Air/fuel ratio (AFR) tuning help

In HP Tuners VCM Editor you edit what the PCM commands: an open-loop or power-enrichment table that targets a specific equivalence ratio or AFR at high load. But what the cylinder actually burns depends on injector data, fuel pressure, MAF/VE accuracy, and fuel quality. You can command 12.5:1 and deliver 13.5:1 if your injector slopes are off or your MAF under-reads — and the engine doesn't care what you commanded, only what it burned.

This is why a wideband is non-negotiable. The narrowband O2 sensors the PCM uses for closed-loop are only accurate near stoich; they're useless for verifying a 12:1 WOT target. TuneVault's entire AFR workflow is built around comparing your commanded table to a logged wideband — if you don't have wideband data, we'll tell you we can't safely sign off on your WOT fueling, full stop.

AFR numbers are fuel-specific: 14.7:1 is stoichiometric for pump gas, but only about 9.8:1 for E85. Switch fuels and every AFR target shifts. Lambda (λ) sidesteps that — λ 1.0 is always stoich regardless of fuel, λ 0.85 is always 15% richer than stoich. Thinking in lambda means a WOT target of λ 0.80–0.85 is portable across gasoline, E85, and blends.

Many HP Tuners OSes let you display and target in EQ ratio (the inverse of lambda) or AFR. The trap is mixing units mid-tune — entering a gasoline AFR into a table the engine is now burning E85 through. TuneVault checks that your commanded targets make sense for the fuel you say you're running, and translates between AFR, lambda, and EQ ratio so a unit mix-up doesn't run you dangerously lean.

Naturally-aspirated engines typically make best power around λ 0.85–0.88 (roughly 12.5–12.9:1 on pump gas), rich enough for a safety and cooling margin without drowning power. Add boost and you deliberately go richer — often λ 0.78–0.80 or lower (high-11s to low-12s on gas) — because the extra fuel cools the charge and suppresses knock as cylinder pressure climbs. The more boost, the richer the safe target.

These are starting points, not guarantees — your knock data and wideband decide the final number. Running a boosted engine at an N/A AFR target is a classic way to make great dyno numbers right up until detonation. TuneVault flags WOT targets that look lean for your stated induction and reminds you it cannot promise the target that makes peak power on your specific combo; it keeps you on the rich, safe side and tells you to confirm with a wideband on every pull.

At idle and cruise the PCM runs closed loop, trimming fuel against the narrowband O2s to hold stoich for emissions and economy — this is where fuel trims live and where your MAF/injector cal gets validated. Under heavy load the PCM switches to open loop and follows your commanded power-enrichment table directly, ignoring the narrowbands. That handoff is defined by tables you can edit (load/RPM thresholds for PE enable).

Understanding the boundary matters: a lean spot at the closed-to-open transition is a real and common drivability and safety issue, often from a poorly-set PE enable point or a commanded table that doesn't blend smoothly from stoich into enrichment. TuneVault checks that your PE transition is smooth and that open-loop targets pick up cleanly where closed loop hands off, so you don't get a momentary lean hole right as load comes on.

E85 changes everything: stoich near 9.8:1, far more knock resistance, and roughly 30–40% more fuel volume needed — which is why injector and fuel-system capacity, not just a table edit, gate how much E85 you can run. "E85" at the pump also varies seasonally between roughly E70 and E85, so your true stoich and your trims move with the blend unless you have a flex-fuel sensor.

TuneVault sanity-checks that your commanded targets, injector capacity, and fuel choice are mutually consistent — there's no point commanding a rich E85 target through injectors that are already maxed. And as always: lambda tells you the truth across blends, but only a wideband tells you what you actually delivered.