What flat shift actually does
With a sequential gearbox, or a dog box driven by a load-cell shifter, you don't need to lift or clutch to upshift. What you do need is a tiny window where the gears aren't loaded, so the dogs can slide out of one gear and into the next. Flat shift creates that window. When the shift starts, the ECU drops engine torque for a short moment, then brings it back once the new gear is in.
The length of that moment is the whole game. Cut too briefly and the dogs are still loaded when they try to engage, which you'll hear and feel as a crunch. Cut too long and the car falls on its face between gears and the shift feels slow and harsh. The Nexus also uses this function on the way down, blipping the throttle to match revs for clutchless sequential downshifts or heel-and-toe style H-pattern downshifts.
Choosing a mode
| Mode | When the cut ends | Best for |
|---|---|---|
| Closed Loop | When Gear Detection sees the new gear, plus a short settle time from the Flat Shift Time table | Cars with a gear position sensor. The most precise option |
| While Active | As soon as the trigger input or conditions drop out | Setups where the shifter switch itself reliably marks the shift |
| Timed | After the time set in the Flat Shift Time table | Simple setups without gear feedback |
Closed Loop is only offered when the ECU supports it, and it needs a gear position sensor set up in Gear Detection. Its advantage is that it doesn't guess. It holds full torque reduction until the gearbox proves it has changed, then waits just long enough for the gear to seat before handing power back.
Two safety timers apply whatever mode you pick:
- Max Shift Time: if the ECU still hasn't decided the shift is over by this time, it forces the recovery and returns torque. This stops a stuck switch or a sensor glitch from holding the engine in a cut while you're in gear.
- Blockout: the lockout period after a cut before another flat shift can begin. It prevents a noisy input from triggering a string of cuts.
What starts a flat shift
You can trigger it from an input, from ECU conditions, or both.
- Conditional Trigger: build the trigger from ECU channels instead of a physical input. See Conditional Trigger.
- Flat Shift Input Enable: turns on a wired input, and Input Select tells the ECU what kind it is:
- Flat Shift Switch: a dedicated input such as an aftermarket shift sensor or switch on the lever.
- Clutch Switch: the clutch switch starts the event.
- Load Cell: an analog load cell on the shifter tells the ECU which way you're pushing. Choosing this adds a tab where you calibrate the upshift and downshift voltages. More in Load Cell.
Wiring for each input type is covered in Flat Shift Wiring.
How power gets taken away
The Torque Reduction Method decides how the ECU removes power. Every method with a cut brings up a cut percentage table, and every method with retard brings up an ignition retard table.
| Method | What happens | Our advice |
|---|---|---|
| Ignition Cut | Spark events are dropped | The common choice. Fast and effective |
| Ignition Retard | Timing is pulled back | Softer. Good for mild setups, but may not unload the box enough on its own |
| Fuel Cut | Injector pulses are dropped | Leaves the engine lean for a moment. We rarely use it alone |
| Fuel & Ignition Cut | Both are dropped | Strong reduction, cleaner than fuel cut alone |
| Fuel Cut & Ignition Retard | Fuel drop plus less timing | Situational |
| Ignition Cut & Ignition Retard | Spark drop plus less timing | Useful when a cut alone is harsh on the driveline |
| Fuel Cut, Ignition Cut & Ignition Retard | All three together | Maximum reduction for high-power cars |
Any ignition cut sends unburned fuel into the exhaust, where it can pop and bang. That is normal on a race car, but keep an eye on exhaust temperature and cat condition on anything that still runs one.
Bringing power back
Recovery Method sets how torque returns after the shift. Options that ramp use the Recovery Time table.
- Instantaneous Recovery: everything returns at once.
- Ramp Out Retard Only: timing fades back in, while any cuts end straight away.
- Ramp Out Cut Only: cuts fade out gradually, while any retard is removed straight away.
- Ramp Out Cut and Retard Together: both fade back in over the recovery time.
Ramping is what makes a flat shift feel smooth instead of like a hammer blow. On a high-grip car, a sudden return of full power can shock the driveline or break the tires loose, so we generally ramp at least a little.
Direction and entry rules
Shift Direction chooses whether cuts happen on upshifts, downshifts or both. Min TPS and Min RPM must both be exceeded before a cut is allowed, which stops the function from firing when you're cruising or rolling through the pits.
Downshift throttle blip
Turning on Enable under Throttle Blip on Down Shift opens the Throttle Blip Amount and Throttle Blip Duration tables. This needs electronic throttle control, since the ECU is opening the throttle for you.
| Setting | What it does | Our advice |
|---|---|---|
| Rev Match Limiter | Applies a soft ignition cut if the blip overshoots the RPM target for the lower gear | Turn it on. It makes an over-generous blip table far more forgiving |
| Min Speed | Road speed required before a blip is allowed | Keep blips out of parking-lot maneuvering |
| Min RPM | Engine speed required before a blip is allowed | Above idle so it never fires during stall-prone moments |
| Delay | Waits this long before blipping | Use it if the clutch switch changes state before the clutch is fully released |
| TPS Correction | How quickly throttle is pulled back per second once target RPM is reached | Tune until revs hold near target without flaring |
| Gear Tolerance Enable | Turns on the Gear Tolerance table | Makes sure the clutch is fully out before the blip starts |
Reading flat shift in a log
Two channels tell you exactly what the function is doing. Flat Shift Direction reads -1 for a downshift, 0 for no shift and 1 for an upshift. Flat Shift State walks through the stages of each event:
| State | Meaning | Torque reduced? |
|---|---|---|
| 0 | Min RPM or min TPS not met | No |
| 1 | Input is still held from before the requirements were met, so the ECU waits for it to release | No |
| 2 | Armed and waiting for a shift | No |
| 3 | Still in the old gear (Closed Loop only) | Yes, full |
| 4 | Mid-shift | Yes, full |
| 5 | New gear detected, holding for the settle time (Closed Loop only) | Yes, full |
| 6 | Recovery, instant or ramped | Fading out |
| 7 | Blockout period | No |
State 4 ends differently depending on mode: Closed Loop leaves when Gear Detection sees the new gear, While Active leaves when the input or conditions drop, and Timed leaves when the Torque Reduction Time has passed. Any mode also leaves if Max Shift Time runs out. Timed and While Active skip states 3 and 5 entirely.
Troubleshooting
| Symptom | Usually |
|---|---|
| Gears crunch on upshift | Cut too short or too weak, or the input triggers after the driver is already moving the lever |
| Car bogs between gears | Cut too long, or recovery too slow |
| No cut at all | Min TPS or min RPM not met, state stuck at 0 or 1, or wrong input type selected |
| Cuts happening in gear | Noisy input. Check wiring and raise Blockout. Max Shift Time is your backstop |
| Downshift revs flare too high | Blip amount too large. Enable the Rev Match Limiter |
Flat shift FAQ
Can I use flat shift with a regular H-pattern gearbox?
The upshift cut is really meant for dog-engagement gearboxes. With a synchro box you still need the clutch. The downshift throttle blip, though, works well on H-pattern cars as automatic rev matching.
Ignition cut or retard?
Start with ignition cut. It unloads the gearbox quickly and predictably. Add retard, or ramp the recovery, if the shift feels too violent.
How do I know my cut time is right?
Log Flat Shift State, gear, RPM and throttle, and look at how long state 4 lasts compared with how long the gear actually takes to change. Start on the long side and shorten it in small steps while every shift stays clean. If you hear any crunch, go back to the last good value. Need help dialing it in? Book a free consultation.
