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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