When engine knock goes super, you can kiss your pistons goodbye

When it comes to gasoline-powered engines, knock is bad news. Did you know, however, that some knock conditions are more destructive than others? In the latest episode of Engineering Explained, host Jason Fenske explores the phenomenon of “super knock” that’s increasingly occurring in modern turbocharged engines. So hold on tight and don’t answer that door—it’s probably your connecting rods making a surprise visit.

Before we dive into super knock, it’s probably best to first explain the regular scenario. Normally, engine knock occurs after the spark plug has ignited on the engine’s power stroke (when the piston is moving downward). It happens due to a “hot spot” within the combustion chamber, creating a secondary pressure wave and that characteristic engine clatter that we all dread. This is generally caused by using fuel with a lower octane rating than recommended for your car. Luckily, this “light knock,” as Fenske describes it, isn’t typically destructive as the rotating assembly is already on its downward stroke when it occurs.

Super knock, conversely, can have catastrophic results. Unlike light knock, the super version exists when the air/fuel mixture ignites before the spark plug fires and propagates throughout the rest of the cylinder. All of this occurs while the piston is traveling upward on the compression stroke. As you can imagine, the force of a combustion event pushing down while the rotating assembly pushes up is a recipe for bent rods and cracked pistons. How does this happen?

Engineers aren’t sure on the specifics, but agree that the super knock condition is caused by some sort of “hot spot” in the cylinder that lights off the mixture prior to the spark plug’s ignition—a scenario called low speed pre-ignition (LSPI). In modern turbocharged engines fitted with direct injection, this usually happens when the engine is at low Rpm and under high load.

The most plausible cause may be droplets of engine oil introduced into the combustion chamber, as it’s been found by Valvoline engineers that lubricants with higher levels of calcium (used as a detergent) show a greater tendency toward LSPI than those oils with alternative compositions. Keep in mind, however, that this is just one of the many suspects identified as potential super knock culprits.

So how can you avoid scattering parts across the ground brought on by super knock in your newer, turbocharged engine? Fenske suggests seeking out engine oils that are API SN+ certified, as these lubricants are specifically formulated to reduce the chance of LSPI. In addition, we think its always a good idea to run gasoline of the manufacturer’s specified octane rating. Especially if it’s the last line of defense between you and a set of bent rods.  

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