Ignition advance, was Re: alternative engines, WARPED

Gary Derian gderian at oh.verio.com
Wed May 19 13:16:15 GMT 1999


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 >>




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