Technical said: "What we all hear when our engines ping or knock is combustion taking place too soon but after the spark plug has fired. Not necessarily a second burn of end gases. If the mixture is fired off too soon and propagates before the piston reaches TDC it pings."
Not so. combustion is always pretty much silent (by comparison anyway) 'Preignition' could theoretically occur (by somewhat changing the definition to "Before the spark-triggered flame front arrives at the location) due to a hot spot, etc. lighting off a normal burn in the as-yet unburned mixture. This too would be relatively silent. If you hear it, it's detonation by its various names. The only difference between ping, knock and broken parts is how large a mass of end mixture explodes

 

mdrew,

Sorry if we (I) came across as arguing. I'm very passionate about cars, but
this is hardly arguing for me.

There's one part of your response that goes against my schooling:

I have been taught that the 'ping' we hear is the shock wave contacting
the cylinder, or chamber...not two flame front colliding.

Splitting hairs? Does it matter? That's where I differ from most.

Nobody in this thread should take my reply as argument. Maybe a little sarcasm
in reference to the 'Jesus' reply.

 

Wow, no offense guys but there is a lot of arrogance going on here...

Cool, so my worry with part-throttle cruise will be pre-ignition.

I have also seen engines that had badly carboned up CC's that would develop "hot spots" around the deposits and pre-ignite the fuel...

Very informative.

Mike

 

Like I stated previously, I'm not arguing the scenarios themselves...only the terminology.

This is the "other" scenario I described. If you advance the timing far enough, you can create this situation. The other scenarios are products of something in need of fixing vs. a tuning issue. And since we have all bumped our timing far enough to hear this, then we know it is indeed audible. The question is what to we call this? The plug fires, but it's the normal combustion taking place...just too soon.

 

Listen carefully: The only sound you will ever hear, no matter when the mixture is lit, by whatever means and at whatever throttle opening is DETONATION.
Preignition makes no more sound than normal ignition, it just silently destroys your engine.
And Adrenaline is right. "Colliding flame fronts" is total bunk. Any dual plug engine (of which there are plenty) has colliding flame fronts every cycle...

 

So then an engine that someone tunes to say 32* advance "pings" not because the flame propagates too soon (before TDC) but because a second instantaneous combustion of end gases ignites (detonates)?

I was taught that the ping is the flame front trying to expand while the piston is still attempting to compress it. It seems unrealistic that there would be enough heat since advancing the timing that far drops cylinder temperatures. i.e. If temperatures weren't high enough to spontaneously *detonate* at 26* then they wouldn't at 32*. Does it have to do with the incomplete burn from the normal combustion (that far advanced, too little heat) leaving behind more unburned end gases?

 

I would estimate your scenario is detonating from heat.

As the mixture expands, the compression of the upward moving piston
against the pressure of combustion increases the temperature within the
cylinder.

This may cause the end gases to detonate, and this is the ping you are
hearing after the spark, but before TDC(?).

 

Sorry, but you were taught wrong. Any textbook I have read supports my previous statements. Also, the earlier the spark occurs, the higher the cylinder temperature and pressure, because the pressure rise from the burn is added to that from the piston's on-going compression of the mixture. This causes the end mixture to reach critical temperature sooner and detonate a larger mass.

 

Either that, or I switched it in my head over the years. As for temperature I was confusing engine temp vs. cylinder temp which is why it didn't make sense to me.

So when we say that timing has been advanced too far, the gases detonate before the combustion is entirely consumed (flame front propagation) vs. a second ignition of gases? The definition of detonation always led me to believe that a second ignition from heat occurred in a sparse mixture of unburned fuel which "detonates" or flashes which causes ping. That's why I couldn't figure out what to call "ping caused by too much timing."

 

The ‘ping’, or whatever someone wants to call the noise, is a product of detonation. Whether it’s colliding flame fronts, or comeing from bouncing off hard parts, who really cares? Does it really matter??? This is all speculation and theorecical babble anyway. Text books are written by men and women and every friggin one of them has their own opinion and twist on what’s going on. And of course, each one is correct, or wrong, depending on what a person wants to believe.

The aspect of this convsation that rally matters, is that detonation and pre-ignition are both harmful to engine components and reduce engine longevity. Both are also completely avoidable by the engine builder. It’s the builder’s responcibilty to get it right while putting the build to paper prior to placing the order for components, not to just ‘wing it’. Pre-ignition, in my opinion is caused by incompetence. Whether it’s the builder’s fault or the owner’s fault, there’s no reason for it to ever occur. Detonation should also not occur if the builder has even a slight grasp of the differences between static and dynamic compression ratio and how they play into cylinder pressure.

 

I hear you. It's sometimes difficult to believe what you read even in texts
these days.

I tend to leaf through every thing, and stick to information that is written by
those with "Ph.d.", "Dr.", or some professional designation wrapped around
their name.

I'm basing my information on what I'd like to believe are credible and accurate
sources.

Whether we're coming across as arrogant to some is probably perception. Those
who have the resources to discuss topics in greater depth tend to come across
as argumentative.

I see it as correcting in the form of passing on education.

There are certain members here that I can learn a great deal from such as
Madbill. I feel privileged to learn something at a higher level and not brush it
off as theoretical babble, or speculation.

Thank you again to the select few that can moderate these topics. I certainly
appreciate learning about even if I'll never need to use it.

 

That's why the title of this forum it "*Advanced* Performance Tech". I personally find the discussion enlightening.

I didn't know incorrect timing was detonation vs. pre-ignition even though I understood the two when caused by something wrong within the engine and not the ignition system being calibrated incorrectly. I feel more "learn-ed" now.

 

99% of the people on this forum do no research, and don't know what they are talking about. They just spout off at the mouth based on bad info from other forums!!!

 

The only way you learn is to "listen", not much of that going on here.
I think posters to this forum should be limited to people who have learned this art, JimmyBlue for instance. He "listens" and only comments if he has something relevant that is based on experience or technical experts.
I would delete this whole thread as it has too much crap in it (including my post).

 

I hope you guys aren't referring to me????

I don't know it all and sometimes make mistakes, but I learned what I do know in tech school and subsequent years working as a dealer tech.

 

I think some people just have a tendency to forget that they weren't born knowing everything.

forums and such provide a good place for people to get answers because they can ask for clarification and details when they don't understand something.

Everyone starts somewhere and for a basic beginner some information is sufficient though untrue due to being incomplete.
for example : a bigger cam makes more power but revs higher.

You grow to find out that the camshaft thing is much more complex.

In the year 726, the sun rising and setting was sufficient, but incorrect. In 2005, the complexity of solar system comes into play. A 726 farmer could be told of the all the complexity of outerspace- but it really doesn't do him any good.

I've seen some serious BS posted here that has gone unchallenged. I don't bother because its not worth it.
I learned a lot of stuff here and learned a lot of if was bs once I read a few books, took a few classes, and did a bit of work in the field. No need to pull a holier than thou attitude (although I get a bit aggravated myself when I hear something like "You need more backpressure so your turbo will spool up faster" type crap.

I had boss deeply involved in the corvette ZR1 program and he would tell me about what pieces of crap they were on the track, their serious deficiencies etc- and when I told this to my friends they wouldn't believe it because corvetteforum.com said they got stronger the more they were driven.

the important thing is to keep an open mind and remember that just because it was said doesn't mean its correct.

The majority of the internet apparently believes that the LS7 is one of the most reliable engines ever built.

 

one other thing-
A lot of statements are made because of assumptions, and not neccisarily unreasonable assumptions. However, many times the corrilation people are looking for is nonexistant or underminded by an overlooked factor.

 

Well, seeing how we’re listing pet peeves, mine is when someone quote’s me, but only part of the quote. Then, others chime in without reading the entire statement, but only a part of it.

To clarify, what I was calling theoretical babble, was the noise one hears. Not the subject content of this discussion.

If anyone doubts my skills or experience, I posted it all above for the world to see. There are some incredibly knowledgably individuals that have done the same. I feel humbled to be in the same company with some, and to share thoughts and experiences with them.

And Technical, keep reading, it makes you even more curious. And that’s when you learn. Then apply what you learn to see how it all plays out. That’s the real joy associated with this craft. – to see a dyno curve come out the way you intended it to when putting the build on paper…..

 

Thanks for the advice. I've been doing that for over 10 years...never gets old.