[Mgb-v8] Carb to dizzy connection

[Carl Floyd]

Some of the British cars may have always used "carb vacuum", but here in the 
States, American Iron used full manifold vacuum for many, many years. The 
ported vacuum advance was a smog bandaid that was applied in the 70s. These 
days the onboard computers handle all that.

Most distributors can be set up either way if you know what you are setting 
out to do. Some engines may prefer one over the other. Usually these is not 
much difference either way once set up properly. They will both the job on 
the street by aiding fuel economy.

I use full manifold vacuum advance on my '79 B because it has a '63 Buick 
215 in it.

Carl

----- Original Message ----- 
From: "Paul Hunt" <paul.hunt1 at blueyonder.co.uk>
To: "Bill Zenkus" <mgb_zenkus at yahoo.com>; "Barrie Robinson" <barrie at look.ca>
Cc: <mgb-v8 at autox.team.net>
Sent: Sunday, October 19, 2008 8:18 AM
Subject: Re: [Mgb-v8] Carb to dizzy connection


> Are you sure you have this right?  Normally the two sources are carb and 
> inlet
> manifold.  Inlet manifold vacuum is near maximum at idle, rises slightly 
> as
> the throttle starts to open simply because the engine gets more efficient,
> then gradually tails away as the throttle is opened further.  Carb gives 
> zero
> vacuum at idle, rapidly rising to a maximum as the throttle starts to 
> open,
> then is the same as inlet manifold i.e. gradually reducing to zero as the
> throttle is opened further.  How much vacuum you get *does* depend on 
> engine
> speed to some extent, but as engine speed is controlled by throttle 
> opening
> (except on the overrun) it's the throttle that controls vacuum, and the
> reducing vacuum with increasing throttle is designed to offset the greater
> tendency of an engine to pink under wider throttle openings by reducing 
> the
> amount of advance on wider throttle openings.
>
> It's *centrifugal* advance that increases as engine speed increases, and 
> that
> takes advantage of the fact that as engine revs increase there is a 
> *reduced*
> tendency to pink, all other things being equal, so it can take more 
> advance to
> improve efficiency of combustion and hence performance.
>
> Which vacuum source is used simply depends on the emissions requirements -
> inlet manifold vacuum gives lower idle emissions as the engine runs more
> efficiently with more advance.  However it has a downside that it makes 
> the
> engine slightly more difficult to start as some advance is applied even 
> during
> cranking.  Carb vacuum is usually preferred, if manifold vacuum were 
> better
> for other than emissions reasons it would have always been used.
>
> PaulH.
>  ----- Original Message -----
>
>
>  The passenger side vacuum port is ""timed" meaning vacuum increases with
> engine speed.
>
>  The drivers side port is the "full" vacuum port meaning the vacuum 
> decreases
> as engine speed increases.
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