Copy and paste:
Find yourself a copy of Timing and Vacuum Advance 101 by John Hinckley.
Here is an excerpt regarding ported vacuum
“PORTED” VACUUM: Now to the widely misunderstood manifold vs. ”ported”
vacuum aberration. After 30-plus years of controlling vacuum advance systems with
full manifold vacuum, that “free” indicator of engine load and fuel mixture, along
came early emission control requirements (seven years before catalytic converter
technology was introduced), and all manner of crude band-aid systems were
introduced to try to reduce hydrocarbons and oxides of nitrogen in the exhaust
stream. One of these crude, but effective systems was GM’s Air Injection Reactor
(A.I.R.) system, which pumped fresh air into the exhaust ports to “afterburn”
pollutants in the exhaust manifolds. The key to making this system work at
maximum efficiency was retarded spark at idle; with retarded idle spark timing, the
“burn” begins late, and is not complete when the exhaust valve opens, which does
two things that were important for emissions. The incomplete burn reduced
combustion chamber temperatures, which reduced the formation of oxides of
nitrogen (NOX), and the significant increase in exhaust gas temperature ensured
rapid “light-off” and combustion of the hydrocarbons in the exhaust gas stream
when the fresh, oxygen-carrying air was introduced from the air pump.
As a result, these engines ran poorly, and an enormous amount of wasted heat
energy was transferred through the exhaust port walls into the coolant, causing
them to “run hot” at idle; cylinder pressure fell off, engine temperatures went up,
combustion efficiency went down the drain, and fuel economy went down with it.
“Ported Vacuum” was easy to implement – they just moved the distributor vacuum
port orifice in the carburetor from below the throttle plate (where it was exposed to
full manifold vacuum) to above the throttle plate, where it was exposed to manifold
vacuum only after the throttle plate opened. This meant that the vacuum advance
was inoperative at idle (retarding idle spark timing from its optimum value), and
these applications also had very low initial timing settings; they were usually set at
four degrees before TDC or less, and some even had initial timing settings as much
as two degrees after TDC. The vacuum advance still worked at highway cruise, but
not at idle, which caused all manner of problems. “Ported Vacuum” was strictly an
early pre-converter crude emissions strategy and nothing more. Don’t believe
anyone who tells you that ported vacuum is a good thing for performance and
drivability – it’s not. Anyone with a street-driven car without manifold-connected
vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and
fuel economy, probably because they don’t understand what vacuum advance is,
how it works, and what it’s for. There are lots of long-time experienced mechanics
who don’t understand the principles and operation of vacuum advance either, so
they’re not alone.
