Oh this is fun. If this thread is intended to define a cause for "detonation" then I think you first need a few definitions and some clarifications to guide the ensuing hypothesis.
First. Lets define two kinds of detonation or knock.
- Knock - will, can, or should be defined as spontaneous combustion of a fuel air mixture caused by a rapid rise in cylinder pressure from a spark ignition event resulting in the auto-ignition of said fuel air mixture ahead of the flame front from said spark event. This is the most typical kind of knock, typically a result of too much spark.
- Pre-ignition - should be defined as the ignition of the cylinder charge prior to the spark event, caused by a variety of not so well understood factors.
Second, we should clarify some operating condition myths.
- The ECU does not adjust desired lambda leaner than 1.0 during a highway driving cycle, except for closed loop toggling. We would saturate the catalyst with oxygen and violate the Nox limit for an emissions test if we did.
- Most "lean spikes" in the exhaust under heavy load are caused by scavenging (high overlap allowing boost pressure to push fresh air in to the exhaust stream), this fresh air pushes the pre-cat O2 sensor lean, however in cylinder air charge actually ends up rich because the ecu fuels for the measured amount of air, even though it is not all trapped in the cylinder.
- The GM performance tune will probably not be any worse for pre-ignition than the standard production calibration because pre-ignition has very little to do with spark advance. Pre-ignition is not detectable by knock sensors because the ignition event occurs ahead of the spark event, and does not produce the same frequency oscillation as typical knock event. In short, there is not a whole lot the ECU can do to stop pre-ignition except cutting off the fuel.
Ok, anyway, on to the meat.
It is well known that knock events in combustion engines are closely tied to fuel quality and pressure rise within the cylinder. That being said, high effective or geometric compression ratios (IE, BOOST) will increase an engines sensitivity to knock, as well as aggressive spark advance. Furthermore the farther you retard spark advance, the more heat you introduce in to the combustion chamber and surrounding components. This means that as you retard spark, you may delay the burn such that it reduces the likely hood of knock, however you will begin to heat up the system causing the sensitivity of knock to increase. One could say that sustaining these high load / retarded spark conditions will just produce a never ending spiral of knock / spark retard / more knock / more spark retard. The ecu is designed in such a way that at a certain point of spark efficiency loss it stops adding boost and you just begin to lose torque. This allows the engine to protect itself against damage from these kinds of events.
- Weather plays a huge role in the capability of the engine controller to deliver a desired torque. The more torque you demand from the LNF, the more sensitive it will be. For instance, dry air knocks a lot more than humid air, hot air knocks more than cold air, cold air is dry, etc etc etc etc.
- All boosted engines will knock! The fact is that you are continuing to cram more air in to a cylinder than it's naturally aspirated limit, it's going to knock. The question of course is how much and for how long. The ECU's knock control system, if operating properly, manages the spark such that it does not continually knock during a driving cycle.
-- There are a few adapts for this system, one even for fuel. Basically if the ECU is convinced that you have put crap in your gas tank it permanently reduces spark to adjust for that driving cycle. The next driving cycle it tries again.
--- The more likely cause for knock after a long highway trip are two fold:
- - - - First: Everything is warm. The combustion chamber and piston, etc, have all gotten hot from driving for such a long time at moderate load.
- - - - Second: The ECU has probably not been operating in a "high" knock intensity area, so more than likely you will not have any spark knock correction.
--- Addressing knock on tip in after a hard accel. This goes to the boost and retarded spark comment. You will knock on tip in after a hard acceleration, you just got everything hot with boost and retarded spark! If you see the knock retard light up after this kind of an event, say you ran up a hill at WOT, and now are driving straight and level and it's knocking, don't be concerned. The knock system is doing it's job! if you don't see the knock system doing anything, then be worried.
While audible knock is concerning, a quick couple of events isn't going to kill anything. it's the sustained knock thats a problem. Of course, if you are concerned about it, try a higher grade gasoline, if available. If you are concerned about the mechanics, pull the plugs and look for corrosion of the side wire, it'll look like cavitation, also borescope the engine and look for nibbling around the edges of the piston, a little is no big deal, if it looks like a bread carving knife you have a problem.
A quick couple of notes about pre-ignition. There are a few things that can cause this, one of which is spark retard. Basically you get the plug so hot it turns in to a torch and ignites the fuel as it is sprayed in to the cylinder. You'll know when this happened because everything will seem fine and then the engine will be laying on the floor disassembled in front of you. :devil:
- The other kind is caused by a mixing of constituents in the combustion chamber, oil, fuel, air, crap, etc. That causes ignition during the compression stroke and results in a huge knock event. Tuners have typically run rich air fuel mixtures to get around this, however with both the stock and GMPP tunes it shouldn't be required.
Personally I have only had one or two "ping" events in my solstice in the 33k miles that i have on it, however I watch the knock sensors pretty frequently and they are indeed working.
Ok in summary. Knock is generally cause by too much spark, lean AFR, or bad gas.
Pre-ignition is generally caused by an overheated spark plug or constituent mixing that allows for combustion before the spark event.
In the hopes of providing actual data for a discussion I would like to entertain some more specific information about the types of events you are seeing Jazz.
- I understand highway, so I'm guessing warm engine, say 95-100C coolant, what kinds of days, hot? cold? muggy? all of the above? Do you have a data trace? I can compare with some data from my car.
I hope that I have provided some insight and not just blabbed :willy:
