[datavirtue]

The gasoline will ignite before it is supposed to, forcing the piston back down before it has travelled all the way to the top of it's stroke. The engine will destroy itself. You can hear "knocking" coming from the engine when this is happening. It's worse than it sounds.

[zdragnar]

I know this is definitely true of older engines that aren't controlled by fancy software, but I suspect at least many consumer cars are capable of adjusting to the difference. I could be wildly wrong here though, and wouldn't want to test it myself.

[kube-system]

Knock sensors are standard on every car made by a global automaker since the early 90s when fuel injection became universal