Pinging - What Happens Inside?

[YellowLark]

I've had a pinging under acceleration with my '66 340-4 engine since the 1970's, when gasoline quality changed. Have read many articles of advice, and tried most of them. Engine has 10.25:1 compression, which is probably the root of the problem. Have recently handled most of the problem with all-MSD ignition and liberal use of octane booster.<P>But, I realize that I really don't understand what is happening inside the combustion chamber at the moment of acceleration, nor have I read a good explanation of it. So, I'm driving along at 30 mph and floor it. Within a second, I have pinging. What has happened within that one second?<P>I can understand the diagrams about too much spark advance, but is that even a factor within such a short time?<P>The carb has opened and vaccuum has dropped. Has compression effectively changed? Has the volume of gasoline in the chamber somehow caused the pinging? Has there been time for the distributor to have advanced the spark timing yet? (the new MSD distributor does not have vaccuum advance, have it set about 14 degrees at idle, 32 total. Anyway, the problem is less with this new ignition.)<P>So, what causes the pinging to start so quickly?

The pinging is caused by the rapid increase in combustion chamber temperature. This is what vacuum advance is designed to minimize. When engine vacuum is high, the engine is at light load, and a lot of timing can be used for good efficiancy. When engine load increases, vacuum drops, and combustion chamber temps go up. The vacuum advance removes ignition lead. A large range of vacuum advance is desirable from an operational standpoint, but it is a bummer for emmissions. A coasting engine would have full advance, and this is not good for the EPA. The best deal is the computer controlled ignitions used on current engines. Computer control also enables EGR to be used as a detonation control. <P> It is important to differentiate between pinging and knocking. Pinging is a light rattle caused by a ragged, unorganized flame front. Light pinging is not too bad on the engine, an engine which has experianced light pinging all its life will have little pit marks on the piston tops.<BR> Knocking is a heavy metallic thud caused by the entire ignition event occuring too soon. The max combustion pressure occurs while the piston is still coming up on the compression stroke. The actual knock is caused by the oil film being beaten out of the rod bearing, and the bearing hammering on the crankshaft. Knocking will destroy an engine in short order.

bob hit the nail on the head. i would like to add, however, that the increase in combustion temps can be caused by a lean/unburnable mixture. the sudden drop in vacuum (actually, increase in pressure toward atmo) causes vaporized fuel to condense, leaning the mixture within hundredths of a second of the jump to WOT. this is why carbs have accel pumps. if you increase the size of the initial edge of the accel pump squirt, you can sometimes fight pinging, due to the cooler/richer intake charge.

Bob and Allan explained it wonderfully!<P>I'll add another bit of info I've read: by the time you actually hear detonation you've got an out-of-control combustion event happening. When Buick introduced the 3.8L Turbo back in 1978 they incorporated a knock sensor that can "hear" detonation well before we humans can. If you had a knock sensor on your 340 it might "hear" that you've got light detonation at part throttle as well. Even with octane boost, on today's unleaded fuels 10.25:1 is really pushing the envelope. Engines built for leaded fuel aren't expecting the relatively quick (and hot) combustion of today's fuels. Lead helped the fuel burn more slowly and fully, with less heat and more efficiency - meaning that the "flame front" Bob mentioned occured for a longer time and covered a broader area of the combustion chamber. Allan's mention of a sudden lean condition at WOT is exactly right; with less fuel to cool the fire, combustion chamber temperature spikes, causing the "pinging" you hear.<P>You might be able to help the problem just a little by "decoking" the combustion chambers. Here's the link to the thread about how to accomplish that:<P><A HREF="http://www.aaca.org/cgi-bin/ultimate...&f=2&t=000921" TARGET=_blank>Decoking; The Click and Clack Method</A><P>By the way, my California GS used to ping once in awhile too, but not to the extent you are describing. I "blew the engine out" when I first brought it home and it seemed to help.<P>Best of luck!<P>Matthew

[YellowLark]

Thanks for the great explanations. I now have a much better understanding of the causes.<P>I did read the decoking thread when it was posted and poured water into my carb, which resulted in much black crap being blown out. Maybe I'll do it again now.<P>Also should mention that changing the spark plugs from AC 45 (original) to the colder AC 43 has helped. Again, this ties into the overheated combustion chamber situation.