Bill,
I sort of disagree with the statements below.
Connecting the manifold vacuum to the vacuum advance
puts a mush stronger vacuum signal on the diaphragm than it was designed for
(20+ inches of HG) vs 10 or so
inches of HG coming from the throttle ports.
This will not yield the same load vs. advance rate, and will likley over
advance the timing at light load.
While I agree most engine LOVE lots of timing at or near idle, the timing needs
to go away as soon as ANY load
comes into play.
Another possible downside, (particularly if you have a more aggressive cam) is
that as the engine RPMs come up, and the
engine VE gets better (so the manifold vacuum gets stronger), the engine can
develop and oscillation of engine speed at idle
due to the changing continuous changing of timing. This I have seen with
poorly setup engine management that puts too much
rate of advance based on manifold pressure (MAP).
Jarrid Gross
-----Original Message-----
>From: William Lewis <william.lewis@ucdmc.ucdavis.edu>
>Sent: May 29, 2007 12:07 PM
>To: alpines@autox.team.net
>Subject: vacuum advance
>
>I have been thinking about putting side draught carbies on my Series II car
>for a long time. One concern I have is that my distributor uses mechanical
>and vacuum advance. I don't really want to loose the vacuum advance and I
>have not seen a side draught carb that has ported vacuum advance. I just
>ran across a web page for drag racers and this guy makes the following
>argument that suggests that you can use manifold vacuum for the distributor
>in lieu of ported vacuum. Any comments from the sages?
>
>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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