DB, arguments are good. This is the best way to learn. I agree with your
description of how the vacuum "signal" is created.
Sorry, I did not express my self properly (you may still disagree, but at
least you will disagree to the right thing!). Engine knock is the result
of very high pressures and temperatures in the "unburnt" zone inside the
combustion chamber. The flame front propagates after the spark in the
combustion chamber in a finite velocity (depended upon the turbulence in
the combustion chamber, NOT upon the fuel grade). The temperature in the
unburnt zone climbs, and if the fuel does not have sufficient octane, the
unburnt zone will explode spontaneously (rather than consumed smoothly by
the flame front) creating pressure waves which then increase the heat
transfer within the combustion chamber walls, and eventually drill holes
through pistons. This mechanism is more prone to occur at high loads when
the peak cylinder pressure and temperature are very high. At part loads,
the peak pressures are quite low, so you can tolerate a lot more spark
advance before knock occurs. But I agree with you, it can happen at part
load too. In my 440's I experience knock only at WOT when I over advance
the timing. I had some part load pinging when I first got my LeBaron, but
the deposits causing it are long gone, converted into green house
gasses. Other experience with part load pinging is on a Fiat engine I
used to operate in Greece. When climbing steep hills, it would ping
sometimes, but that was also a high load condition. I do not recall if
that engine would also ping at WOT with the settings causing it to ping at
part but still heavy load. I guess its possible to have an overly
aggressive vacuum advance to the extend the engine would ping. From my
limited experience, I think its not very likely if the engine is free of
deposits...
D^2, 2x68s
PS, Dick, may be you should drive at WOT more often to clear up deposits! ;)
At 10:09 AM 8/19/2002 -0700, you wrote:
>I hate to pull rank here, as I know I'm in danger of starting an argument
>with our young friend from Texas, but in my experience, ping usually occurs
>at part throttle, not full throttle unless the car is seriously out of tune.
>
>The reasons are clear, pure physics. (Boyle's law and the Venturi effect to
>you, d-squared)
>
>The vacuum is supplied by an orifice just above the throttle butterfly, so
>that at idle, there is no vacuum advance (the butterfly is not open
>significantly), but as the throttle is opened, the orifice begins to see
>more and more of the intake manifold vacuum, finally dropping off again as
>the throttle approaches wide open, due to the loss of restriction between
>the outside air and the intake manifold.
>
>Thus, the vacuum advance has a strong effect at part throttle, advancing the
>timing at part load, but this disappears as the load increases and the
>vacuum falls toward zero.
>
>Dick Benjamin
>
> > Chris, you may be more familiar than I with the early 60 big blocks, but
>the
> > vacuum advance advances only at part load. Engines typically ping at full
>load
> > where there is zero vacuum, so the vacuum advance setting has no effect.
>I
> > think it may be unlikely that an engine without deposits will ping at part
> > throttle if proper octane gas is used. More advance at part load could
>help
> > gas mileage and reduce coolant temperature a bit by reducing exhaust
> > temperature.
> > D^2
>
>
>
>
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