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post #1 of (permalink) Old 02-16-2002, 12:50 PM Thread Starter
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Detonation

Can someone give me a quick explanation of what ignition "detonation" is. I've seen it mentioned a few times as something that is very harmful. What is it and how do you know if you're experiencing detonation. How do you correct it, I assume by retarding the timing. I completed the TR ignition upgrade and had to adjust my timing to 12 degrees and I'm not sure if this is a danger for my engine.

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TEXAS1AL

84 CJ-7; 258 I6; Restored-to-Stock Condition, TR Ignition Upgrade
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post #2 of (permalink) Old 02-16-2002, 02:58 PM
 
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Re: Detonation

detonation is when the spark ignites the fuel before the end of he compression stroke in essence the piston is going up while the explosion is trying to send it down not good at all usually it is experienced while giving the engine a load or under heavy acceleration sounds like a pinging or slight knocking causes can be too high compression poor fuel or bad ignition timing

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post #3 of (permalink) Old 02-16-2002, 04:06 PM
 
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Re: Detonation

He's got the correct idea.

If you have too much ignition timing, meaning too may crankshaft degrees before the piston reaches Top Dead Center (TDC), then you will experience detonation.

If all things were equal, you could fire the ignition at TDC.
But they are not on planet Earth.
If you wait until the piston reaches TDC to fire the ignition, then it's already on it's way back down during the Power stroke before the burning Charge makes any pressure, and you have wasted much of your Power stroke.

The idea is to light the fire a little sooner, (before TDC, on the compression stroke) so the fire has a chance to consume all of the fuel in the chamber. By doing that, the Heat produced by the fuel expands the gasses in the chamber rapidly.
If you have the time you start the fire correct, the pressure will peak just after the piston passes TDC and starts down so the pressure can have maximum time to push on the piston and make horsepower.

If you fire the ignition too soon, the pressure will peak too soon, and try to push the piston back down the way it came.
That's VERY BAD for parts! (Not to mention your wallet!)
<hr>
I'm sure you have heard detonation before.
Sometimes called 'Spark Knock'... Like when you are going up a hill in too high of a gear at too low of an RPM or hauling too much of a load...
You give it a little gas and it goes away...
(That's because you opened the throttle blades a little more, the vacuum went down, and some of the vacuum advance was lost, less advance means the ignition was firing closer to TDC and not way too soon.)

It's a rattle, knock, sputter, or just usual noise when you are cruising along with the throttle steady in a higher gear, and you hear something that goes away when you shift down, slow down, or give the engine more throttle.
<hr>
If you find your engine is making noise like 'Spark Knock', back the timing down a couple or three degrees at a time until it goes away.
I'd also take two or three (at least) degrees away after that to make sure you don't have detonation problem you can't hear, or buy one of those knock sensors you can just mount on the dash....
<hr>
We are coming up on a generation that has never heard 'Detonation'...
Every fuel injected vehicle I know of has a 'Knock Sensor' and retards the timing to get rid of it before human ears can even detect it!

MSD makes a gadget (timing control) that you can adjust your total timing from the drivers seat. It will work with the stock '78 to '90 Jeep distributor/module.
MSD also makes a remote knock sensor. That's the gadget I spoke of earlier that allows you to detect detonation you can't even hear yet. MSD P/N 8964
Combine the two on your dash, and you can maximize your ignition timing without detonation no matter the fuel quality, load, engine speed, or atmospheric conditions...
Easy to install, and damn near bullet proof! (Under $250 for both)








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post #4 of (permalink) Old 02-16-2002, 05:32 PM
 
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Re: Detonation

When you hear a layman's explanation of an internal combustion engine they talk about the spark igniting the fuel and this small "explosion" pushing the piston downward. I cringe at that explanation. When you ignite air and fuel under normal operating pressure, you get combustion, the heat of the combustion (albeit rapid) causes expansion of the gases and the increased pressure drives the piston downward. Detonation is when the combustion process turns supersonic. During detonation, a flame front or shock wave travels at the speed of sound through the mixture, producing pressure well above the normal. These pressures are so high, they can collapse pistons, break ring lands, blow out head gaskets, split cylinder walls, etc. So it's the rate of combustion that determines whether it's detonation or not. The exact cause is another story. It's typically heard as a marbles falling in a tin can noise in a gasoline engine. If the compression ratio is too high for the type of fuel you are burning, or if ignition occurs too early in the compression stroke due to too much advance or another source of ignition (pre-ignition, such as from a glowing hot spot) the pressure applied to the yet unignited mixture will convert the normal process of combustion into detonation, which is like the difference between burning a stick of dynamite in your fire (harmless) and setting it off with a blasting cap. Same amount of energy, but the delivery of that energy is totally different. Two flame fronts from different sources, spark and pre-ignition, can also result in detonation when they run into each other. This I gathered from a discussion with scientists at our atomic energy labs who where studying the phenomenon of detonation as it relates to industrial / mining fires. They set up a demonstration where a flame front traveling down a tube would turn supersonic /detonate when it reached an obstruction, which one might hypothesize increased the pressure on the burning mixture. Because it is related to pressure, detonation caused by lugging is more likely to occur when the piston is traveling slowly (low rpms) (allowing pressure to build too fast) heavy throttle application (fuller cylinder=more pressure=more effective compression) and too early of spark (pressure builds too high as the piston is still compressing the mixture).

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post #5 of (permalink) Old 02-16-2002, 06:56 PM Thread Starter
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Re: Detonation

Thanks for the GREAT explanations. Excellent info for the hip pocket. I'm going to check my timing again, I think it's a tad too far advanced.

TEXAS1AL

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post #6 of (permalink) Old 02-17-2002, 02:29 AM
 
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Re: Detonation

I always loved that 'Super Sonic' theory using a fuel that isn't capable of supersonic burn rates.
(Just plain too lean is the problem most of the time....)
One of these days someone is going to prove, or disprove it.

I've spent several hundred hours in the Dyno rooms, and I've sure never been able to get gasoline to burn at supersonic rates.
(nitro-methane is another story!)
I have been able to prove first hand that every single case of 'Detonation' we've ever come up against was ignition too soon for the crank speed, or too lean of fuel mixture.

Anything is possible I guess...
<hr>
I always like to see the term 'Pre-Ignition' thrown around.
That's a sure way to get a sh*t fight started in a dyno room!
True 'Pre-Ignition' is very rare these days!
I haven't seen or even heard of a case of true pre ignition in at least 10 years....

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