Over-revving to avoid engine damage: Why downshifting matters

Ignorance, carelessness, or simple laziness are the main reasons for one of the most common mistakes made during acceleration. Pressing the accelerator to the floor without downshifting first is ineffective and harmful to the engine. We explain why this happens.

When drivers are learning to drive, they are informed that they should downshift before performing an overtaking manoeuvre. A lower gear provides more power and torque, which translates to better acceleration and safer manoeuvre execution. Turbocharged engines generate more power at lower revolutions per minute (RPM). Therefore, drivers often press the accelerator without downshifting on empty roads with good visibility, which is incorrect.

A combustion engine is not efficient at low RPM. The acceleration process takes longer, and energy losses are greater. But that's not the end of the problems – Japanese companies Toyota and Denso have noticed that such acceleration can lead to pre-ignition. What does this mean?

In such conditions, flammable oil and fuel particles can accumulate in the combustion chamber and not be expelled with the exhaust gases. They glow in the chamber, and during the next compression, they ignite the mixture before the spark plug does. Studies have shown that the higher the RPM, the lower the likelihood of pre-ignition. That's why it's sometimes worth "blowing out" the engine at higher RPM.

Pre-ignition of the mixture results in enormous stresses in the power unit. In the best-case scenario, this can lead to spark plug damage; in the worst-case scenario, it can lead to piston cracks, rod damage, and the need for a major engine overhaul.