Low RPM Knock – Worse Than @High RPM?
May 27, 2025Is low RPM engine knock more damaging than engine knock which occurs at high RPM?
Short Version
Whether knock occurs at high or low RPM, both have the ability to destroy an engine if the detonation/engine knock is bad enough. However, there are some key differences between low RPM and high RPM.
A Quick Sidebar on Detonation
Detonation occurs when part of the air fuel mixture is ignited before the flame front from the spark plug has had time to reach it. In other words, because detonation occurs after the spark event but before the flame front, detonation is uncontrolled and unpredictable. If it’s uncontrolled and unpredictable, we can’t tune for it and if we can’t tune for it, we need to stop it from happening.
Why?
Unpredictable explosions in the combustion chamber lead to unpredictable cylinder pressures and if the pressures are unpredictable they could cause excess cylinder pressure which could cause engine damage. To reduce the chance of engine damage, we need the fuel burn to be predictable.
If this detonation were predictable then theoretically we could tune the engine for it (like a compression ignition engine) and it could lead to better engine performance, but the technology is not there yet, at least for petrol powered engines. You can read more about the potential of harnessing detonation in this article.
Low RPM Engine Knock
Getting back to the title of the article, is low RPM knock potentially more damaging than high RPM knock all else being equal?
I would say yes.
Because the engine is turning slower, if we experience detonation at low RPM, the detonation has more time to increase cylinder pressure when the piston is still travelling up the cylinder, (before TDC).
Whereas at high RPM, the window to build cylinder pressure is shorter so there is more chance for some of the increasing pressure from the detonation to occur after TDC.
Another downside of low RPM detonation is that the peak forces act for longer at undesirable crank angles. Because the crank is turning slower.
In Summary – Low RPM Detonation/Engine Knock.
All else being equal I think I’d rather knock at high RPM than low RPM but as the technology stands, it’s better to avoid both.
Engine Knock Detection System – KSC3
Engine Knock Detection System – The KSC3 Monitos engine knock severity in real-time, visually and/or via a 0-5v signal to engine ECU/datalogger
What Is The KSC3?
A suppl…
