In hot blowing they cut the ignition when the driver lifts off the throttle, but continue to inject some fuel through the engine’s valves into the exhaust to increase the energy into the gas. This fuel is not igniting inside the engine cylinders, but pass trough and ignites on the hot exhaust, increasing the amount, speed and temperature of the airflow exiting towards the diffuser. So they end up with more downforce. To do that they have to retard the ignition and kill the torque, because if you don’t then the engine is going to create
torque and the engine is going to keep going when the driver lifts off the throttle. Clever
engine maps prevent the engine pushing the car on in these conditions by retarding the ignition by as much as 35-40% on the over-run. By retarding the ignition when the driver lifts off, the fuel is no longer burnt inside a closed combustion chamber, but instead the fuel and air burn in the exhaust pipe, the expanding gasses blow out of the exhaust exit as though the engine is running . This creates a more constant flow of exhaust gasses between on and off throttle. This maintains the performance of the blown diffuser and keeps the downforce up when it's most needed. In this way they managed to avoid the main problem of an exhaust blown diffuser whereby when a driver lifts off the throttle for a corner, the downforce goes missing when you most need it and the rear stability changes.
It's not something you can do for all duration of the race as the engine temperatures go sky high, which damages the engine, but it gives that vital fraction of a second which keeps Red Bull ahead of the rest in qualifying. The problem here is that the engine mapping uses more fuel and creates excessive heat in the exhaust pipes and at the exhaust valve. Renault reported that both Red Bull and Renault used 10% more fuel in Melbourne compared to last year, most likely due to these off-throttle mappings.
