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Boosted Engines (Background)

General background on how a tune changes when an engine is boosted.

Not in the game

Engone currently simulates naturally aspirated engines only. This page is general background on how a tune changes when an engine is boosted. Nothing here can be done in the game today.

What it is

A boosted engine forces air into the cylinder at more than atmospheric pressure, packing far more air mass into each cycle. That changes almost every part of the tune, because the tables you already know — VE, lambda and ignition — all use manifold pressure as their load axis.

Why it matters

The tuning principles from the rest of this wiki still hold; they just apply to a higher range of load:

  • The load axis goes above atmospheric. On a boosted engine MAP exceeds barometric pressure, so the VE, lambda and ignition tables need load cells above about 100 kPa.
  • Lambda target goes richer. A denser, hotter charge needs a richer mixture for charge cooling and knock margin. A starting point: about 0.02 richer as soon as the engine is boosted, then a further 0.06–0.10 richer for every extra 1.0× of atmospheric pressure in the manifold — and don't go below roughly λ 0.78–0.80.
  • Timing comes down. Cylinder pressure scales with MAP², so the knock limit's load penalty, 18° × (MAP/baro)², is about 18° at full naturally aspirated load and about 40° at 1.5× atmospheric. Pull timing steeply in the boosted cells, and run higher octane.
  • Compression ratio is lowered on boosted builds, for the same reason.
  • Intake air temperature rises with boost. Compressing air heats it; a hotter charge lowers the knock limit by about 0.08° per °C above 30 °C and reduces air mass. An intercooler is the remedy.
  • Fuel supply. More air needs more fuel — injector capacity and the fuel pump have to grow with power.
  • Damage. Over-pressure stresses pistons and head gaskets. An overboost cut (a fuel cut when boost exceeds target plus a margin) is a standard protection.
  • Closed loop still shouldn't be trusted at WOT, and the boosted cells are the highest-risk part of the map.

How to tune it

Treat the boosted cells as their own tune: richer, with less advance and more octane than the same engine unboosted, and with fuelling and cooling checked before you add pressure rather than after.

Watch out

The boosted cells are the highest-load, highest-risk cells in the map. Add fuel and pull timing as you add pressure, not after.

In the game

Nothing on this page can be done in the game today.