Every now and then I get to thinking…

So let’s talk about spark knock. I’m trying to visualize this. If I understand correctly, this occurs when the timing is so advanced that when the *plug* ignites the mixture, the flame front travels to the piston before it reaches TDC creating the ticking sound we hear.

Now if this understanding is incorrect, my following thoughts may not be valid.

Now I’ll define pre-ignition as what occurs when the mixture is ignited by some source other than the spark plug, whether it be cylinder pressure, a hot spot in the combustion chamber or piston, or some combination thereof.. The result of which is also the familiar ticking sound.

(I’m not sure how to define detonation.)

Now then, let’s assume that at 4,000 RPM I have 32 degrees advance and am experiencing spark knock. If I back off the timing to 27 degrees the spark knock stops. If I understand correctly, I have reduces the ignition timing to the point that the flame front is no longer ‘running into’ the piston on the compression stroke.

Now here’s a big assumption: The spark advance is what it is at 4,000 RPM no matter what the engine load. (I’m fairly certain of this assumption on non computer controlled vehicles with a distributor and mechanical advance, but I believe this is false on modern day vehicles, so for the sake of this discussion let’s take this assertion to be true.)

Back to the 32 degrees advance: If I slide it in first, let out the clutch and *ease* it up to 4,000 RPM, I have no spark knock. On the other hand, if I mash the go pedal to the floor I begin to experience ‘pinging’ as I cross the 4,000 RPM threshold.

What’s going on here? I say pre-ignition, probably due to the increased cylinder pressure from WOT operation. Do you agree?

Now then, let’s say I back off the timing to 27 degrees and do the same WOT run up. This time I experience no spark knock all the way to my 1-2 shift.

Here is my real question: What just happened here? Did I just make up a situation that cannot occur? I don’t think so because I can reproduce the results experimentally. What I don’t understand is with the timing advanced, I’m experiencing pre-ignition. Therefore, the SPARK PLUG is not igniting the mixture, something else is. But, if I back off the timing (which should have made no difference) the pinging goes away.

What am I missing here? Assuming no hot spots in the combustion chamber, the mixture should light off at a given temperature (cylinder pressure) give or take due to insufficient octane. If so then why the hell does backing off the ignition timing (assuming cylinder pressure/temperature remains the same) stop spark knock?

 

There is so much to look at here when tuning a boosted application. In your situation, there are two main things to look at; air charge temps (under boost) and fuel octane you are using.

Now with these two things being the factor, a dyno tune will allow you to see just how much timing your motor can take before it reaches, what many refer to as, the "buffer zone". Basically, it's a point to where the motor doesn't make any more power with added timing; you want to keep it slightly below this point. On a dyno, you can see when the motor gets close to this zone because HP gains will be very minimal (if any at all). If you're doing road tuning, with a datalogger, when you get close to this point, the calculated engine load increases will be minimal (if they change at all, or get worse). With the change in seasons/air temps, the ECU can be programmed to pull more timing at higher air charge temps, and even engine coolant temp, which is a good thing.

 

If you're speaking of an engine manufactured within the last 15 years by Ford... you have other things to consider.

There are more sensors involved... knock sensors right off hand.

The ECU can also calculate the dynamic load with the given data from the sensors.

If you are out of the operating window, you'll experience the knock if the ECU cannot 'fix' the problem. It's trying, even when you force the timing to run outside of the window - perhaps the slower acceleration curve is giving the ECU time enough to compensate.

-Kerry

 

Here's how I learned it, in order of severity, from least destructive to most, and one leads to the next if not corrected:

Spark knock (pinging): This is where the mixture is ignited normally, but then as the flame front expands out the end gas is spontaneously ignited before a full burn. OR a hot spot ignites the mixture and starts a second flame front which collides with the first. This is usually caused by too much timing or too low octane.

Autoignition: This is where things get so hot in the combustion chamber (usually due to the above described spark knock) that the mixture spontaneously ignites BEFORE the spark plug fires.

Detonation: This is where things get so hot that the entire mixture spontaneously ignites at once. Kaboom!

Runaway detonation: This is regular detonation which occurs earlier and earlier each cycle, and quickly destroys a motor.

In any case, it is not the flame front pushing down on the piston before TDC that's making your noise.

In your situation, backing off the timing lowers the amount of time the combustion chamber and other components such as the spark plug ground strap, are exposed to ignition temperature. This directly lowers the temperature of the combustion chamber and other components, preventing spark knock from occuring. If more timing yields more power, and it's pinging, you need fuel with a higher octane if you want to run the boost you're running. Another option would be to switch to a lower temperature thermostat to lower the combustion chamber temperature by lowering the coolant temperature.

 

Here is an article I found about reading spark plugs. I've never heard my engine spark knock but I have seen the tiny black specks which caused me to back off my timing. You need a good magnifying glass (at least a 10X) to really see the porcelain.
"The porcelain shows up preignition/detonation, it will not accurately reveal jetting/air/fuel ratios. To look for the first/beginning signs of detonation, search the white porcelain for tiny black specks or shiny specks of aluminum that have fused to the porcelain. When detonation occurs, part of the air/fuel mixture explodes instead of burning, the explosion is heard as a "metallic knock", this audible knock is the result of a sound shock-wave, this shock wave travels back and forth across the clearance volume "disrupting" the cooler boundary layer gases that cover the entire clearance volume area. This disruption allows "more" heat to be transferred into parts, especially, domes/piston tops,...along with the very rapid rise in pressure like a hammer blow, pistons can get torched with melted sides and holes !!! With the early signs of detonation, the shock-wave will also rattle rings causing the tiny amounts of oil that now gets by rings, to be fused to the white porcelain as tiny black specks, also fused as specks are soot that was clinging to clearance volume surfaces in the relatively "still-air" of the boundary layer. One step beyond the black specks, will be tiny specks/***** of aluminum coming off the pistons that will be fused to white porcelain,....the next step to be reached is occasional pieces of the porcelain being broken-off as detonation gets worse, ETC. Soon after that are holes, blown head gaskets, broken connecting rods, ETC"

 

Now we're talking. Isn't this a facinating discussion? I know my examples were simply mythical (but did exist in my past with a N/A 355 SVO). I honestly strive to understand (actually visualize) what is happening in the combustion chamber during these situations.

 

You like reading! Here's a link to some articles you may be interested in then:

http://www.kb-silvolite.com/article.php

I find the one on "Combustion Science and Theory" particularly interesting.

 

Thanks Rusty!

 

Reading that article makes me nervous...

Pretty interesting reading, though.

-Kerry