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Performance Limited By Knock

Engine knock, that annoying rattling sound that sometimes comes from under the hood of your pride & joy, is a killer. We have all heard the stories of blown head gaskets and broken pistons. Maybe it has already happened to you, too. But just what is knock, sometimes also referred to as detonation?

The knocking sounds you hear are the cylinder walls set into oscillation by intense pressure waves, caused by abnormal combustion.

Normal combustion is a controlled burn that starts from the spark plug and spreads outward, causing a pressure rise in the combustion chamber. The pressure is then converted into torque on the crankshaft. Ideally, the peak cylinder pressure occurs about 10-15 degrees after TDC as the piston is on its way down.

Detonation is a form of abnormal combustion that starts off right, but at the last millisecond, something goes wrong. The remaining fuel-air mixture, called the "end gas" explodes all at once instead of burning in a controlled way.

Resultant engine damage is caused by an instantaneous pressure rise that can exceed 1500 psi. This is more than double the normal combustion pressure and will blow head gaskets, break piston ring lands and hammer the rod bearings. Another form of damage seen is that the tops of the pistons will melt. High pressure waves produce localized hot spots which soften and erode the piston material.

High octane fuels are more resistant to detonation because they contain compounds that slow down the chemical chain reaction we call combustion. If left unchecked, these chain reactions would quickly multiply in the same way a nuclear explosion occurs.

All fuels, regardless of octane, have a knock limit. This is reached when the temperature of the "end gas" reaches the "auto-ignition point". Combustion chamber designers can use high swirl inlets and large "quench areas" to cool the end gas. Centrally located spark plugs and compact chambers can reduce the combustion time, inhibiting the heat transfer to the end gas. Listed below are some other factors that affect end gas temperatures:

To reduce detonation

  • Intake charge temp. reduce

  • Coolant temp. reduce

  • Compression ratio reduce

  • Manifold pressure reduce

  • Spark timing retard

  • Air-fuel ratio richer up to about 9.2-1, then worse

  • Humidity increase


  • Get cold fresh air to the inlet

  • Intercool turbos

  • Use O2 meter to set mixture

  • Water/alcohol injection

  • Retard spark ( traditionally in proportion to manifold pressure)

  • Sense detonation with knock sensor & retard timing

The article goes on to reveal a secret of figuring out which cylinder is knocking. The frequency is high enough that you "listen" and you can hear it before the next cylinder fires! Then just remember to retard that cylinder on the next power stroke.

BTW, there have been many attempts to "look" at combustion. One way was to put optical (sometimes fiber optic) ports in the combustion chamber. I've seen spark plugs with optic probes down the center electrode.

Article by:
J&S (makers of a knock sensor based igniton system)
Box 2199
Garden Grove, CA 92642-2199

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