What a trans-brake does
A trans-brake is a solenoid valve in an automatic transmission that applies first and reverse at the same time. With two gears fighting each other, the output shaft can't turn, so the car stays put no matter how much power the engine makes. The driver holds the button, brings the engine up against the converter, and releases. The solenoid lets go of reverse, and the car leaves in first with the converter already loaded.
Letting the ECU run the trans-brake means it can limit RPM while the brake is on, control exactly when the solenoid releases, and pulse the solenoid for staging. You'll find the function under the Transmission section in NSP.
Wiring the solenoid
Trans-brake solenoids often draw a lot of current, so wire yours to a suitable HCO (high current output) rather than a standard low-current output. Check the solenoid's current draw against the output rating before you commit to a pin. Wiring details are in trans-brake wiring.
The settings
| Setting | What it does | Our advice |
|---|---|---|
| RPM Limiter Method | How the ECU holds RPM while the trans-brake is on: None, Fuel or Ignition | Ignition cut is the more common choice, and it's where we start |
| RPM Limit | The RPM held while the trans-brake is active. Only shown when the method is Fuel or Ignition | Set it where your converter and turbo (if any) make the best launch, found by testing |
| Delay | Time between the driver releasing the trans-brake input and the ECU releasing the solenoid | Start with the smallest delay and only add it if you need consistency or reaction timing |
| Bump/Creep Frequency | How fast the solenoid is pulsed when bump or creep is used | Match to your solenoid and test how smooth the roll-in feels |
| Bump Time | How long a single bump pulses the solenoid | Short. You want a small, repeatable roll forward |
The duty cycle used while bumping or creeping is set in the bump/creep duty cycle table.
Bump and creep: staging at full RPM
Here's the problem bump and creep solve. It takes a moment for the engine to reach full launch power against the brake, and a turbo car may need several seconds to build boost. If the driver waits until fully staged to bring the RPM up, they're either slow or they break the beams early. Ideally you'd pre-stage, bring the engine up to launch RPM, and then roll into the stage beam without dropping any power.
That's what bump and creep do. Instead of releasing the trans-brake to move forward, the ECU pulses the solenoid on and off at the set frequency and duty. The transmission slips just enough to roll the car forward a little while the engine stays loaded at the launch RPM.
- Bump is a timed pulse. Press the button and the ECU pulses the solenoid for the Bump Time and then stops. It's repeatable, which is what most drivers want.
- Creep is driver-controlled. The pulsing lasts as long as the driver holds the button, so they can decide how far the car rolls.
A typical sequence: roll into the pre-stage beam, apply the trans-brake, bring the engine up to the RPM limit, then bump or creep forward into the stage beam and release when the tree drops.
Trans-Brake Enable Condition
The function only works when the enable condition is true. It's built from three parts:
- Select: the ECU channel to check.
- Maths: the comparison. The usual six are available: =, ≠, >, <, ≥ and ≤.
- Value: what the channel is compared against.
For example, you could require vehicle speed to be equal to 0, or the gear selector to be in drive, so the trans-brake can't be triggered at the wrong moment. Pick something that fails safe: if the channel reads wrong, the trans-brake should stay off.
A trans-brake puts enormous load on the transmission, converter and driveline. Make sure the transmission is built with a trans-brake valve body, test at low RPM first, and confirm the solenoid releases every time before you ever try a full-power launch.
Troubleshooting
| Symptom | Usually |
|---|---|
| Car creeps with the brake on | Weak solenoid supply, worn transmission clutches, or too much RPM for what the brake can hold |
| Trans-brake does nothing | Enable condition not met, or output not assigned |
| Output dropping out or damaged | Solenoid wired to a standard output instead of an HCO |
| Bumps are inconsistent | Bump Time or duty too high, or frequency wrong for the solenoid |
| Engine bounces off the limiter harshly | Limit set too close to where boost builds fastest, or try the other cut method |
Trans-brake FAQ
Is a trans-brake the same as launch control?
No. Launch control limits RPM, while the trans-brake holds the car still. Drag cars often use both. See launch control.
Why add a delay between button release and solenoid release?
Some racers use it to fine-tune their reaction time or keep launches consistent. Most start at zero.
Where do I start with RPM limit?
Low. Build up in small steps on the brake, watching converter slip and transmission temperature, then settle on the RPM that launches best at the track. If you want help planning it, book a free consultation. For the Elite version, see trans-brake (Elite).
