Detonation vs. Pre-Ignition

[klamathpro]

Originally Posted by Import Power

Your summation is incorrect. One of the main jobs of a sparkplug aside from igniting the mixture is HEAT REMOVAL from the chamber. It is this HEAT (along with pressure) in the chamber that causes detonation. The colder the plug the more heat (and faster) the plug can pull out of the chamber and move it to the cooling system for removal. This chamber cooling is what colder plugs are all about. And this is why colder plugs lower chances of pre-ignition detonation induced by a heat spot caused by the plug itself.


Colder plugs also pull heat off thier tip faster which does help pre ignition as well.

Mind telling me how a colder plug can "pull heat" away and into the cooling system? Colder plugs simply eliminate the plug from being a source of heat, not draw heat away or remove heat. The spark plug has many points of heat spot potential and that is why a colder plug works well in preventing pre-ignition and detonation induced pre-ignition.

What I was saying was the fact that true detonation occurs away from dead center usually rules out the plug as being the main culprit. This is because the gases have moved away from the center of the chamber after the explosion. However that hot plug could pre-ignite the next cycle which would occur at or near the chamber's dead center. Which is why I never noticed a reduction in knock by going to colder plugs because our knock sensors detect true detonation and not pre-ignition. Detonation simply occurs away from the spark plug. As quoted under pre-ignition: "A glowing spot somewhere in the chamber is the most likely point for pre-ignition to occur. It is very conceivable that if you have something glowing, like a spark plug tip or a carbon ember, it could ignite the charge while the piston is very early in the compression stoke." Which is where a colder plug benefits.

I'm not saying there isn't a benefit to having colder plugs, heck I bought two sets off of you. I'm saying that colder plugs cannot draw heat away and cannot cure knock problems. Retarding timing reduces knock. But colder plugs sure help with pre-ignition and detonation induced pre-ignition and you can count on me buying them again in the future.

[Import Power]

Plugs pull heat out of the chamber and place it into the head. That heat is then carried away by the cooling system. This is not my opinion or thoery, it's known fact. It's one of the jobs of the spark plug.

[klamathpro]

Then this is a fact I've never heard of. I just don't understand how you can imply that the spark plug could actually be some sort of heatsink for a 1500 degree chamber. The chamber walls transfer the heat to the cooling system, not a puny ceramic stick. If one of the jobs of the spark plug were to be a heat sink, then the water cooling paths would be focused around the plug. The water paths of all engines circulate around or near the chamber walls. The job of the modern spark plug is to do two things: (1) Create the spark and (2) draw it's own induced heat away; not draw away the heat in the chamber. It's a potential Heat Spot, not a heat sink. And having a colder plug reduces that heat spot because it stays colder.

Originally Posted by Import Power

Plugs pull heat out of the chamber and place it into the head. That heat is then carried away by the cooling system. This is not my opinion or thoery, it's known fact. It's one of the jobs of the spark plug.

[bluemax_1]

IPO,

I think klamathpro is right. The reason for plug heat ranges is because there is an optimal heat range for the spark plug in the engine. If the plug is not a cold enough range, it isn't conducting enough of the heat it absorbs from the combustion and can either create a hot spot for pre-ignition or potentially burn the electrode tip.

While colder plugs are more effective at conducting heat to the head, it is the little heat that they've absorbed to keep them in proper operating temp range that they're supposed to conduct away. Their effect as a heat sink to draw heat out of the chamber is much less significant.

Too cold and the plug doesn't stay clean vaporizing/burning off fuel that comes in contact with it and it will start to develop carbon deposits, which of course can lead to the potential for hot spots and pre-ignition and if the gap is small enough, carbon deposits eventually bridging the gap.


Max

[mjannusch]

Agree with klamathpro.

[mehlosTT]

Agreed with Matt.

[Highboosted]

Agree with IPO. Colder plugs have a shorter ceramic projection, resulting in faster head transfer to the head, decreasing chamber temps. Spark plugs themselves will spark no matter the temperature. Now bluemax is also right, in that the "range is for proper clean burnoff, but you would be suprized the amount of heat a spark plug removes, which is REALLY important for aircraft engines because they are air cooled and prone to shock cooling and seizure when power is decreased at altitude. We have a test bench for spark plug's heat dissapating capabilities for testing aircraft plugs.

[mjannusch]

What conducts heat better? A short little ceramic projection, or a large block of aluminum which is in direct contact with the coolant?

The plug's temperature range adjusts how much heat the plug itself can dissipate to control its own temperature to prevent itself from being a hotspot in the chamber, it doesn't really dissipate enough combustion heat to the head to reduce the chamber temps to large extent compared to the large exposed surface area of the head itself.

BTW, I'm not trying to argue with anyone - I just think on a liquid cooled aluminum head motor the head is more efficient at removing heat than the plug is.

[VR4neophyte]

Outstanding article. So detonation is more often the culprit rather than pre-ignition. I'm sure it is detonation that caused my engine carnage in July. Good sticky.

[Bret Brinkmann]

Wow this thread is awsome!

I have to say that ALL of the heads I have ever seen (just got done swapping heads for another member), the exhaust valves had the sandy erroded look as described in the signs of detonation section. I even see this on valves ot of stock engines. I had to polish the valves to make for a smooth enough sureface for my suction cups to hold onto so I could lap the valves. The intake valves had a lot carbon deposits on them. This says to me that the temperatures across the head are NOT the same. It looks like the intake valves aren't getting hot enough to burn off the deposits increasing the potential for hot spots (from leaky injectors/running rich/oil in the intake charge from leaking turbo seals or the PVC system) and the exhaust valves are getting too hot causing detonation.

I see several problems here. The main ones being a lack of maintenance and tuning capibility. With the intake valves, make sure your injectors aren't leaking and have excellent spray pattterns. In my conversion with a representitive from RC Engineering, they recommend cleaning the injectors every 80K. I'd like to see this service be added to the 60K service personally. All ECUs have the capibility to adjust injector duty cycle. But only to a certain degree. In my auto classes I was taught that amotn of adjustment is about +/- 14%. So if increase the injector size (OEM size is 360cc/min) past 410cc/min injectors (360 plus 14 percent equals 410.4), then you just maxed out your ECUs ability to compensate the ijector duty cycle toward the negitive side. Your engine may still run desent, but you will start to get lower idle quality and a driveabilty issues. Some sort of fuel controller will be nessisary at this point, but even a fuel controller on a stock 3/S can give a worth while power increase if you know how to tune it properly. (Spend a lot of time in the ECU tuning forum, on the dyno, and looking at gauges.) EGR can also introduce carbon build up to the manifold and combustion chamber bt

Keeping oil out of the engine seems a bit harder. Valve stem seals come to mind. It isn't always in the budget to replace them but until you do you may as well as save the mod money for maintenance money instead. I have seen several cars with various mods run like crap at the track because of leaking valve stem seals. I don't know what exactly it does to the combustion process (not just octain rating but also causing abnormal/incomplete combustion), but I know it sure doesn't help power production. Properly maintaining your 3/S WILL prevent power losses and make it easier to tune as well as run smoother.

The other source of oil that comes to mind is through the PCV system. A restricted PCV valve will cause an engine to injest oil through the rings and/or valves stem seals. Cleaning or replacing the PCV valve can help prevent this. You can still get a certain amount of oil throgh the normal operation of the PCV cyctem though. An oil catch can can help. It is important to remember that the catch can has to have enough air flow capasity through it or it will act as a restriction to the PCV system. I helped one member diagnosis a hugh crank case pressure proplem not too long ago that we traced back to the addition of an oil catch can. It did not provide enough air flow capasity for the PCV system and resulted in crank case pressureization. The dip stick was shooting out, the front main was pooring out oil, and the engine was spitting out light blue smoke. The low in power as a result of this situation was so bad it was practically worthless to even try and accelerate.

I'm not sure what we can do about the exhaust valves sort of swapping them out for ones that run at a cooler operating temp or transfer heat to the head faster. I suppose if we keep them from getting so hot in the first place (ie keep the EGTs down), then that may help. One way or another the failures and signs I personally am seeing and hearing about suggest detonation and not pre-ignition is what we should be mainly concerned with and trying to eliminate.

That being said, plugs are more likely to cause pre-ignition and not detonation. At least directly anyway. True the head and cylinder walls are the main means by which heat is transfered into the coolant but the plugs still make a significant difference, even if they are not THE MAIN difference. Still, I have seen more evidence of detonation around the exhaust valves than the plugs themselves.