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Dual Vaccuum Advance
Technical Charts
Q. Why do I even need a spark advance
diaphragm?
A. Vacuum advance helps keep cylinder temperatures lower by effectively ''fine
tuning'' the spark advance curve.
Q. What? How?
A. Well, it seems that since an explosion takes a certain amount of time to fully
expand, the peak of this expansion should occur just slightly before the piston reaches
top dead center of the compression stroke, in a four stroke engine.
Q. Ya. So?
A. Think of a pendulum, or a person on a swing, the optimum moment to push would
occur just before the swing completely stops to reverse its direction. It's the same
principle.
The higher the person swings, his velocity increases, if you are going to push,
you will have to jump earlier and higher, to get the timing exact.
Q. Does vacuum advance have an effect on exhaust emissions?
A. Yep. That's why Jerry Brown approved the Nox Kit and smog checks back
in the eighties.
Q. ''Linda Ronstadt '' ??
A. No, No, No. Not knocks kit, a Nox KIT !
Q. Whaat ?? Nitros Oxide????
A. Nox kits included a sticker that was placed on the speedometer that explained
that driving at sustained speeds over 55 mph could cause engine damage.
Q. Huh?
A. You will burn your valves. You see, the effective portion of the Nox kit was
a couple of rubber plugs that were used to plug the ported vacuum line that feeds
your advance unit. Also the initial ignition timing was retarded.
Q. Retarded sounds about right. Do all distributers have spark advance?
A. Some distributers have centrifugal advance, some use only vacuum, some use
both, some use none. Computerized advance. Spark advance varies greatly by application.
Many factors determine what the optimum advance curve should be, compression, fuel
octane rating, vehicle weight, cam profile, gear ratio, windshield slope, elevation,
and shoe size are a few examples.
Q. Dude, this stuff is over my head. How can you tell I need my vac. advance
replaced?
A. Just pull off the vacuum line and apply suction, two things should happen.
First the diaphragm will hold vacuum, and with the distributer cap off, you will
see the breaker plate move. If it doesn't move, replace the diaphram.
Q. My diaphragm won't hold vacuum, how do I replace it?
A. Remove those two screws, magnetize a small screwdriver or use tiny needlenose
pliers to remove the clip at the breaker plate, and pull out your spark adv. unit.
Your new unit will have a chart included that lists most distributers number
that is stamped in the distributer housing. Follow the directions while adjusting
with a small allen wrench through the vacuum opening the recommended number of turns.
I use a timing light that is adjustable with the engine running to compare the
actual amount of advance with the Ford specifications.
Just remember, these specifications were for stock, new engines running on a
better quality of fuel than is available from any filling station nowadays. Fords
specs.were a bit conservative, they were designed to be a little on the safe side
to prevent pinging.
The old-timers use a vacuum gauge, timing light, and a good ear to set the timing
as advanced as possible for performance without excessive ping, or pre-ignition,
or ''kick back ''on starting .
Q. How much do you charge to replace my spark advance diaphragm and set my
timing?
A. Usually about thirty-five scoots. Plus parts.
Shasmu08