How the transbrake and torque reserve work on the Dodge Challenger SRT Demon

The 2018 Dodge Challenger SRT Demon ushers in a couple of firsts for production cars, and today, we’re here to take a look at two of them in greater detail. Both the transbrake and the torque reserve system are production car firsts and they help the Demon launch with incredible force at the dragstrip.

Our host Jason Fenske of Engineering Explained breaks down both systems to help understand what’s actually going on to ensure the Demon has an optimal amount of torque on tap for every quarter-mile run.

It all starts with the torque reserve. The system effectively holds onto 8 psi of boost from the 2.7-liter supercharger by closing off the supercharger’s bypass valve. The supercharger is always spinning at a rate that depends on engine rpm because it doesn’t have a clutch disconnect. By closing off the bypass valve, the air is not recirculated and it’s on reserve for a launch instead.

From there, the fuel flow and spark timing are retarded to allow the system to hold the engine between 950 and 2,350 rpm. But, something has to hold the car back from actually moving before the driver launches the car, right?

That’s where the transbrake comes in. The transbrake locks three clutches in first gear and one clutch in second gear in the 8-speed automatic transmission to hold the car in place ahead of a launch. The driver does not have their foot on the brake with the transbrake engaged.

The driver instead modulates the throttle between the previously mentioned 950 and 2,350 rpm and holds onto a shift paddle Since two gears are being held in the transmission, the car does not move and it prevents the reserved torque from making its way to the rear wheels.

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As some hot rodders point out, most transbrake systems hold onto first and the reverse gear. Dodge chose to hold first and second gear for safety reasons. In the unlikely event the transmission fails, the car will still move forward, rather than potentially launching in reverse.

To launch the car, the driver simply releases the shift paddle, and the car is off like a shot.

Jason gives a lot more information about both systems in the video above. Check it out.

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