spark knock, pre-ignition, pinging, gurgling, etc.

[twmattox]

Alright, I have never had the courage to ask this...but here it goes...

What is the difference in all of these? Are they different names for the same thing??? What are the symptoms? How do you accurately diagnose? I know many people say to drive and listen...but I can't always hear the engine when I am driving.

Honestly, every now and then I get a cross between a ping and a gurgle (like when I heavily lug the engine), but I can't always reproduce it, and am not certain it is pre-ignition.

 

[n9viw]

First, in spite of it being the wiki, http://en.wikipedia.org/wiki/Engine_knocking .
Next was gleaned from http://www.clubwrx.net/forums/showthread.php?t=25686 :

"Causes of Detonation
Trying to isolate a detonation problem in a highly modified or even slightly modified engine requires you to determine if the combustion problem is being caused by a change you made to the engine, is inherent to a design flaw, or is derived from an external cause such as carbon deposits or poor fuel. Obviously the area with the largest variable is the fuel quality. Octane determines a fuel's ability to resist auto-ignition, or in lay terms, its ability to wait for a spark to initiate a flame. For our purposes we will explore knock independently of fuel octane rating.

When knock occurs, there is uncontrolled combustion with multiple flame fronts, with only one of them being initiated by the spark plug. During knock, cylinder pressures experience an extremely rapid rise and spontaneous burning and release of the end gases' energy. It is not uncommon to see a pressure rise of up to 8 bars per degree of crankshaft angle, or up to 50,000 bars per second. Anything that will either increase heat or retain heat in the combustion chamber will promote auto-ignition.

Common problem areas are sharp edges of metal either on the piston or in the combustion chamber. For instance, if the piston has a valve relief cut into it, there is usually a very defined edge that the cutting tool leaves. This sharp edge is greatly prone to super heating and will actually retain enough heat that it will start to glow. If the fuel should hit this glowing edge either prior to the lighting of the spark plug or even after ignition, it is very likely that another flame front will initiate. If this unintended ignition occurs very early in the compression stroke, then the piston will be forced up against the increased pressure of the burning gas and will result in a form of abnormal combustion referred to as preignition. When this happens, the end result, if severe enough, is that the connecting rods bend. Turbocharged engines are specifically prone this. Carbon deposits that build up in the combustion chamber or on the piston top will have the same effect, along with the decreased volume at TDC, which in turn raises the compression ratio.

As stated earlier, anything that will raise temperatures in the cylinder will help promote detonation. This rise in temperature could occur many different ways: very lean air/fuel ratios, heating of the charge air prior to entering the combustion chamber, compression of the air molecules from forced induction, or poor heat dissipation through the cooling system. Increase of cylinder pressure will also prompt detonation. Camshaft profile, compression ratio and quench area all come into play. Design criteria as to combustion chamber shape and mixture motion, spark plug location and piston design can all make an engine more or less inherently prone to abnormal combustion. Naturally, spark timing and fuel quality round off the list."

 

[twmattox]

See, here is where I get confused...

If I follow correctly, detonation is caused by having the timing advanced too much causing the combustion to occur prior to the piston being at the maximum point of compression. Pre-ignition is caused by something else (hot) causing combustion to occur prior to the piston being at the maximum point of compression. So, in essence both are the same result just different causes. To make it even more confusing, pre-ignition may be occurring because timing is advanced too far (though not far enough to cause detonation???) causing the combustion chamber temperature to be higher than normal.

So, back to part of my original question... some people talk about it pinging, some about it knocking, some have even mentioned a coffee can with marbles. Are all of these sounds for the same thing (combustion occurring before the piston is at maximum compression)? How can you tell if you are dealing with a detonation or a pre-ignition issue?

Finally, and most importantly, every now and then, I get a noise that may be called a gurgle knock (get on an incline at a dead stop and try to take off in 2nd gear...that's the noise). Actually, it sounds a lot like an old school diesel. Usually happens when the engine is lugged and only occurs for a second or two. Like when in 5th gear doing 50 (1800rpm) and goosing the gas.

Basically, I just don't want to end up with a hole in my piston...
Thanks

 

[n9viw]

First of all, if you don't want to wind up with a hole in your piston, DON'T DO THAT.
Second, that sounds like detonation to me, and as the article says, a little is okay, but a lot is bad. Preignition can happen any time, revved or not, under load or not- it just has to be hot enough to burn. Preignition and dieseling go hand in hand; often when you have one, you have the other as well.
My wife's CJ-7 has some detonation issues (it's a high-altitude engine), but I managed to control it by removing the vacuum advance line completely and backing off the timing a bit. There's no noticeable change in the way it runs, either at idle or at speed, apart from the lack of knock. It still does it some if I really hammer on it trying to pass someone, but not near as bad as it was. Sounded a bit like valve clatter.
Back your timing off a couple degrees and see what happens.

 

[twmattox]

Valve clatter??? See, another noise I don't recognize (I'll google it).

I have to work very hard to get mine to knock. If I tromp the pedal, it won't do it. I have to be in a very high gear and rapidly goose then let off the gas multiple times just to make it do it (found this out playing one day). Believe me, I am not trying to do it...just heard the noise and was curious what it was. One friend said spark knock...another said pre-ignition...someone said engine knock...I just got confused with all the "slang" terms for these conditions...

Thanks for helping me understand...

 

[Duffman]

Slight gurgle on accelerating...

Out of curiousity, I was wanting to remove the vaccuum advance tube just to see what effect- if any, I may notice because I still have a tiny bit of "lug" sometimes when I accelerate. Timing is set at about 6-deg and all else runs great.
I have a new MC2100 and new HEI and I read that you don't "need" to run the vaccuum advance line. I still have been runing with it, but was curioius if there could be any damage done to the pistons from changing the advance/retarding of the ignition.
I could easlily trial and error it, but ya'll sound wise beyond my years here. Any advice?

Thanksa so mucha,
Duff

 

[twmattox]

From my understanding, and according to the MOPAR Performance Jeep Engines book:

The mechanical advance is set at the maximum advance required for the engine. The addition of the vacuum advance is 2 fold. 1) on computer controlled engines, they over advance the engine and use a knock sensor to allow the computer to retard timing to maximum best and 2) on non-computer controlled engines, they allow additional timing during acceleration, when the engine is able to handle the added advance

So, can you unhook the vac advance...yes. Will it hurt anything...no. However, unhooking the vac advance should not get rid of any lug...more like the opposite. I would make sure your vac line is connected to ported (not manifold vacuum) and possibly bump your idle timing up to 8* and test drive (carefully).

 

[sxer]

This is my understanding of the difference between preignition and detonation.
Ideally, when the spark plug fires, the flame starts in the center of the piston and has a controlled burn to the outsides of the cylinder creating equal pressure across the piston. Detonation is when this burning process is uncontrolled. When there is an ignition source, the mixture explodes and burns uncontrolled causing unequal pressure across the piston. Octane is a measure of a fuels resistance to detonation. That is why when you raise compression or advance timing you need more octane. Compressing the fuel more raises the temperature of the mixture and brings it closer to its ignition point. That is also why running higher octane fuel without raising compression or timing will not help you. It burns slower than a lower octane fuel.
Preignition is caused by anything that is hot enough to ignite the mixture before it is supposed to be ignited.
I believe the reason for the "pinging" when lugging the engine is because the engine speed is too slow for the amount of timing being called for. By having your foot into it the engine is calling for a timing setting for high RPM where actually the RPM is very low.
The result of extended detonation will be cracked ring lands and possibley piston skirts, probably not a hole in the piston.
When I was racing snowmobiles, when we raced in higher altitude, we would lower the octane as much as we dared as the engine made more power and ran faster. Sometimes we went too far and seized an engine.
Heat is horsepower, but too much heat means your day is done.
Bob

 

[twmattox]

By having your foot into it the engine is calling for a timing setting for high RPM where actually the RPM is very low.

Makes a lot of sense...thanks