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
----- Original Message -----
From: <dardal@xxxxxxxxxxxxxxx>
To: <mailing-list@xxxxxxxxxxxxxxxx>
Sent: Monday, August 19, 2002 9:45 AM
Subject: Re: IML: 1960 Vacuum Advance
>
> > Note on vacuum advances from the mid-60's: The most
> > common ones available are set up for the emission
> > controlled needs from 1965 on - called C.A.P. They
> > provide nearly double the advance (up to 17.5 degrees)
> > and if installed will cause your early 60's engine to
> > ping like crazy. Be sure you are betting the right
> > one.
> >
>
> 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
Back to the Home of the Forward Look Network