Transient enrichment corrects a temporary lean condition that occurs when the throttle is opened quickly. This lean spot is caused by the delay in fuel vaporization and manifold dynamics—airflow increases immediately, but fuel delivery and mixture homogeneity lag behind.
The RM-Primis ECU offers two distinct modes for handling this:
This is the default mode when the **"Transient enrichment" checkbox is unchecked .
This method monitors the rate of throttle opening (TPS delta per second).
When the TPS change rate exceeds a user-defined threshold , it triggers a fuel enrichment event .
The enrichment amount is based on the current TPS/S rate and RPM using a 2D TPS/Sec table .
The fuel enrichment then decays over time , controlled by the “enrichment decay per cycle” value.
X-axis : Throttle change rate in degrees/second (TPS/S)Y-axis : RPMCell Values : Percent enrichment (e.g., 100.0 = full enrichment, 0 = no enrichment)
When active, this table defines exactly how much extra fuel is added during a throttle spike at a given engine speed and throttle speed.
TPS/S Threshold : Minimum TPS change (°/s) to activate enrichment. Prevents accidental triggers from noise or idle corrections.Enrich. Decay/Cycle : Number of ignition cycles over which the extra fuel fades out. A higher number results in a slower decay.
Good for simple setups , especially engines with short intake runners or EFI conversions.
Easier to tune and understand for beginners.
Offers quick responsiveness and deterministic behavior .
Use when you're not confident measuring or simulating fuel film behavior (wall wetting).
This mode is enabled when the “Transient enrichment” checkbox is checked .
X-Tau is a fuel wall wetting model that simulates how fuel behaves during rapid throttle changes:
When fuel is injected, not all of it enters the cylinder—some adheres to the intake manifold walls.
During throttle changes, airflow and wall film dynamics change, causing more or less fuel to enter the cylinder than expected.
X-Tau models this effect with two parameters:
X-Factor : How much fuel sticks to the wall.Tau : How quickly this wall film evaporates into the airflow.
Fuel enters wall film : A percentage (X-Factor) of injected fuel is assumed to stick to the walls.Film evaporates (Tau) : That stuck fuel evaporates over time, contributing to mixture delivery with a defined time constant.Transient enrichment : The ECU adjusts injector timing dynamically to compensate for the difference between injected fuel and evaporated film.
When X-Tau is activated:
The middle table (previously "TPS/Sec") becomes the Tau table .
This table defines Tau values (decay rate) as a function of engine RPM and possibly TPS or MAP.
X-axis : Usually RPMY-axis : Possibly throttle or manifold pressure (depending on axis selection)Cell Values : Tau values—lower numbers = faster evaporation, higher numbers = slower decay (longer film presence)
Temperature-based table that defines how much fuel sticks to the wall at different coolant temps.
Higher values at lower temperatures (e.g. 40%) simulate increased wall wetting during cold starts.
Recommended for engines with:
Long intake runners Port injection systems Large fuel film effects
Useful in OEM-level tuning or if you're looking for maximum realism and driveability.
Best paired with precise injector data , stable idle, and good AFR logging.
Feature
Time-Based Mode
X-Tau Mode
Trigger
TPS/S rate > threshold
Internal fuel film dynamics
Enrichment control
Table-based (TPS/S & RPM)
Physics-based wall wetting model
Decay control
Fixed cycle decay
Time constant (Tau) per condition
Best for
Simpler setups
Advanced, accurate modeling
Tuning difficulty
Easy
Moderate to high
Suitable for
Any engine
High-performance or OEM-like setups
Start with 100% enrichment at >1000 TPS/S and taper down.
Set TPS/S threshold to ~15–25 to avoid noise.
Use ~40–60 decay cycles as a baseline.
Start with X-Factor around 30–40% at cold temps, 20–25% when warm.
Tune Tau to reflect engine behavior: lower Tau for snappier engines, higher Tau for engines that hold film longer.
Use logs to match AFR spikes/dips during throttle changes and adjust X-Factor or Tau accordingly.
On the far right of the screen is a Coolant Temperature-based multiplier for transient enrichment:
At cold temps, compensation increases enrichment further (e.g. 175% at -20°C).
Helps smooth out throttle when the engine is cold and fuel vaporization is poor.