The only difference between
ported and manifold itself is that
manifold adds timing at idle while
ported does not. If you have your
carburetor setup right and not too much
throttle exposure. So which one should
you use. Where should you hook your
vacuum line for your vacuum advanced
canister? Onto your carburetor ported or
manifold? Then to clear up one more
misconception about all this. A lot of
will move their vacuum line from
ported to manifold, manifold deported and
say well it runs better. Either way know if
you go to ported. You have to realize for
manifold you took ignition timing and
took it away until you have enough
throttle input. What you have to do is
advance your initial mechanical timing.
Make whatever assessment you want to
make and ultimately it's your choice.
What you choose to use.
Most all street engines
that have mild cams or stock cams work
the best on ported vacuum, They don't
require a whole lot of ignition timing
at idle. Because the dynamics of the
camshaft is just making really good
cylinder pressures at idle. Versus a high
lift cam with a lot of overlap. Doesn't
have as much cylinder pressure at idle
and requires more initial advance. If
you try and run manifold vacuum on a
mild cam or stock style engine. What
you're going to have happen. There will
be so much ignition timing being added
in from that vacuum advanced canister. That in order for your carburetor to idle. At the know ideal rpm. The throttle has to close way too much on the carburetor or you need to have way too little initial mechanical advance.
Then when you step on the throttle hard. You take that timing and throw it out
the window. You know just your
engine responds, very lazily off the line.
because now you don't have enough
ignition timing. So with ported you can
mitigate a lot of those issues. Of
course there is a dynamic relationship
between your ignition timing and the
functionality of your carburetor at idle
Again remember ported we need to add
a little bit more initial mechanical
timing. What you're looking for on your
carburetor settings. Is a throttle
position in between the point of where
your transfer slots are exposed. You
have a little bit of vacuum signal on
those. But not so far that you get to the
point. That is actually your ported port for your vacuum advance. If you get too much
throttle exposure and you're seeing
vacuum on that port at idle then you
have too much throttle exposure. Then you need to increase your initial mechanical timing.
If you go too far.
which most can easily do with full
manifold vacuum. You will see that now no
transfer slots are exposed. The
purpose of those transfer slots in
conjunction with your accelerator pump.
Is to handle the transition from idle to
your main jet circuit. They work a lot
better if they have a little bit of
vacuum signal. Because fuel is already
flowing. When you hit the throttle you know it just continues to draw more from that circuit.
If you have no exposure at idle. Basically you have a flat spot, between the time it takes from fuel to get going on that circuit before it gets to the engine. That's kind of
the form and function of how all this
works in a Holley and Edelbrock are
actually exactly the same. In this regard
the ported port. On Edelbrock
being up here because it's above the
throttle blades, in the same way it also
has transfer slots. You can set
everything up with the same mentality
between the both carburetor styles.
There's really no difference. I also
want to make it clear that ported vacuum
is not the end-all be-all.
there are compromises you have to make
with ported vacuum. You know even
though the advanced. (The initial
mechanical timing relatively to an idle.
The advanced is still a little late and
it's going to favor a richer idle
mixture. )
You have to make on both sides. I'm not
really in favor with either of them. But
for street engines I've seen that ported
vacuum works really well.
That's maybe why I think the Thermo quad. It was a stroke of geniuses. Didn't have to do much to it. It would adjust to it's demands. It would purr like a kitten, then you woke it up it would roar like a lion.
