I have a rough idea of what it is. But what exactly is the cause, what does it do, and how do I prevent it?

 

You'll usually see it in the force inducted section because this is the "enemy" to the force inducted crowed or any engine. Basically what it is, is pre-ignition meaning a given cyclinder or cyclinders have so much pressure and heat in them that they will ignite fuel before the spark plug will. The pistons, especially ours do not like this and are likely to destroy them. To prevent this, usually you would lower your compression or run better fuel like race gas. Im pretty sure thats how it goes but if not I know the more advanced members will correct me, wouldnt be the first time but het thats how you learn right?

 

Here's a discussion from Fel-Pro:
Note that pre-ignition and detonation are NOT the same thing.
It's too long to post all of it so here's the link with pictures:
http://www.federal-mogul.com/cda/con...3_7525,00.html

Detonation & Preignition
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WHAT IS DETONATION?

Detonation (also called "spark knock") is an erratic form of combustion that can cause head gasket failure as well as other engine damage. Detonation occurs when excessive heat and pressure in the combustion chamber cause the air/fuel mixture to autoignite. This produces multiple flame fronts within the combustion chamber instead of a single flame kernel. When these multiple flames collide, they do so with explosive force that produces a sudden rise in cylinder pressure accompanied by a sharp metallic pinging or knocking noise. The hammer-like shock waves created by detonation subject the head gasket, piston, rings, spark plug and rod bearings to severe overloading.

Mild or occasional detonation can occur in almost any engine and usually causes no harm. But prolonged or heavy detonation can be very damaging. So if you hear knocking or pinging when accelerating or lugging your engine, you probably have a detonation problem.








A DOZEN WAYS TO PREVENT DETONATION
(half here/ half in the link)
1. Try a higher octane fuel. The octane rating of a given grade of gasoline is a measure of its detonation resistance. The higher the octane number, the better able the fuel is to resist detonation. Most engines in good condition will run fine on regular grade 87 octane fuel. But engines with high compression ratios (over 9:1), turbochargers, superchargers, or with accumulated carbon deposits in the combustion chamber may require 89 or higher octane fuel. How a vehicle is used can also affect its octane requirements. If a vehicle is used for towing or some other application where the engine is forced to work hard under load, a higher octane fuel may be necessary to prevent detonation.

If switching to a higher octane fuel fails to eliminate a persistent detonation problem, it probably means something else is amiss. Anything that increases normal combustion temperatures or pressures, leans out the air/fuel mixture, or causes the engine to run hotter than normal can cause detonation.

2. Check for loss of EGR. The Exhaust Gas Recirculation (EGR) system is one of the engine's primary emission controls. Its purpose is to reduce oxides of nitrogen (NOX) pollution in the exhaust. It does this by "leaking" (recirculating) small amounts of exhaust into the intake manifold through the EGR valve. Though the gases are hot, they actually have a cooling effect on combustion temperatures by diluting the air/fuel mixture slightly. Lowering the combustion temperature reduces the formation of NOX as well as the octane requirements of the engine. If the EGR valve is not opening, either because the valve itself is defective or because its vacuum supply is blocked (loose, plugged or misrouted vacuum hose connections, or a defective vacuum control valve or solenoid), the cooling effect is lost. The result will be higher combustion temperatures under load and an increased chance of detonation.

Refer to a service manual for the configuration and hose routing of your engine's EGR system, and the recommended procedure for checking the operation of the EGR system.

3. Keep compression within reasonable limits. Typically ratios of 9-9.5:1 are acceptable for engines with cast iron heads and most mild street cams, while aluminum headed engines can tolerate 10-10.5:1. The effect of static the compression ratio on detonation is greatly dependent on the duration and intake closing point of the camshaft. Larger cams can tolerate higher static ratios because they bleed off some of the compression. The easiest way to determine if a particular camshaft is compatible with a particular compression ratio is to measure the cranking compression. On pump gas, anything over 180 psi can be a problem.

Another option would be to use a gasket shim with the stock head gasket to reduce compression.

Retarding the cam timing can also lower cylinder pressures to reduce detonation at low r.p.m., but doing so hurts low speed torque which is not recommended for street engines or cars with automatics.

For supercharged or turbocharged applications, a static compression ratio of 8:1 or less may be required depending on the amount of boost pressure, and the duration of the camshaft.

Another point to keep in mind is that boring an engine's cylinders to accept oversized pistons also increases the static compression ratio. So too does milling the cylinder heads. If such modifications are necessary to compensate for cylinder wear, head warpage or damage, you may have to use a thicker head gasket if one is available for the application or a head gasket shim to offset the increase in compression.

4. Check for over-advanced ignition timing. Too much spark advance can cause cylinder pressures to rise too rapidly. If resetting the timing to stock specifications doesn't help, retarding the timing a couple of degrees and/or recalibrating the distributor advance curve may be necessary to keep detonation under control.

5. Check for a defective knock sensor. Many late model engines have a "knock sensor" on the engine that responds to the frequency vibrations characteristically produced by detonation (typically 6-8kHz). The knock sensor produces a voltage signal that signals the computer to momentarily retard ignition timing until the detonation stops. If the "check engine" light is on, check the vehicle's onboard computer system using the prescribed procedure for a "trouble code" that would correspond to a bad knock sensor (code 42 or 43 for GM, code 25 for Ford, or code 17 for Chrysler).

A knock sensor can usually be tested by rapping a wrench on the manifold near the sensor (never hit the sensor itself!) and watching for the timing change while the engine is idling. If the timing fails to retard, the sensor may be defective -- or the problem may be within the electronic spark timing control circuitry of the computer itself. To determine the cause, you'll have to refer to the appropriate diagnostic chart in a service manual and follow the step-by-step test procedures to isolate the cause. Sometimes a knock sensor will react to sounds other than those produced by detonation. A noisy mechanical fuel pump, a bad water pump or alternator bearing, or a loose rod bearing can all produce vibrations that can trick a knock sensor into retarding timing.

6. "Read" your spark plugs. The wrong heat range plug can cause detonation as well as preignition. If the insulators around the electrodes on your plugs appear yellowish or blistered, they may be too hot for the application. Try the next heat range colder spark plug. Copper core spark plugs generally have a broader heat range than ordinary plugs, which lessens the danger of detonation.



PREIGNITION

Another condition that is sometimes confused with detonation is "preignition." This occurs when a point within the combustion chamber becomes so hot that it becomes a source of ignition and causes the fuel to ignite before the spark plug fires. This, in turn, may contribute to or cause a detonation problem.

Instead of the fuel igniting at the right instant to give the crankshaft a smooth kick in the right direction, the fuel ignites prematurely (early) causing a momentarily backlash as the piston tries to turn the crank in the wrong direction. This can be very damaging because of the stresses it creates. It can also localize heat to such an extent that it can partially melt or burn a hole through the top of a piston!

Preignition can also make itself known when a hot engine is shut off. The engine may continue to run even though the ignition has been turned off because the combustion chamber is hot enough for spontaneous ignition. The engine may continue to run-on or "diesel" and chug erratically for several minutes.

To prevent this from happening, some engines have a "fuel cutoff solenoid" on the carburetor to stop the flow of fuel to the engine once the ignition is turned off. Others use an "idle stop solenoid" that closes the throttle completely to shut of the engine's air supply. If either of these devices is misadjusted or inoperative, run-on can be a problem. Engines with electronic fuel injection don't have this problem because the injectors stop spraying fuel as soon as the ignition is turned off.








CAUSES OF PREIGNITION

Carbon deposits form a heat barrier and can be a contributing factor to preignition. Other causes include: An overheated spark plug (too hot a heat range for the application). Glowing carbon deposits on a hot exhaust valve (which may mean the valve is running too hot because of poor seating, a weak valve spring or insufficient valve lash).

A sharp edge in the combustion chamber or on top of a piston (rounding sharp edges with a grinder can eliminate this cause).

Sharp edges on valves that were reground improperly (not enough margin left on the edges). A lean fuel mixture.

Low coolant level, slipping fan clutch, inoperative electric cooling fan or other cooling system problem that causes the engine to run hotter than normal.

 

For the left brainers (or is it right brainers?) like me who need mental pictures:

Detonation: Like little bunker busters going off in the combustion chamber. Destructive. Blown gaskets, hammered out bearings, cratered piston tops. Usually takes a little time to do it's full damage.

Preignition: A medium yield nuke going off in the combustion chamber. WICKED destructive. This is the thing that bends connecting rods, snaps cranks like peanut brittle and blows the entire side out of the block's bore. Truly unnatural things happen to engine parts subjected to preignition and it happens in a faction of a second. By the time you're in preignition it's way too late to do anything.

Extended running in detonation can often cause preignition.

Can I just say how much fun it's been over the years learning the difference by looking at the reamins of scattered engines? Oh, the humanity! Heh heh.

Failure analysis is fun.... as long as it's someone else's failure.

 

Yea, basically it's like hitting the top of the piston with a sledgehammer as it's traveling up the cylinder. The wastegate actuator hose blew off on me on a WOT blast once last year while running pump gas, and it it spiked to almost 30psi. I normally can only get about 17psi before it knocks, and luckily the computer caught it in time and killed the spark/fuel before it popped a gasket or worse. It does have a frightening resemblence to clashing metal. To much of it can do this:

http://www.gnttype.org/techarea/pict...ocks/DOTC.html

A very expensive little bugger to deal with.

 

Unless the preignition is the result of prolonged detonation, isn't preignition usually caused by a hot surface like very hot carbon deposits or oveheated spark plug electrodes? Preignition is the equivalent of an advanced spark and may cause detonation, as you said. To me, that's your "little bunker busters" (maybe an oxymoron itself?)...the hot spots.

Is it still generally acepted that detonation is due to autoignition of the end gas, which is that part of the charge which has not yet been consumed in the normal flame-front reaction, as espoused by Taylor and Taylor in The Internal-Combustion Engine? To me, this is the low-yield nuke (very low-yield if you are into nukes!) in the combustion chamber: the reaction takes place at nearly constant volume, creating the very strong pressure waves that do the physical damage. I've heard it compared to getting hit in the teeth with a baseball bat.

Maybe we're saying the same thing but calling one thing the other. Or maybe I just didn't understand you analogy.

I guess I'm saying that I think you reversed the definitions.

It's right brainers who prefer pictures, BTW.

Hmmm.

 

holy crap

 

Just trying to inject a little humor. Obviously, the big brains could give a much better technical explanation of it than I could.

I'll just throw in that my understanding is that detonation happens somewhere between TDC and a few degrees after. Multiple flame fronts consuming the mixture very quickly, pressure waves crashing into stuff, etc.

Preignition, however, happens BEFORE TDC- sometimes long before TDC. Imaginine lighting it off and THEN trying to shove it up to TDC. Yikes. Every time I've seen it it's becuase there was obviously something that could have gone "glow plug" inside the combustion area. Whether a sharp edge hanging out in normal combustion or something that got too hot from the excessive heat caused by running in detonation shortly before.

I won't get too deep into it becuase like I said- others understand the process better than I do. I've seen the end result plenty of times, though!