One of the keys to achieving power is the correct ratio of fuel for a given quantity of air. Either too much or too little fuel results in an engine that is down on power, delivers poor economy and has a big question mark against its long term durability. The primary job of the engine tuner is to ensure that the engine management system has been programmed to deliver fuel in the correct quantities at all times.

Fuel Mixture is measured in units of either Lambda or air fuel ratio (AFR). Also referred in tune-speak as either lean (small amounts of fuel to air) or rich (large amount of fuel to air). What is the correct mix of fuel and air? While there is no such thing as an absolute across the board setting, the generally agreed “golden rules” of fuel mixtures are:

1. At low power outputs (no boost) catalytic converter equipped cars require a fuel mixture of Lambda 0.99 (AFR 14.64) for best emissions, and is the target fuel mixture when the OE ECU is operating in closed 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 expense of cat converter operation and will increase exhaust temperatures when cruising.

2. Medium power outputs (the transition between on and off boost) sees best power achieved at around Lambda 0.89 (AFR 13.1 to 1).

3. At high power outputs things get interesting, as this is the area where the greatest potential for engine damage exists. Version I to VI engines with stock internals running around 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 previous model, dictating significantly richer fuel mixtures of around Lambda 0.75 (AFR 11.0 to 1) or lower to achieve best power safely.

Typically, air fuel ratios can be leaned slightly for a small increase in power on engines equipped with forged pistons, due to their strength and greater heat resistance when compared to OE cast pistons.

Measuring Fuel Mixtures

There are only two ways to check fuel mixtures accurately; the preferred method is to use a high quality 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 measure 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 method 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 measure 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 between the engine and turbocharger causes a significant reduction in sensor accuracy.

Want to release the true potential of your turbo charged Subaru? Subaru tuning for WRX & STI performance Upgrades is our passion at Scooby Tuning. We offer Subaru ECU tuning for WRX, STI & Legacy GT open source road tuning, as well as remote etunes, designed to bring out the bottled up power in your Subaru with safe and effective tuning.

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