That Damn Pinging!

Nearly ALL late model Trucks ping. It can be controlled by retarding the spark to TDC- however, you will lose efficiency. What ping, or *detonation*, actually is, is a matter of some debate in automotive engineering circles. The phenomenon is caused by an auto-ignition of the end gas produced by normal combustion of the fuel-air mixture. It is often mistaken for *preignition*, a state in which a portion of the fuel-air charge begins to ignite before the spark. Make no mistake, they are *totally* different phenomenon, and are completely unrelated in their cause. (Preignition is often caused by super-heated carbon deposits in the combustion chamber.

What happens is, the piston compresses the mixture, the spark fires, the flame front spreads outward, and then a series of secondary reactions takes place in the super-heated, high pressure, product gasses. These reactions, the actual mechanism of which are unknown, are extremely exothermic (hot), and hence cause a very rapid increase in pressure. This pressure increase causes the gas surrounding the reactive volume to expand rapidly in what amounts to a shock wave. (Not to dissimilar to the mechanism which produces thunder from a lightning bolt.) This shock wave in turn "bounces" around in the cylinder and combustion chamber which act as a resonant cavity. As they do so, energy from the shock waves is transmitted to the cylinder walls. *This* is the sound you hear.

To give you some indication of the difference between normal ignition and ignition with detonation, the peak pressure in the combustion chamber attained by normal combustion is somewhere around 800 psi (normally aspirated, spark ignition engine burning gasoline). The peak pressure, on the other hand, attained during knock can be as high as 2800 psi. Unfortunately, this incredibly high pressure is no good for producing work to drive the engine. It is of a highly oscillatory nature (5000 Hz or so) and so the average value of the pressure is not much higher than 800 psi.

What the oscillations do do, is increase the amount of heat transfer from the end-gas to the cylinder walls, combustion chamber, and piston face. This lowers the overall temperature of the end-gas and hence there is less energy available to do useful work. What this increased heat transfer can also do, is damage the soft face of the piston. (However, this is typically limited to very high compression engines.)

What causes it? No body knows for sure, but high compression ratios(9:1 or greater), highly advanced timing( greater than 15BTDC), low octane fuel(87 or less), and a lean mixture are all contributors. These contributors are roughly in order of severity, most to least. Let's figure out why late model J trucks ping.

Compression ratio 8.5:1, not that. Static timing 5deg BTDC, probably not that. 87 Octane or better at most pumps, probably not that. Since most late model trucks run lean (for fuel economy and emissions reasons), that is the most likely reason for ping.

It seems that most trucks knock after they have reached operating temperature, and then under significant acceleration. What that points to is a component affected by heat causing the mixture to lean out, or allowing the timing to advance to much under load.

Upon examination, everything that could point to the timing changing to cause ping rules itself out, which leaves the fuel mixture.   Perhaps the fuel heats up in the bowl and expands enough to lower it's density. Then, volumetrically speaking, the air flow through the carburetor would pull out to little fuel mass. Most likely is a poorly engineered power valve in the carburetor.

As the throttle is opened under load and the rpm's increase over 2000 rpm [or so] the fuel:air mixture requirements for an engine increase. Venturis, having a linear response to air flow (in this regime) do not allow for the increased fuel requirements necessary under heavy loads. The power valve, which is typically connected to the accelerator through a mechanical linkage of some kind, is designed to open and allow additional flow through a second set of jets. It would seem that the guys who designed the system cut the power valve response to try and eek a few more mpg's out of the 360. (I think that it's safe to say that AMC wasn't forced out of business due to the ultra-thrifty 360 equipped J truck)