What the link delay does
When the Nexus ECU is running your transmission, a shift has two halves. The engine side is the flat shift event: a torque reduction (timing retard and/or a percentage cut) and, for downshifts, possibly a throttle blip. The gearbox side is the Transmission Control outputs switching to the new gear. Those two things rarely want to happen at exactly the same instant, because it takes time for clutches to fill or a shift barrel to move.
The Flat Shift Link Delay table sets the time gap between them. The Flat Shift Link Option decides which event goes first. The table only appears in NSP once Flat Shift Link Option has been enabled inside the Transmission Control function.
How it fits with the other flat shift tables
| Table | What it controls | How we set it |
|---|---|---|
| Flat Shift Link Delay | When the torque reduction starts relative to the transmission outputs | From the lag between the shift command and the first RPM change |
| Flat Shift Duration | How long the torque reduction lasts | From how long RPM takes to settle at its new-gear value |
| Flat Shift Retard / Percentage Cut | How much torque is removed | Adjusted last, for the feel and component load you want |
Getting the delay right comes first. A perfectly sized torque cut that arrives early or late does little good.
Setting it up on an automatic
Choose your axes
A reasonable layout is Transmission Control Current Shift on one axis and Line Pressure Control Target on the other. The idea is that different shifts, and different line pressures, take different amounts of time to complete. If the transmission has a line pressure sensor, the measured pressure is often a better axis than the target.
Tune line pressure first
Line pressure changes how quickly a shift happens. If you tune the link delay and then rework line pressure control, the delay values will no longer be right. Get line pressure where you want it, then do this table.
Log real shifts
Set up a log that includes:
- Transmission Control Current Shift
- Engine RPM
- Every channel used as an axis on your Line Pressure Shifting Target table
Then drive, and make the car shift under as wide a range of conditions as you reasonably can: light throttle, moderate, full, and at different speeds. A chassis dyno is often not ideal for this, because the load doesn't behave the way the road does through a gear change.
Read the log
- Find a shift and put a cursor where the Current Shift channel changes. That's the moment the ECU told the gearbox to shift.
- Move forward to where Engine RPM first starts to move toward the new gear. That's the moment the gearbox actually started changing ratio.
- The time between the two is your link delay for that shift and load. Enter it in the matching cell.
- Continue to where RPM settles at its new-gear value. The time from the start of the RPM change to that point is what goes in the Flat Shift Duration table.
- Repeat for each shift and load area you logged, and smooth the table between them.
Worked example
Say a 2-3 shift at moderate line pressure shows the Current Shift channel changing, then RPM starting to fall a short while later, then settling a bit after that. The first gap is your link delay for that 2-3 cell. The second gap is your duration. At higher line pressure the same shift will usually happen quicker, so you'd expect a smaller delay in that column. Treat any logged value as a starting point and confirm it with more logs.
Then tune the torque reduction
With timing sorted, adjust the retard and percentage cut tables. On a street car the goal is for wheel torque to hand over smoothly from the old gear to the new one. That makes the shift comfortable, and it also reduces momentary wheel spin and chassis upset through the change. On high-power cars you'll often need more retard and a heavier cut, mainly to keep loads on clutches, bands and shafts in a range the transmission will live with.
Pneumatic sequential gearboxes
On an air-shifted sequential, the delay depends mostly on how fast the actuator moves.
- Regulated air supply: actuator speed is consistent, so the table can often be a single value.
- Unregulated air supply: pressure varies as the tank drains and recharges. Put pneumatic pressure on an axis and reduce the delay as pressure rises, because the solenoids and actuator move faster with more pressure behind them.
Troubleshooting
| Symptom | Usually |
|---|---|
| Table isn't visible in NSP | Flat Shift Link Option not enabled in Transmission Control |
| Engine flares before the shift completes | Torque cut ending too soon: check delay and duration |
| Harsh bump at the end of the shift | Torque reduction arriving late, or duration too short |
| Shift feel changed after a line pressure retune | Link delay needs re-logging for the new pressures |
| Inconsistent air-shift timing | Unregulated supply without pressure on an axis |
Need help reading shift logs? Book a free consultation and send us a log.
