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

Will Kooiman wkooiman at earthlink.net
Sun Mar 18 12:17:02 EDT 2012


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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