Tech Talk #92 – The Detonation Investigation


By David Reher, Reher-Morrison Racing Engines

“The burning fuel-air mixture becomes an acetylene torch that can melt pistons, cylinder heads, and blocks.”

In the days before electronic controls, it was common to hear the sound of detonation in a street engine during hard acceleration – a distinctive knocking or metallic ringing sound.

Now that cars are equipped with knock sensors, the sound of detonation is seldom heard. In a race car with open headers, it’s virtually impossible to hear knock, but the damage and destruction caused by detonation can be extensive – and very expensive.

Detonation is the uncontrolled self-ignition of gases in the cylinder. What’s so serious about a little self-ignition? Simply this: those gases burn with an explosive intensity that hammers pistons and melts parts.

During normal combustion, the spark plug ignites a flame front that moves smoothly across the cylinder. Hydrogen and carbon atoms in the fuel combine with oxygen in the cylinder in a chemical reaction that releases energy. The resulting increase in heat and pressure causes more oxygen and hydrocarbon molecules to collide in a chain reaction, like the balls on a billiard table. This creates the cylinder pressure that pushes the piston and transfers the fuel’s energy to the connecting rod and crankshaft.

That’s how it’s supposed to work. However, during abnormal combustion, things get messy. Like any chemical reaction, combustion takes time. Even though the flame front only has to travel few inches from the spark plug to the edges of the cylinder, the rapid rise in temperature and pressure can cause the spontaneous ignition of residual end gases, creating secondary flame fronts. When these uncontrolled flame fronts collide, the resulting shock waves hammer engine components unmercifully.

Instead of a gradual rise in cylinder pressure, the untimed combustion of fuel during detonation produces intense pressure spikes. The “knock” and “ping” that can be heard when an engine detonates is the sound produced by these pressure waves. Detonation also strips away the boundary layer that insulates internal engine components from the searing heat of combustion.

If detonation persists, localized hot spots ignite the fuel-air mixture before the spark plug fires, escalating the vicious cycle of uncontrolled combustion. In this pre-ignition stage, cylinder temperature skyrockets and the burning fuel-air mixture effectively becomes an acetylene torch that can melt pistons, cylinder heads, and blocks.

A racer’s first line of defense against detonation is high-octane fuel. Octane rating is simply a measure of a fuel’s  resistance to detonation – it is not about horsepower or heat energy. Octane does have a significant impact on how an engine performs because it is the enabler that can allow an engine to reach its peak potential before encountering detonation.

Turbulence in the cylinder can help to forestall detonation by speeding up the combustion process. Clap your hands together, and the air between your palms shoots outward; the same thing happens when the piston approaches the cylinder head. In a wedge-style cylinder head, fuel and air molecules caught between the head and piston are expelled into the combustion chamber, creating turbulence that helps the flame front to travel quickly across the cylinder while the quench area between the piston and head helps to cool the end gases before they can self-ignite. The geometry of the intake port can also induce a swirling motion that promotes rapid combustion as the intake charge enters the chamber.

Detonation in a racing engine is often the result of an overly aggressive tune-up – too much spark advance or not enough fuel. An early warning sign of detonation is a loss of crankcase vacuum. Trace detonation can hammer the cylinders hard enough to break the ring seal and cause crankcase vacuum to decrease. Retarding the timing or richening the fuel-air mixture may prevent further damage.

While reading spark plugs might seem old-fashioned in this digital age, it’s still the best way to see what is actually happening inside an engine. If an engine is detonating, you will often see “pepper” on the spark plug’s porcelin insulator – small deposits of piston material. You may also see the sharp edges of the ground electrode strap beginning to erode into a radius like the end of a welding rod – another indication of excessive heat. When you spot these early warning signs, it’s imperative to get some heat out of the motor either by retarding the spark timing or enrichening the fuel mixture. Generally it’s best to use a combination of the two.

Readings from oxygen sensors are a good reference point to gauge cylinder temperatures, but they are not absolute answers. The 02 readings can be affected by the location of the sensors, their age, and whether they have been contaminated by deposits. It’s impossible to say whether an engine should have an O2 reading or 13.7 or 12.3 because the numbers depend on many variables and factors. The best advice I can offer is to use the oxygen readings to establish a baseline for your specific combination, and then be aware of any significant changes.

Inspecting parts during an engine teardown can also reveal the operating conditions inside an engine. I look at the free gap on the used piston rings and compare the gaps between the cylinders. When there is too much heat, the ring starts to collapse and the free gap diminishes. A cylinder with a collapsed ring is likely to be the first one to give trouble. When that engine goes back on the race track, it’s going to need an adjustment in the tuneup.

I’ll post my annual disclaimer that I am not an engineer, a chemist, or a physicist. I’m a racer and an engine builder, and my comments are based on decades of firsthand experience and observation with gasoline-burning engines. I do know one thing with certainty – detonation is to be avoided!