Air And Fuel

April 11, 2013 By Mike Lee
Last updated on July 27, 2015

Ok we’ve decided to put these two together here (incorporating the induction system).

Air And Fuel Mixture (Ratios)

Also known as AFR – Air-Fuel Ratio. It is important to note that the correct amount of air and fuel is needed to run your engine without damage. A chemically balanced mixture is known as a stoic mixture, which is approximately 14.7:1 (14.7 to 1) mixture of Air to Fuel. Increasing the fuel ratio will make the mixture rich, decreasing it will make the mixture lean. Most cars will run with a stoic mixture when cruising, however stoic mixtures create lots of heat when combustion takes place, therefore, under load and acceleration, your cars computer/carburettor will richen the mixture slightly, which in turn is cooler in combustion and will help avoid detonation and damage to your engine.

Air Flow
Please see our separate article on performance air filters. The rest of this section will deal with the various induction components that follow the air filter, followed by the fuel section.

Air Flow Sensor
On electronic fuel injected cars the air filter will be attached to an air flow sensor, also known as an air flow meter, mass air flow meter (MAF), speed density sensor/meter and probably several other names.

The job of the air flow sensor is to sense and calculate the amount of air flow and it’s rate. It does this through either a hot wire MAF or vane air flow sensor. There are one or two other ways, but for now we will concentrate on these two most common types.

The Hotwire Mass Air Flow Sensor (MAF)
Air Flow Meter Air Flow Meter Mesh
This uses a wire to measure the air flow rate, it is protected by a wire mesh (see picture above) that stops dirt from getting onto the wire, which could damage it. Some tuners suggest removing this mesh to increase the air flow, which can be increased by around 2%. However you have to weigh up the chance of dirt damaging your MAF.

Vane Air Flow Sensor
Vane Air Flow Meter
This uses a swinging flap that is tensioned by a spring, this is more restrictive than the Hot-Wire, but also more reliable.

We would tend to leave these alone. If you do go for a larger air flow sensor however, don’t forget to make sure that your ECU can either cope with this or get it reprogrammed.

Throttle Body
Throttle Body
Many people increase the throttle bore by reducing the thickness of the inside wall of the body, although you will then need to replace the flap plate with a larger one. Ideally though the bore needs to be the same size as the air intake duct.

Larger throttle bodies are available, but may require different idle controls. Another thing you can do to increase air flow through your throttle body is to smooth away any rough surfaces and edges, just like when porting a cylinder head. Warning: fitting a larger throttle body will increase the throttle response to a degree that may make partial throttle movements jerky as smaller movements will be amplified. To get round this, multi plate throttle bodies are available. These have two plates, one opens at low rpm, while the other then opens when you accelerate harder.

More than one throttle body can also be added to a normally aspirated engine, one to each end of the plenum, but this creates other problems and fitting can be tight.

Intake Manifold (EFI)
(See carburettor section below for intake manifolds for carbs):
Intake manifold Plenum And Runner
An EFI manifold is known as a dry flow manifold (only air flows through it).

The plenum distributes the air to the cylinders. Stock plenums aren’t generally that efficient and some cylinders will end up running either a little lean or a little rich. The size and shape of the plenum is therefore important in determining the way the air is distributed.

The runners are tubes running from the plenum to the cylinder head. The length of the runners affects the amount of power at both high and low revs, while the diameter affects when peak power occurs.

Now depending on your cars use, you can alter the diameter to suit your needs. Although you will need to speak to an expert on what best suits you. Generally larger diameter runners will put the peak power to a higher rpm, but will reduce low rpm power (might be beneficial to a race car or even drag car). The same goes for the length of the runners, these have an effect of where the power comes in and for how long.

E36 Engine

Fuel Delivery
Your stock fuel system is adequate for your car, however when you start adding extra air flow, whether that be larger throttle bodies, intakes, performance air filters, turbocharger or supercharger, you will require more fuel. The duration and so the amount of fuel delivered through the injectors are controlled by the engine management system on cars so equipped. The EMS will take into account the amount of air flow, air density, engine load and temperature to calculate this. However sensors and the EMS will have a limited amount of variables and may need to be reprogrammed to account for increases in air and fuel flow, this will not be the case for a carburetted engine (see below).

Too much fuel = runs rich
Too little fuel = runs lean
Too much of either can lead to engine damage.

Fuel Pump
The fuel pump must be able to supply enough fuel beyond the maximum (but to a limit) required for full throttle application. Increasing the fuel pressure will require you to increase the fuel flow rate of the fuel pump. You can buy a larger fuel pump for track use, but for road use a second switchable pump added in parallel is more desirable.

Mechanical fuel pump
Mechanical Fuel Pump
(carburetted cars). Mechanical pumps are located on the engine block or cylinder head, they have a lever that is in contact with a lobe on the camshaft which pulls a diaphragm in the pump down, drawing fuel into the pump.

Electric fuel pump 
Electric Fuel Pump
(carburetted and fuel injection cars). These pumps create positive pressure and push the fuel along the fuel lines. Early fuel injected cars had fuel pumps outside the fuel tank, others had one outside and one inside and many newer cars now just have one in the fuel tank. Electric fuel pumps can replace mechanical pumps in older carburetted cars.

Fuel Filter
Clear Fuel Filter Fuel Filter
Just make sure you have a nice clean one, the filters can be removed and cleaned by reversing the flow of fuel the other way and then blowing through (you can use a small amount of compressed air).

Fuel Pressure Regulator
Adjustable Fuel Regulator
This regulates the fuel pressure. Adding more air as we have mentioned earlier requires more fuel at the right time. Turbocharged and supercharged, cars with additional boost will benefit from this, normally aspirated cars with increased air intake will also. Adjustable fuel regulators are available, but require setting up properly. Get a specialist to do it.

Fuel Injectors
Ferrari Injectors V12
Firstly, dirty or clogged injectors will make your car run badly. Have them checked or use an injector cleaner additive. If they’re really clogged or blocked up, you can remove them and put them directly in the additive for a good clean, although this can be a pain on newer cars, as usual you may have to remove half a ton of piping, wires and covers to get to them.

If you have made some major mods to your engine, then you may require larger injectors to cope with the added fuel demand, which in turn will require a higher fuel pressure. However you may not need larger injectors if your current injectors are adequate given just the additional fuel pressure.

Note: depending on how modified your engine is, you may need to get a modified or a completely different air flow metre to cope with changes to the fuel and induction system. With mild tuning you may be ok. Also don’t forget about the ECU, mild tuning maybe ok for self teaching ECU’s, but a reprogram may be required for the more modified engines.

Old Carburettor
There are literally hundreds of carburettor types, makes and models out there. One mistake many people make, is the same old going too big. All this does is give you a car that has reduced performance and bad throttle response. The size of the carburettor should be roughly based on the following formula:

(max rpm x Cubic Inches of engine) / 3456 x VE

The VE (Volumetric Efficiency) is the amount of air that your engine can draw in from its total capacity. So if you have a 302 cubic inch engine and it has a VE of 85% then the VE = 0.85 (257 cu).

As mentioned there are literally hundreds of types of carburettors to choose from, just be sure to get the right one for your application. They can be a work of art as well and make your engine bay a treat for the viewer, how about a set of triple Webers or more?

Intake Manifold
4 Barrel Intake Manifold
A carburettor manifold is a wet flow manifold (it has fuel and air flowing through it)

The same principle of smoothing and porting applies to carburetted manifolds as to EFI manifolds, however the way in which this is done is more critical as fuel is mixed with the air and therefore the air flowing through is heavier. Runner velocity is important and will affect the engines performance.

There’s a great deal of aftermarket choices for the V8 crowd here and even a few smaller engines as well.

Popular choices are the dual plane intakes as these have a high vacuum signal, which helps suck air into the cylinders. There are single plane intakes also, but it all depends what you’re after. Picking the wrong one will affect your engine power and driveability.

Jetting is the process of fitting different jets or altering the various jet adjustments required, unlike an EFI that has a computer to adjust fuel and other requirements, a carburettor has to be set manually. There are a number of jets to adjust, for such things as, altitude adjustment, temperature, fuel density, low-speed and idle mixture control, throttle valve control, low to mid throttle fuel mixture control, fuel mixture control for the midrange and for the three quarters open to full throttle conditions. As you can see it’s not as simple as plug and go. Although some aftermarket companies do try to set their carbs up to run straight out of the box, you have to be prepared to make adjustments for your particular engine mods.

Like all tuning upgrades, you need to match your carburettor (and its settings) for your intake manifold, camshaft, cylinder head and any other mods to each other and for the type of driving you do and the environment your car lives in.

MGC GT Twin Carbs, Intake and Runners

Final Air And Fuel Advice
Air Advice

  • Playing around with your throttle body, runner length/diameter is a specialist job, if you want it done properly, find someone with a flow bench and the right tools and knowledge

Fuel Advice

  • We would start at making sure your current system is in top condition and if possible get a baseline of its performance. Then replace what needs replacing and start with increasing the fuel pressure first

Both Air And Fuel Advice

  • As usual, it is worth looking at finding direct replacement parts from the same manufacturer as the engine you have. You maybe able to find parts that are of a higher spec than yours and will do the job nicely with little hassle in installing them
  • We always say this, but we’ll say it again, don’t get carried away, larger isn’t always better!
  • Make sure all other components are able to cope with the changes (sensors, ECU, airflow metre etc)
  • Understand that if you get the fuel and air mixture wrong, you may very well damage your engine, get it down to a specialist, get the right advice for your car and its mods done or to be done