:
: Here's a duplicate of something I posted on the
: slantsix.com site. I had a request to post
: it here FYI too.
:
: Vacuum advance in a nutshell: 1) A vacuum
: amplifier will do nothing except readjust or
: delay your timing curve. The speed with
: which the advance "comes on" is
: plenty fast with just manifold/carb vacuum.
:
: 2) Vacuum advance only advances the timing when
: you are at part-throttle cruise (when
: clyinder pressure is low) and/or at idle.
: You can have vac
: advance at idle by using a direct manifold
: vacuum source, which some cars had, or you
: can have no vac advance at idle by using the
: above-throttle-plate vacuum port on your
: carb. The more you push on the pedal, the
: less vac advance you get. The lowest amount
: of vac advance is when you are at WOT, so
: you don't ping. Your advance curve will be
: almost identical whether you use the
: manifold port, or the carb port - the only
: substantial difference being the one (at idle
: advance) I just mentioned above.
:
: 3) The number stamped on the arm of the vac pot
: is the number of degrees of cam rotation
: advance that it produces, double this number
: for crank rotation advance (which is what
: you look at with a timing light).
:
: 4) You can adjust the point at which the vac
: advance comes on and off by sticking an
: allen head wrench into the end of the pot
: and turning. You
: can adjust between about 4-5 in Hg, to about
: 13-14 in Hg. I think it's CW for lower vac,
: and CCW for higher, but you might want to
: check this. Some
: pots have no adjustment. Setting the pot for
: lower vacuum means that you will maintain
: advance with more opening of the throttle,
: and you are more likely to ping. Setting for
: higher vacuum means the vac advance with
: pull off with less throttle (less pinging).
: Note that when you are at WOT you pull 0 in
: Hg of manifold vacuum, and at idle you pull
: about 15-20 in Hg. Parts stores sell
: "Standard" brand pots that are
: adjustable in several advance flavors
: (number of degrees).
:
: I hope this helps all you power tuners out
: there,
:
: Lou

: Here's a duplicate of something I posted on the
: slantsix.com site. I had a request to post
: it here FYI too.
:
: Vacuum advance in a nutshell: 1) A vacuum
: amplifier will do nothing except readjust or
: delay your timing curve. The speed with
: which the advance "comes on" is
: plenty fast with just manifold/carb vacuum.
:
: 2) Vacuum advance only advances the timing when
: you are at part-throttle cruise (when
: clyinder pressure is low) and/or at idle.
: You can have vac
: advance at idle by using a direct manifold
: vacuum source, which some cars had, or you
: can have no vac advance at idle by using the
: above-throttle-plate vacuum port on your
: carb. The more you push on the pedal, the
: less vac advance you get. The lowest amount
: of vac advance is when you are at WOT, so
: you don't ping. Your advance curve will be
: almost identical whether you use the
: manifold port, or the carb port - the only
: substantial difference being the one (at idle
: advance) I just mentioned above.
:
: 3) The number stamped on the arm of the vac pot
: is the number of degrees of cam rotation
: advance that it produces, double this number
: for crank rotation advance (which is what
: you look at with a timing light).
:
: 4) You can adjust the point at which the vac
: advance comes on and off by sticking an
: allen head wrench into the end of the pot
: and turning. You
: can adjust between about 4-5 in Hg, to about
: 13-14 in Hg. I think it's CW for lower vac,
: and CCW for higher, but you might want to
: check this. Some
: pots have no adjustment. Setting the pot for
: lower vacuum means that you will maintain
: advance with more opening of the throttle,
: and you are more likely to ping. Setting for
: higher vacuum means the vac advance with
: pull off with less throttle (less pinging).
: Note that when you are at WOT you pull 0 in
: Hg of manifold vacuum, and at idle you pull
: about 15-20 in Hg. Parts stores sell
: "Standard" brand pots that are
: adjustable in several advance flavors
: (number of degrees).
:
: I hope this helps all you power tuners out
: there,
:
: Lou

: I just want to add that if you convert to EFI,
: use a throttle body, and use your existing
: ignition timing, that you'll have to use
: manifold vacuum for your distributor vacuum
: advance. This is because there is no venturi
: in an EFI throttle body. I'm using manifold
: vacuum, have excellent off idle throttle
: response, and have experienced no
: difficulties.
: If you're convinced that you want ported vacuum
: for your distributor vacuum advance, and
: still want EFI, you could use a modified
: carburetor for your throttle body. However,
: because of the required airtight ductwork,
: and the fact that a carburetor is typically
: taller than a throttle body, hood clearance
: may be your biggest obstacle with that
: approach.
:
: Bob D

: Bob,
:
: Just FYI, some throttle bodies do have ported
: vacuum. My Holley TBI does. Personally, I
: like the manifold vacuum anyway. You don't
: need a venturi for ported vacuum, only a
: port on the air side of the throttle
: blade(s). Most carbs I have seen have the
: ported vacuum hole just above the throttle
: blades and not in the venturi.
:
: Cheers,
:
: Lou

Do you use the temp switch that shuts vacuum off until the engine warms up?