[DeTomaso] Vacuum advance, ported or manifold, age old question answered

SG sdlgibson at sbcglobal.net
Sun Mar 18 18:05:53 EDT 2012


Hooked up correctly, you shouldn't have any ported vacuum present at idle. Only as rpm increases and levels out at cruise.  Manifold vacuum will increase your vac advance at idle which is what you want in order to run cooler and cleaner.  It takes a lot of initial timing to correctly burn low octane and junk fuel.  You also need to run as much initial advance as the engine will stand, and limit the mechanical to the total that you want.  If your vac advance canister pulls in too much timing at cruise it is easy to get way too much timing, so it needs to be limited too.  Ported vacuum came about when they were trying to control emissions back in the day by trying to make engines run hotter etc.  The distributor needs to be set up correctly for the needs of each engine and vacuum advance should be used on everything but an all out race engine.  With vac advance an engine will normally run cooler and get better fuel mileage.
                                           Shaun Gibson

--- On Sun, 3/18/12, Will Kooiman <wkooiman at earthlink.net> wrote:


From: Will Kooiman <wkooiman at earthlink.net>
Subject: Re: [DeTomaso] Vacuum advance, ported or manifold, age old question answered
To: DeTomaso at realbig.com
Date: Sunday, March 18, 2012, 11:17 AM


One advantage of ported vacuum is it helps maintain a steady idle.

With manifold vacuum, the timing gets advanced as vacuum increases, which
increases the idle, which increases vacuum, and so on (and vice versa).  It
runs quieter and cooler with manifold vacuum, but the idle rpms tend to hunt
up and down.

With ported vacuum, the timing is constant (at idle), so it does a much
better job of maintaining steady rpms.  Plus, with ported vacuum, it sounds
better - less like a Toyota.

The hunting doesn't happen when you are moving because your rpms are
determined by your right foot.

-----Original Message-----
From: detomaso-bounces at realbig.com [mailto:detomaso-bounces at realbig.com] On
Behalf Of Mikael
Sent: Sunday, March 18, 2012 10:45 AM
To: 'SG'; DeTomaso at realbig.com
Subject: Re: [DeTomaso] Vacuum advance, ported or manifold,age old question
answered

When starting the car it creates vacuum and advances the timing. Problem is
that the starter can only run the engine at a very low rpm, can't remember
exactly, but well below the idle. At that low rpm the ignition can happen
when the piston is still on its way up, making it hard for the starter to
overcome. (If this happens, push down the accelerator and keep it there
until the gas has cleared the manifold, that'll allow the engine to turn
faster, then release accelerator)



Mikael



Fra: SG [mailto:sdlgibson at sbcglobal.net] 
Sendt: 18. marts 2012 15:31
Til: DeTomaso at realbig.com; Mikael
Emne: Re: [DeTomaso] Vacuum advance, ported or manifold, age old question
answered




Does manifold vacuum have anything to do with starting....or just initial
advance?   Hmmmm


Shaun Gibson

--- On Sun, 3/18/12, Mikael <mikael_hass at mail.tele.dk> wrote:


From: Mikael <mikael_hass at mail.tele.dk>
Subject: [DeTomaso] Vacuum advance, ported or manifold, age old question
answered
To: DeTomaso at realbig.com
Date: Sunday, March 18, 2012, 2:10 AM

I found this chart on the web, two map sensors and a throttle position
sensor clearly shows that except at idle, ported and manifold vacuum is the
same. So when you need to decide whether to use ported or manifold vacuum,
let your idle decide. If vacuum advance helps your idle and doesn't make it
hard to start, go directly to manifold. If the additional advance makes
starting hard, use ported vacuum instead.



Now all we need to sort out is the meaning of life...



Vacuum Chart

http://www.gofastforless.com/ignition/vacuum_chart.jpg





Mikael

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