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Home GlossaryAir-Fuel Ratio (AFR)

Fuel & Electronics

Air-Fuel Ratio (AFR)

The mass ratio of air to fuel during combustion. Stoichiometric (theoretically perfect) gasoline AFR is 14.7:1. Tuned JDM turbo engines run rich (11.5:1 to 12.5:1) under boost to lower combustion temperatures and prevent detonation. E85 stoichiometric is 9.7:1 — more fuel volume but cooler burn.

Related Terms

Detonation (Pre-Ignition / Knock)

Uncontrolled secondary combustion in a cylinder that creates pressure spikes and damages pistons, rod bearings, and ringlands. Caused by too much spark advance, too lean an AFR, low octane fuel, or overheated intake charge. The defining engine-killer of poorly tuned JDM turbo builds. Modern tuning relies on knock sensors and conservative timing maps.

Standalone ECU

An aftermarket engine management system that fully replaces the factory ECU, offering complete control over all engine parameters. Popular options for JDM builds include Haltech, Link, AEM, and Motec for high-power or heavily modified applications.

Wideband O2 Sensor

An air-fuel ratio sensor (commonly Bosch LSU 4.9 or NTK UEGO) that reads accurately from 10:1 to 20:1 AFR. Replaces the narrowband sensor used by stock ECUs which only signals "rich vs. lean." Required for any tuned forced-induction build. Mounted in the exhaust collector or downpipe.

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