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What is Detonation?

 

Detonation, also known as pinging, pinking, or knocking, is a serious motor destroying event.  It sounds different on many cars but it can sound like slapping metal wires against the motor, a rattle in the dash, shaking popcorn kernels in a coffee can, and when it’s real bad it sounds like marbles in a coffee can.  It has a rhythmic sound much like a diesel (technically its very similar to a diesel).  When you hear detonation GET OFF THE GAS IMMEDIATELY!  Detonation is not the same as pre-ignition although they often occur at the same time.  Detonation can be caused by too much spark advance, high IATs, lean mixture, dry air, high ECTs, low octane, etc. It occurs when the spark ignited flame front compresses the reaming air fuel mixture till it reaches critical temperature and pressure to auto ignite the mixture on the other side of the cylinder. The two flame fronts then collide with a giant explosion causing the audible knock sound.  The cylinder temperatures and pressures increase drastically beyond the normal combustion temperatures and pressures causing burned holes in the pistons, bent or broken rods, damaged crankshaft bearings, and complete and total destruction.

 

Pre-ignition occurs when the mixture ignites on some hot spot (spark electrode, spark plug threads, a sharp point, carbon build up, etc) before, or at the same time the spark fires. The flame fronts do not always collide, the destruction comes from extended burn time in the cylinder. Pre-ignition does not produce the classic knock or ping, more of a dull thud, but it is often caused by detonation and therefore associated with the pinging sound. Pre-ignition is more dangerous than detonation because it occurs earlier in the engine cycle causing more temperature build up due to the extended burn times. It can also cause “runaway advance” where the hot spot gets continually hotter causing the explosion to occur earlier and earlier in the engine cycle. It can also be caused by detonation events heating up a hot spot to the point where the detonation then turns into pre-ignition to cause some serious destruction.  This is why its very important to get off the gas when you hear or feel anything “funny” at WOT.

 

Detonation is affected by many things including ECT, IAT, octane, timing, compression ratio (or boost), RPMs, and AF.  Increased coolant temps or air intake temps will increase the chances of detonation.  Too much advance can also cause detonation resulting from increased burn times.  Too much compression will cause detonation that can be eliminated with increased octane or in the case of forced induction by lowering the boost and or intake air temps.  Octane is the easiest factor to control detonation.  Octane is the fuels resistance to auto ignition (detonation and pre ignition).  Sufficient octane will solve nearly all minor detonation problems.  High octane fuel burns slower than low octane, and high octane is much more resistant to higher pressures and temperatures.  Engines are much more prone to detonation while “lugging” when at high loads and low RPMs.  This is mostly due to the extended burn times at low rpms and usually a motor will not detonate under the exact same conditions, but with a higher RPM.  Lean AFR mixtures are also very prone to detonation and rich mixtures can sufficiently cool the cylinder to reduce or eliminate detonation.  A motor might run great at 11.5:1 and detonate like crazy at 12.5:1 on the same octane.  The goal to eliminating all detonation is to keep things cool and use sufficient octane for your appropriate boost and timing requirements.  Water methanol injection can drastically reduce detonation by cooling the chamber as well as adding octane.  Methanol has a pump octane rating around 100.  The formula for calculating your octane level when mixing gas is:

[(Volume 1 * Octane 1) + (Volume 2 * Octane 2)] / (Volume 1 + Volume 2)

 

Volumes must have the same units such as gallons.  Example:  Mixing 7 gal of 91 octane with 3 of 100.

[(7*91)+(3*100)]/10=93.7

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Last updated: 03/07/10.