[DeTomaso] Vacuum Advance - ported or full vacuum source

[Mikael]

Dennis, some facts first:
1. Choosing between the two does in no way affect your total timing. That is
of course only true if you set your initial idle the right way, with vacuum
line disconnected and plugged
2. The vacuum off idle and cruising and WOT etc, they're the same regardless
of whether you use manifold or ported vacuum. I've seen a test of this with
a Throttle Position Sensor and two Manifold Air Pressure sensors (one
connected to the manifold, the other to ported vacuum), and the two values
were the same at all rpm, except at idle.

This means that the only thing you have to consider when choosing between
the two, is what happens below 1000 rpm. Normally an engine idles better
with more vacuum. So normally one would use manifold vacuum. That's what
they did in the 60s when idle quality was more important than
anti-pollution. In the 70s they started to use ported vacuum even though it
made the idle worse (they had to put on solenoids to boost idle when the A/C
kicked in), to lower emissions. So always start with manifold vacuum, try
that out first. What would make you go to ported instead then? One of two
things. If either the engine pings at idle (never experienced that) or if
the engine is hard to start because the early ignition makes the cylinder
ignite when the piston is still on the way up. In those cases you have to
reduce ignition advance at idle. This can be done by going to ported vacuum,
but then you lose all vacuum advance. Another middle way is to keep the
manifold vacuum, but adjust the vacuum advance, most modern distributors
have adjustable vacuum advance using an allen wrench. Beware you can't
adjust the rate/speed of advance per vacuum unit, only the maximum advance.

So, there's a bit of trial and error to do here. Go full vacuum advance, and
if no pinging or start problems, keep it.

BTW, all this and more is described in my book Tuning Made Easy (gotta make
a living)

Mikael

-----Oprindelig meddelelse-----
Fra: dennis617 at aol.com [mailto:dennis617 at aol.com] 
Sendt: 27. august 2012 16:41
Til: detomaso at list.realbig.com
Emne: [DeTomaso] Vacuum Advance - ported or full vacuum source


Does anyone have an opinion on which source I should use for my vacuum
advance on my Mallory Unilite distributor, full vacuum directly from the
intake manifold or ported vacuum from the Holley 650 double pumper?  I have
a mostly stock engine with an Edlebrock Performer intake manifold.  I have
recieved conflicting information from Holley and Mallory, Holley claims that
I should use the ported vacuum at the carb and Mallory said I should use
full vacuum from the intake manifold.