Updating GCU Firmware

The GCU (Gear Control Unit) firmware controls the pneumatic sequential gearbox shifter. Firmware updates improve shift speed, reliability, and diagnostics.

Updating Your GCU

You will need a USB-B cable (printer-style) to connect the GCU to a laptop. We like these cables. IMPORTANT: some GCUs were manufactured with slim clearance to the USB port. You need a right-angle USB-B plug that is as slim as you can buy, for maximum compatbility. The linked cable is one of the best we’ve found.

1. Download the release zip for your version below.
2. Unzip it to a folder on your computer.
3. Connect the GCU to your computer via USB-B.
4. Open the GCU Flasher app:
   • macOS: Double-click GCU Flasher.app
   • Windows: Double-click GCU Flasher.bat
5. Select the correct firmware for your car:
   • Gen2 (CAN / drive-by-wire): release_CAN-X.X.X.hex
   • Gen1 (KLine / throttle cable): release_KLine-X.X.X.hex
6. Select your serial port from the dropdown (click Refresh if it doesn’t appear).
7. Click Flash Firmware and wait for the “Flash complete!” message.

Note

Not sure which version you have? Gen1 cars have a throttle cable at the pedal box; Gen2 cars have an accelerator pedal sensor (no cable).

Viewing the Serial Console

The GCU Flasher includes a built-in serial console for diagnostics. Click View Console to connect to the GCU’s serial output at 115200 baud. Click Disconnect to close the connection. The console is useful for verifying the GCU is running correctly after a flash or for troubleshooting shift issues — see GCU Debugging for the output strings to look for and what they mean.

Setting the Compressor Cut-In and Cut-Out

Firmware 3.1.1 and newer lets you choose the pressures at which the air compressor starts and stops. They ship at 70 and 90 PSI, and you only need to touch this if you want a different band.

The best time to do it is right after flashing, while the GCU is still connected.

1. Click View Console. The Compressor row fills in with the band the GCU is currently running.
2. Change the two numbers. The first is cut-in (the compressor starts), the second is cut-out (it stops).
3. Click Save.

The flasher tells you what it stored. The setting lives on the GCU itself, so reflashing won’t wipe it.

Cut-in must be at least 50 PSI, cut-out at most 130 PSI, and cut-out at least 10 PSI above cut-in. Anything outside that is refused, and whatever band you had keeps running.

Note

The GCU will not save this once the car has shifted. Writing to its memory briefly stops it watching the paddles, so it only allows the write before the first shift of that power cycle. If you have already been shifting, cycle the ignition and set the band before pulling a paddle.

Changing the band changes when the compressor runs, nothing else. Shift timing still tracks actual tank pressure, so the car shifts the same way.

If the flasher tells you the firmware is too old, flash 3.1.1 or newer first. Earlier versions have a fixed 70/90 band.

Releases

3.1.1 — July 2026

You can set the compressor cut-in and cut-out pressures yourself now. They still ship at 70 and 90 PSI, so nothing changes unless you change it. See Setting the Compressor Cut-In and Cut-Out above.

• Set it from the flasher: click View Console and the Compressor row shows the band the GCU is running. Change the numbers, click Save, and it stores them. The setting survives a power cycle and a reflash.
• Bad numbers get refused: cut-in has to be 50 PSI or higher, cut-out 130 or lower, and cut-out at least 10 PSI above cut-in.
• Only before you’ve shifted: saving briefly stops the GCU watching the paddles, so it only allows it before the first shift after power-on. Set the band in the paddock.
• Shift timing doesn’t follow the band: shift length still tracks actual tank pressure. Moving the band changes when the compressor runs, nothing else.

📥 Download: rush_GCU_3.1.1.zip

3.1.0 — March 2026

Much faster sequential shifting. Each shift now completes in about 180ms (previously 325ms), allowing approximately 5.5 shifts per second instead of 3. You’ll notice the difference on fast downshift sequences.

• Faster shifts: Reduced the wait time between shifts by over 70%. The air cylinder returns fast enough that the old delays were unnecessary.

📥 Download: rush_GCU_3.1.0 (1).zip

3.0.6

Shifts adapt to tank pressure. The GCU now automatically adjusts shift timing based on how much air is in the tank.

• At low tank pressure, shifts take a bit longer to make sure they complete reliably.
• At normal pressure (70–90 PSI), shifts behave as before.
• At high pressure, shifts are quicker since there’s more force available.

You don’t need to do anything — this happens automatically. The result is more consistent shifts regardless of tank pressure.

📥 Download: rush_GCU_3.0.6.zip

3.0.5

Diagnostic LED improvements.

• Startup self-test: When the GCU powers on, the red, yellow, and green LEDs light up in sequence. This confirms all three LEDs are working.
• Compressor indicator: The yellow LED now flashes while the air compressor is running. Useful for confirming the compressor is cycling on and off correctly — if you suspect a compressor issue, watch the yellow LED.

📥 Download: rush_GCU_3.0.5.zip

3.0.4

First stable release with countershift. Countershift sends a brief reverse air pulse after each shift to help the gear fully seat, reducing the chance of a missed or incomplete shift.

• Separate shift timing for upshifts and downshifts
• Improved shift light behavior

📥 Download: rush_GCU_3.0.4.zip

3.0.3 and earlier

Contact Rush Auto Works for firmware files for older versions.