boom!

 

Lucky,
A real simplified explaination is that the fuel in the cylinder "explodes" instead of burning smoothly like it is supposed to. If you can imagine what a handfull of marbles rattling around in the motor would sound like, that is what to listen for.
There are many causes, but the most common one is not enough fuel octane for the compression ratio of the motor.
Hope this helps,

------------------
Bill Koustenis
Advanced Automotive Machine
Waldorf Md

1971 Heavy Chevy - original owner
Team Chevelle #100

 

Good expaination BillK!

Another way to visualize it is a diesle engine fires the fuel mixture much the same way. Thus the diesle "knock". The difference is, the diesle is built to take the punisment.
Did I spell deasle correctly?

 

And dont confuse detonation with preignition. They can be related, but most of us, thankfully, only hear the preignition part, like the marbles BillK. refers to and some have called "spoons under the hood".

I think pure detonation is more like the "BOOM" gorbe is kidding about.

Did I mix them up Bill? Preignition being the rattling sound by fuel igniting early and detonation being a full force ignition overpowering the metal containment surfaces??

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Gene Chaas
Gold Member 62/ACES
67 SS 427

 

A term used by a majority of drivers to describe the sound is "knocking" or "engine knock". As Bill says, it's a rattling sound. Depending on engine conditions, fuel quality, fuel octane, etc, the condition of preignition or knocking (which can result in detonation) can many times be minimized or eliminated by retarding the timing a few degrees and/or using a higher octane gasoline or fuel additive.
Probably one of the most common causes of this condition to begin with is having too high of a compression for today's gasoline. Back in the "old" days of leaded, premium (ethyl, to us really old timers) gasoline, there were some gas stations which sold gasoline with an octane rating as high as 103.

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Tom Parsons

 

Bill, I guess thats why every time I hear detonation I feel like a kid again!---

 

My understanding is that detonation is the knock or rattling sound from too much timing, too lean a mixture, or not enough octane, when the spark plug ignited mixture flame front spreads too rapidly in relation to the piston position before TDC. It can be affected by rpm, load, etc. Pre-ignition is when the mixture is actually ignited way in advance by a source other than the spark plug, like a glowing sharp edge, etc., and obviously can be very destructive in a short time.

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von '69 300 Dlx SS TC #15 ACES #1575

 

Based on personal experience, it sounded like my 396 turned into a diesel engine for about 3 seconds until I figured out what the heck that noise was and let off the gas. I believe it literally is the sound of your connecting rod bearings banging on the crank journal, ouch!

Red Label

 

Some people say detonation is the point where too much timing advance results in a power loss. For example if you run the motor on the dyno with 38 degs then bump it up to 40 and the power drops off then there may have been detonation even though you can't hear it.

 

Detonation makes a
"knocking" or "pinging" sound which occurs when the normal flame front runs into a secondary flame front. This produces the knocking or pinging sound that you may have heard. Pre-ignition occurs when the air/fuel mixture ignites before the regular plug firing occurs. Preignition and detonation are very closely related and one can lead to the other.

Causes for detonation may include a lean air/fuel mixture, insufficient octane, improper ignition timing, lugging the engine, excessive carbon deposits, excessive milling of the heads or block.

Causes for pre-ignition include incandescent carbon particles, improper heat range spark plugs, spark plugs not firmly tightened, sharp edges in combustion chamber, hotter than normal valves, head gasket protusion into cylinder bore, overheating, ignition crossfire.

To say that detonation is the point where too much timing advance results in a power loss would not really be accurate. When you pass the peak power point advancing the spark (mbt) sometimes you can go several degrees without any detonation. The reason that the power goes down is simply that the location of peak pressure is occurring before the point that the piston has the most leverage on the crank, ideally 10 - 15 degrees atdc.

[This message has been edited by Unclepennybags (edited 11-12-2000).]