models in your manufacturers range for suitable uprated parts. A turbo charged 1.6l engine will easily require as much fuel as a naturally aspirated 2.5l engine.

Generally you should over specify a fuel injector capacity to give a little overhead to play with and we're sure you'll be adding some more power mods over time.

We note that the safe duty cycle (time the injector is open between each bang phase) is generally 80% of it's rated cc (a highly tuned motorsport engine will be approaching 85%) so although the flow rate per minute might be 100cc it's only running 80% of the time so you would not have enough fuel if your engine needed 100cc per bang cycle.

As they get older they can struggle and cars sometimes require a little more fuel at times, and under certain load.

An efficient turbo charged engine will make more power per unit of fuel than an inefficient heavy v8 engine. This requires a different fuel injector capacity and is called the Brake Specific Fuel Consumption.

injector size in lbs./hr. =  (Max. HP x BSFC) / (number of injectors x duty cycle)

In any case running a 50cc injector on an engine with a peak 50cc fuel requirement is not a good idea.

Other factors when tuning fuel intakes

Manifold shape is critical. The grooves and bumps on the surface provide more area to aid the evaporation of fuel into the intake charge mixture.

For this reason you should avoid the temptation to polish the intake manifold, concentrate instead on the head of the engine and exhaust outlet if you have an overwhelming need to polish something.

Return pipes and fuel filters can get clogged up. This can cause all manner of problems in the fuelling and generally cause surges and make the engine run too rich. All fuel lines should be inspected and renewed if they show signs of fatigue.

When increasing the fuel pressure the fuel lines are typically the first thing to go. If they are not in perfect condition the extra pressure will highlight the weak spots and cause a leak.

Air locks in the fuel lines are very undesirable and pumps can burn out or fuel starvation can happen. Never run a car with fuel injection on empty, the damage and inconvenience caused is really not worth it.

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One Response to “Fuelling uprating the fuel system”

  1. TCJBOLDIE says:

    It is perfectly fine to polish the inlet manifold UPSTREAM of the injector and it is fine to deburr the manifold downstream of the injector but ensure you do not use a too fine a stone of flapwheel as there needs to be marks similar to a cylinder hone marks to promote better atomizing of the fuel air mix for better moire complete combustion and power

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