AFR explained for beginners

AFR is air-fuel ratio: the mass of air the engine breathes for each unit of fuel it burns. 14.7:1 is the chemically "perfect" mixture for petrol — the one that leaves nothing over on either side. It is the number your car targets when it is cruising, because it is what the catalytic converter needs.

It is not the number that makes power, and it is not the number that keeps a turbocharged engine alive at full throttle.

Richer makes more power, up to a point

Under load, engines make their best power slightly rich — somewhere around 12.5:1 to 13:1 on a naturally aspirated engine, richer still under boost. A little extra fuel cools the charge, which lets you run more timing before knock, and it makes sure every bit of oxygen finds something to burn.

Go too rich and power falls off gently: fuel that does not burn is just weight. It costs you a few percent and blackens the tailpipe. It does not hurt anything.

Leaner is where the danger is

Go lean and two things happen. Power falls off — faster than it did going rich — and combustion temperatures climb. Lean mixtures burn hot, and hot is what turns a normal burn into detonation. On a turbocharged engine there is more of everything in the cylinder, so the same leanness costs more.

This asymmetry is the whole lesson. Rich costs a little power. Lean costs power and the engine. If you are going to be wrong, be wrong rich.

What the factory targets

At full throttle, a stock turbo car typically runs around 12.0 to 12.5:1 — a fair way rich of 14.7 — precisely because the manufacturer is buying margin against the worst fuel and the hottest day. That is not a number a tuner needs to move much. What a Stage 1 does is hold a similar target while boost and timing rise, and make sure the fuel system can deliver it.

How you can see it

The simulator on this site has an AFR slider and a knock gauge. Set any engine to stock, then drag the mixture lean, one notch at a time. Watch the power fall, and then watch the knock index start climbing — sooner and faster on the turbo engines than the naturally aspirated ones. Now drag it rich instead. Power falls, the gauge stays quiet.

That is the asymmetry in one minute. It is the reason the first question a tuner asks is what fuel you run, and the reason the first thing they look at in your datalog is the mixture under load.