Ignition advance, was Re: alternative engines, WARPED

[Ken Kelly]

Good response! However, there were a few engines in the 70's
that didn't used ported advance. The Oldsmobile 350 "R"
engine from 76-78 was an example. My 77 Olds 350R always
failed the NJ State Emmissions test by 10% high HC because
it used manifold vacuum for advance at idle. If I plugged
the vacuum advance, and reset the idle speed to stock, it
would pass at about half the state HC level, but the milage
and drivability were terrible! It was a common problem with
these engines, and the emmission test. However in stock form
it was one of the most responsive late 70's engine, and it
had great gas milage. After owning it for 17 years and 17
inspections I got to be able to switch it back and forth in
5 minutes.

		Ken

Gary Derian wrote:
> 
> There are two kinds of ported vacuum on carbs.  The EGR vacuum works like
> Aaron described.  Its purpose was to gradually feed more EGR as the throttle
> is opened.  Its port is a tall slot in the throttle bore.  As the throttle
> is opened, the throttle plate rises along the slot which feeds more vacuum
> to the port.  The other type of ported vacuum is just a small port above the
> throttle.  As soon as you go above idle, it applies full manifold vacuum.
> Its purpose is to reduce timing at idle to just the static advance.  Not
> considering emissions, some engines idle smoother with less advance.  After
> 1968, all engines used ported vacuum advance.  Before 1968, some did and
> some didn't.  Both types of ported vacuum are limited to the maximum
> manifold vacuum available and go away as one approaches full throttle.
> There is also venturi vacuum which is way smaller than manifold vacuum but
> it increases with air flow.  The Ford small in line 6's from the early 60's
> used some kind of all vacuum distributor connected to both manifold and
> venturi vacuum (I think).  It was claimed to provide improved economy.
> Venturi vacuum is also used in some EGR control schemes.
> 
> Gary Derian <gderian at oh.verio.com>
> 
> > Hi all,
> >      I could be wrong at this, but what I have seen is, ported vacuum is
> dead
> > at idle, because it is above the butterfly....and as throttle is
> increased,
> > vacuum is generated at the "port" and does not go away at full
> > throttle,,,,,great for emissions but terrible for performance,,,,,the same
> > for MPG....
> > -Carl Summers
> >
> >
> >   You sure about that?  I have always understood that a vac advance can
> >  reads ported vacuum in order to affect a curve that begins as the
> throttle
> >  is opened and increases as more throttle is applied, up to the point at
> >  which all vacuum (manifold and therefore ported as well) fades under
> heavy
> >  throttle openings, at which point the advance will decline again and the
> >  engine will see only mechanical advance.  This is apparently to provide
> >  extra advance for economy at part throttle.  Anybody who can further
> >  illustrate this concept, please do because if I am wrong here I'm going
> to
> >  have some serious rethinking to do!
> >
> >
> >   Aaron Willis
> >   ICQ #27386985 >>