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In Reply to: what happens when your car pings too much? posted by chrystobal on July 30, 2000 at 17:54:25:
As for the mechanical side... Pinging/knocking or detonation (proper engineering term) occurs when the fuel in the combustion chamber is burned before the piston or pistons reach TDC. The noise that you hear is your poor engine fighting itself... not a good thing. Not only is it bad for the engine you also loose power. If the fuel ignites before the piston is at the top the combustion will force the piston the wrong way. There is not enough force to make the engine stop and spin the other way due to the large ammount of rotational inertia in an engine. It does stress and fatigue all connected components because your BMEP is opposing the rotation of the engine. BMEP = brake mean effective pressure == amount of pressure exerted on the piston face during the power stroke.
As for the thermodynamic side... Higher octane fuels need more advance because they burn more slowly than lower octane fuels. More advance = more time to burn. If you set up your timing curve for high octane fuel and then fill up with some bunk gas you can get screwed. BTW, DIS's with knock sensors are great! So if the fuel is combusted and then compressed even more by the piston that is still traveling up you have hot gas and then some really hot gas because of the decrease in volume. (pV=mRT)
As for the somewhat fluid mechanical side... the gas sits in the combustion chamber for an instant longer than should becuase there is a longer time before the exhaust valve opens and flows out.
Bottom line... Burned exhaust valves, burned head, burned combustion chamber, burned piston faces, broken cranks, conecting rods, etc. are the worst case senerios. Best case is your car will run a lot hotter, with poor performance.
Hope that was somewhat helpful. Drop me a line if you need any specifics,
Mike
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