Speed-Density¶

The method Engone uses to estimate air mass from manifold pressure, temperature, and volumetric efficiency — no airflow meter required.

What it is¶

Speed-density works out how much air enters the engine by combining engine speed (RPM) with air density (from manifold pressure and temperature) and the VE table. The core chain of equations:

\[\text{airDensity} = \text{densitySTP} \times \frac{\text{MAP}}{\text{Baro}} \times \frac{\text{stdTemp}}{\text{IAT}}\]

\[\text{actualAirMass} = \frac{\text{displacement}}{2} \times \text{airDensity} \times \frac{\text{VE}}{100}\]

\[\text{airflow} = \text{actualAirMass} \times \frac{\text{RPM}}{60}\]

Reading them plainly:

Air density starts from a standard value and scales with manifold pressure (MAP relative to barometric) and inversely with intake air temperature (IAT) — hot air is thinner.
Actual air mass is the air drawn per intake event: half the displacement (one stroke fills half on a four-stroke) times density times VE.
Airflow scales that by how often it happens — engine speed.

Reference conditions: air density 1.225 kg/m³ at 15 °C and sea-level pressure. Barometric pressure is fixed at 101.3 kPa (see Barometric).

Why it matters¶

Air mass is what fuel is metered against. Every term here changes how much fuel the injectors deliver. The IAT term is the one players most often overlook: hot intake air carries less mass, so it needs less fuel — ignore it and the engine runs rich when heat-soaked.

There is no temperature term in the VE table on purpose — the IAT term here already corrects density; adding another would double-count heat.

How to tune it¶

Speed-density itself isn't tuned directly — you tune its inputs. The big lever is the VE table; the temperature and pressure terms come from sensors and the build.

Rule of thumb

If fuelling drifts the same direction everywhere as the day warms up, suspect the IAT/density term, not the VE table.

In the game¶

The throttle box shows MAP and MAF — the ECU's air-mass flow estimate, in g/s — and the FUEL cell shows the fuel the ECU delivers. The IAT bar in the TEMP box is the temperature input. The fuelling you see is the output of this calculation.