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Re: vacuum advance

To: "William Lewis" <william.lewis@ucdmc.ucdavis.edu>,
Subject: Re: vacuum advance
From: "Louis & Laila" <bwana@c2i2.com>
Date: Tue, 29 May 2007 17:10:53 -0700
Outstanding post!
>
> http://www.gofastforless.com/ignition/advance.htm
>    You're probably thinking, "Sure there is no manifold vacuum at WOT but
> aren't I supposed to use ported vacuum for the vacuum advance." Hold onto
> your hat, THEY ARE THE SAME THING! Except ported is shut off at idle. 
> There
> are a lot of misconceptions when it comes to the ported vacuum source.
> After hearing 20 different theories I decided to hook up two vacuum 
> gauges,
> one to manifold and one to ported, then drive my car and watch it. I found
> out they are the same, except the ported is shut off when the throttle is
> closed. Even then I had a hard time convincing guys so I hooked up a 
> couple
> MAP sensors and a throttle position sensor to a data logger and recorded
> them while driving then dumped it into a spreadsheet and made a chart. As
> you can see, there is a direct relationship between throttle position and
> vacuum. When the throttle is closed vacuum is high, when the throttle is
> open vacuum is low, and ported vacuum is the same as manifold except when
> the throttle is closed. So which one do you want to hook it to? I prefer
> manifold vacuum. This pulls in more timing at idle which is good since
> there is virtually no load. Your motor will idle smoother and cooler with
> the extra timing. One night I was at the drags and my car was running hot
> in the staging lanes, I swapped the vacuum advance from ported to manifold
> then it would idle all night at 1750. Believe it or not the purpose of
> ported vacuum is to raise the temperature at idle, to lower NOx emissions.
> If you're like most hotrodders that is of no concern to you. If you have a
> big cam with a choppy idle then a vacuum advance hooked to manifold vacuum
> can really help. It will idle smoother and requires less throttle to
> maintain speed. Often a big cam requires you to open the throttle so far
> that the curb idle adjustment needles won't work. Hooking the vacuum
> advance to manifold vacuum will allow you to close the throttle some which
> may be enough for the idle mixture screws to work. Someone told me he
> noticed less dynamic braking with the vacuum advance hooked to manifold. I
> didn't notice it on my car but it makes sense. If the motor is running 
> more
> efficiently with the added advance it will make a less effective brake. So
> which should you use? Try both and see which you like best.




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