Understanding Tuning!

[BoostedFC]

I pulled this from another forum I belong to & thought it would be useful here.

HELP/TUNING/TROUBLESHOOTING THREAD

NARROWBAND vs. WIDEBAND air/fuel meters and O2 Sensor Background:

O2 Sensors:

Most cars produced after the 1980s, and all since 1996, have at least one oxygen sensor. It is a part of the emissions control system and is also a part of the engine management system. Oxygen sensors help the engine run efficiently.
Different fuels have different stoichiometric values, e.g. methanol 6.4:1 and ethanol 9.0:1. Theoretically gasoline burns completely at an air to gasoline ratio of 14.7:1. This value is gasoline's "stoichiometric" value.

When the air-fuel-ratio (AFR) in the combustion chamber of your engine has less air than the stoichiometric value, then the AFR is said to be "rich," rich in fuel. If there is more air than stoichiometric, then the AFR is "lean," lean on fuel. The AFR variance indicates the deviation of the actual AFR from the theoretically required ratio for complete combustion. The value of this variance is represented by the Greek symbol called "lambda," and is calculated by dividing actual induced air mass by the theoretical air requirement.
Variations from the stoichiometric air-fuel ratio result in pollutants. Excess, unspent fuel in the combustion process results in hydrocarbons (HC) and carbon monoxide (CO). Excess air causes increased nitrogen oxides (NOx). Catalytic converters help reduce the HC, CO, and NOx emissions if the engine is operating around the stoichiometric AFR.

Oxygen sensors measure AFR post-combustion. They are positioned some distance down the the exhaust pipe in order to ensure that the sample they measure is representative of the AFR in the cylinders. Oxygen sensors can identify variations from the ideal AFR and tell the engine management system to adjust the ignition and injection processes accordingly. So what is the difference between narrowband and wideband O2 sensors?

Wideband O2 Sensors -- What is the difference from narrowband O2 sensors?

Narrowband O2 sensors are designed only to measure the stoichiometric air-fuel-ratio (AFR) for gasoline, i.e. 14.7:1. Wideband O2 sensors have a broader effective range of sensing. Narrowband sensors can only tell you when the AFR is 14.7:1. Although it can also tell you when you are richer or leaner, it cannot tell you by how much.
A wideband O2 sensor can. Designed to measure a broader range of AFR (9.65:1 to 20:1), Wideband O2 sensors are more effective instruments for tuning your engine. They can detect variations of the AFR better than stock narrowband O2 sensors. The result is that you can tune your engine and modify your management system according to your use and performance level. But who needs to measure a wide band of AFRs?

TUNING AIR/FUEL RATIOS AND TIMING:

Programmable engine management allows the selection of pretty well any air/fuel ratio and any ignition timing at any load and rev point. But what are the "right" settings that should be used? Here's a little guide to the settings that will give good results. Remember every car reacts differently to changes so this should just be used as a guide to get you in the ball park.
Before a programmable management system can be effectively tuned, the air/fuel ratio needs to be measured. The air/fuel ratio will need to vary in different conditions, and so the meter needs to be accurate across a wide range of ratios. While the oxygen sensor found in the factory management systems of all cars can determine rich/lean scenarios, it is not accurate enough to be used in the tuning of programmable management. Which was explained a lil more above with the types of O2 sensors.

A well-tuned engine used in normal road conditions has an air/fuel ratio that is constantly varying. At light loads, lean air/fuel ratios are used, while when the engine is required to develop substantial power, richer (ie lower number) air/fuel ratios are used.
Bosch state that most spark ignition engines develop their maximum power at air/fuel ratios of 12.5:1 - 14:1, maximum fuel economy at 16.2:1 - 17.6:1, and good load transitions from about 11:1 - 12.5:1. However, in practical applications, engine air/fuel ratios at maximum power are often richer than the quoted 12.5:1, especially in forced induction engines where the excess fuel is used to cool combustion and so prevent detonation.

There is no one air/fuel ratio where all emissions are minimised. At an air/fuel ratio of 14.7:1 oxides of nitrogen peak, while hydrocarbons and carbon monoxide (CO) increase substantially as the air/fuel ratio richens.

High Load

A naturally aspirated engine should run an air/fuel ratio of around 12 - 13:1 at peak torque. The exact air/fuel ratio can be determined by dyno testing, with the ratio selected on the basis of the one that gives best torque. Rich air/fuel ratios can be used to control detonation, and this is a strategy normally employed in forced induction engines. Thus, on a forced induction engine, the mixture should be substantially richer: 11.6 - 12.3:1 on a boosted turbo car and as rich as 11:1 on an engine converted to forced aspiration without being decompressed. As is also the case for ignition timing, the air/fuel ratio should vary with torque, rather than with power.

Most factory forced induction cars run very rich full load mixtures, with 10:1 being common. This is done for engine and cat converter safety reasons - in case an injector becomes slightly blocked, or the intake air temperature rises to very high levels. These cars will normally develop more power if leaned out. Note that emissions testing does not normally take place at full throttle, so full load emissions can be high without legal problems.
In the engine operating range from peak torque to peak power, a naturally aspirated engine should be slightly leaner at about 13:1, with the forced induction factory engine about 12:1 and an aftermarket supercharged engine staying at about 11:1.

Acceleration

During acceleration the engine requires a richer mixture than during steady-state running, with the extra fuel provided by acceleration enrichment. Under strong acceleration, the air/fuel ratio will typically drop 1 - 1.5 ratios from its static level. The amount of acceleration enrichment that is required is normally found by trial and error, and this is best done on the road rather than the dyno. The acceleration enrichment should be leaned out until a flat spot occurs, then just enough fuel to get rid of the flat spot should be added. This approach usually gives the sharpest response. Note that both over-rich or over-lean acceleration enrichment will result in flat spots, and that a greater amount of acceleration enrichment is needed at lower engine speeds than higher speeds.

Over-run/Deceleration

In road-going vehicles, deceleration enleanment is used to reduce emissions and improve fuel economy. This normally takes the form of injector shut-off, with the shut-off often occurring at mid-rpm (such as 3000-4000 rpm) and the injector operation re-starting at 1200-1800 rpm. High rpm injector shut-off can, in some cases, have the potential to cause a momentary lean condition.