Spark knock is when the gas/air mixture in a cylinder ignites before the sparkplug does. Thats why its often called pre-ignition knock.dgksleeper said:I don't mean to sound like a dumba-- but can someone help me and describe this spark knock noise. I hear something after the car has warmed up.
If the gas/air mixture is igniting before the sparkplug, then would using a gasoline with a higher octane rating help? I was thinking that since a higher octane rating means a higher compression ratio before it ignites, it would prevent the self-ignition.Huff said:Spark knock is when the gas/air mixture in a cylinder ignites before the sparkplug does. Thats why its often called pre-ignition knock.
Often it sounds like the engine has marbles in it. In the case of my Caliber it very much sounds like a diesel engine at idle or when just starting to accelerate from a stop. If you've heard a diesel you'll recognize the issue in your Caliber if it exists.
It's not a compression ratio thing for the octane. Higher octane fuel has a higher flash point, so it doesn't burn as easily and doesn't create as much heat when burned, which reduces spark knock. Spark knock is usually caused by the fuel igniting before the spark plug fires due to hot spots in the combustion chamber, not the amount of compression.Sleazy said:If the gas/air mixture is igniting before the sparkplug, then would using a gasoline with a higher octane rating help? I was thinking that since a higher octane rating means a higher compression ratio before it ignites, it would prevent the self-ignition.
But once again, it's from excess heat in the combustion chamber, and yeshigher compression creates more heat, but it's still the hot spots in the combustion chamber causing the pre-ignition. Compression creating heat is the whole concept behind deisel motors. Our motors are not high compression motors. They were designed to run on 87, so it could be anything from just low quality gas(not low octane) to small bumps or ridges in the combustion chamber that needed smoothed out,to just too much timing in the computer.Fingernipp said:Compression will often do it as well. try putting 87 octain into anything running at 11:1 or higher. Many people are familiar with the dieseling effect heard from a car after shutting it off when running much too low of an octain on too high of compression engine. It will keep running on a cylinder or 2 using just detonation alone to keep it going. As you know compression create heat and low octain fuel being more volitile than high octain has a lower flash point.
Actually, our 2003 Saturn manual pretty much says the same thing. My guess is that most newer cars have such high compression ratios, that the engines are always on the verge of ping. Says the same thing that occasional light is OK, but prolonged or heavy ping should be addressed.quadrajet said:I have never seen an owners manual for anything that even mentioned spark knock, let alone it being "not harmful".