Why lift-off needs less fuel
With the throttle open, a film of fuel builds up on the intake port walls. Close the throttle quickly and manifold pressure drops, that film evaporates into the airflow, and the engine gets more fuel than the air warrants. The result is a rich spike, sometimes a stumble, and poor fuel economy on the overrun. Disenrichment pulls fuel out briefly to cancel it. It's the lift-off partner to enrichment, which adds fuel when you open the throttle.
The two tables
| Table | What it controls | Typically mapped against |
|---|---|---|
| Disenrich Rate | How strongly disenrichment responds to a closing throttle event | How fast the throttle is closing and the throttle position it started from (up to 3D) |
| Disenrich Amount | How much fuel a disenrich event removes | RPM (up to 3D) |
The two are multiplied to get the final disenrichment, so a sharp lift from wide open at high rpm gets far more than a gentle lift from part throttle at low rpm.
The Amount values aren't true volumetric efficiency, but they act like a VE figure: bigger numbers remove more fuel. As a guide, values around 25 to 50 are typical at low rpm, rising to around 100 to 200 at high rpm.
How we tune it
- Get steady-state fueling right first. Disenrich can't fix a base map that's already off.
- Log lambda, throttle position and rpm while doing sharp lifts at a few different rpm points.
- If lambda goes rich after a lift, increase Amount at that rpm. If it goes lean or the engine stumbles, reduce it.
- Use the Rate table to shape how the response changes between gentle and sudden lifts.
For the bigger picture, see fuel transient throttle and transient coolant temperature correction.
