One of the keys to achieving power is the exact ratio of fuel for a given quantity of air. Whether too much or too slight fuel results in an engine that is down on power, delivers poor economy and has a big ask mark against its long term durability. The primary job of the engine tuner is to ensure that the engine administration law has been programmed to deliver fuel in the exact quantities at all times.
Fuel aggregate is measured in units of Whether Lambda or air fuel ratio (Afr). Also referred in tune-speak as Whether lean (small amounts of fuel to air) or rich (large amount of fuel to air). What is the exact mix of fuel and air? While there is no such thing as an absolute over the board setting, the ordinarily agreed "golden rules" of fuel mixtures are:
Manifold Absolute Pressure Sensor Operation
1. At low power outputs (no boost) catalytic converter qualified cars require a fuel aggregate of Lambda 0.99 (Afr 14.64) for best emissions, and is the target fuel aggregate when the Oe Ecu is operating in complete loop. Improvements in fuel economy can be made by leaning fuel mixtures off to Lambda 1.05 (Afr 15.4 to 1), but this is done at the price of cat converter operation and will increase exhaust temperatures when cruising.
2. Medium power outputs (the transition in the middle of on and off boost) sees best power achieved at colse to Lambda 0.89 (Afr 13.1 to 1).
3. At high power outputs things get interesting, as this is the area where the many possible for engine damage exists. Version I to Vi engines with stock internals running colse to 1.2 bar boost typically make good power reliability at Lambda 0.78 (Afr 11.5 to 1). At the same boost pressure and with stock internals, Subaru Wrx Version Vii and later models have redesigned cylinder head combustion chambers for a very different set of burn characteristics and fuel distribution compared to the old model, dictating significantly richer fuel mixtures of colse to Lambda 0.75 (Afr 11.0 to 1) or lower to accomplish best power safely.
Typically, air fuel ratios can be leaned slightly for a small increase in power on engines qualified with forged pistons, due to their impel and greater heat resistance when compared to Oe cast pistons.
Measuring Fuel Mixtures
There are only two ways to check fuel mixtures accurately; the adored recipe is to use a high potential aftermarket wide band air fuel ratio meter in conjunction with a five wire Bosch Lsu type lambda sensor. This type of sensor and meter can accurately portion fuel mixtures from Lambda 0.69 (Afr 10.1 to 1) to Lambda 1.30 (Afr 19.1 to 1) and beyond. Hence the name wide band sensor.
An alternative recipe for cars with flash compatible Ecus, fuel mixtures can be read directly from the Ecu data stream using Delta Dash, with a few exceptions. The Oe lambda sensor can "see" lean mixtures well, but cannot portion fuel mixtures richer than Lambda 0.76 (Afr 11.2). Additionally, at power outputs higher than stock, sensor placement is an issue, as exhaust back pressure in the middle of the engine and turbocharger causes a considerable reduction in sensor accuracy.
Any other recipe of measuring fuel mixtures such as cheap Diy meters, reading tea leaves or consulting a psychic have no place in modern high performance engine tuning.
comprehension The Subaru Wrx Fuel law