What the throttle follower does
When you lift off the pedal, the throttle closes almost instantly but the engine is still spinning fast. The idle controller only reacts once RPM gets near target, and by then the engine may be falling too quickly to catch. You get an RPM undershoot, a stumble, or a stall.
The throttle follower solves this by adding an offset to the idle output whenever the pedal is pressed. On the way back down, that offset doesn't vanish instantly; it decays over a time you set. The engine gets a little extra air while it settles back to idle, like a dashpot on an old carburetor. It's part of idle control and has two tables, Throttle Follower Target and Throttle Follower Decay, both only available once the throttle follower is enabled.
Throttle Follower Target
This table is the amount of offset the follower asks for. It's designed with RPM on the X axis (left to right) and Pedal Position Source on the Y axis (bottom to top), but either axis can be switched to any ECU channel. Press F3 with the table selected to edit or add axis values. If you change an axis value, NSP adjusts the cell values so the overall shape of the table stays the same.
Rules we follow when filling it in:
- Zero pedal around idle must be zero. The table is always active when the follower is enabled. Any value at 0% pedal near idle RPM becomes a permanent addition to idle airflow and fights the idle controller.
- Zero pedal at higher RPM can be small, but only small. A little here helps a coasting engine come down gently. Too much and it won't return to idle properly.
- More RPM or more pedal usually wants more offset. That's the typical shape, rising toward the top right.
- Tune by feel. You can rough it in on a dyno, but blip-and-lift behavior on the road is what matters. We finish this on a road drive.
DBW vs idle valve
On a drive-by-wire car, the idle control output is the throttle blade, and the target is a percentage of the DBW Max Position setting. The follower can't push the throttle past the maximum throttle position, so if idle control is already asking for something close to the max, there's little room left for the follower to add.
On a cable throttle with an idle valve, the target adds to the idle valve output instead, so the valve opens further while you're on the pedal.
Throttle Follower Decay
The decay table sets how long it takes for the follower's offset to fade away. It only works in one direction: while the throttle is opening, the follower output simply matches the target. As soon as the throttle moves toward closed, the decay takes over and the output ramps down over the time you set rather than dropping straight to the new target.
What that looks like in a log depends on the throttle type:
| Drive by wire | Cable throttle + idle valve | |
|---|---|---|
| What decays | The actual throttle blade position | The idle control output to the valve |
| In the log | Throttle position closes more slowly than the pedal | Idle output falls more slowly than throttle position |
| What the driver feels | A softer lift-off, less driveline shunt | RPM settles to idle instead of dipping under it |
A worked example: a car with a lightweight flywheel stalls about one time in three when you clutch in and lift off at 3000 RPM. The log shows RPM dropping through target and the idle controller only reacting at the bottom. Adding a moderate target at mid RPM and mid pedal, then lengthening the decay, keeps enough air in during the drop that RPM lands on target. Too long a decay and the same car will hang at a high idle for a second or two after lifting, which tells you to shorten it.
Troubleshooting
| Symptom | Usually |
|---|---|
| Stalls or dips hard when lifting off | Target too low where you lift from, or decay too short |
| RPM hangs high after lifting, then drops | Decay too long, or target too high at zero pedal and high RPM |
| Idle sits high and the controller keeps pulling it down | Non-zero target at 0% pedal near idle |
| Follower seems to have no effect on a DBW car | Idle control is already near the DBW max position, so there's no room for the offset |
| Can't find the tables | Throttle follower isn't enabled in idle control |
Before tuning the follower, make sure the base idle is right. The closed loop base table and the PID gains should already hold idle steadily, and idle up handles loads like A/C. The follower is for the drop back to idle, not a fix for a poor idle.
Throttle follower FAQ
Does the throttle follower affect driving at part throttle?
It adds idle airflow while you're on the pedal, which is usually small compared with the throttle opening. Its main effect is felt when you lift off.
Should I use it on every car?
Not necessarily. Stock-ish engines with a heavy flywheel often return to idle fine without it. It earns its place on light flywheels, big cams and DBW cars with harsh lift-off.
