dual vacuum advance distro to single vac adv

Hey! First post on this forum! Just got a 71 Vert, 302 2v. Practically everything still stock and original!

so, anyway, looking to go electronic distributor, perhaps Pertronix. My current distro has dual vacuum advance ports: one to manifold vac and the other to that ported vac switch. So if I go to single vac distributor, what happens?

I searched in this forum first and did not really find the answer.:huh:

Can't really answer your tech question, but why not leave as original since rest of car is and just add Pertronix and your good to go?

Dual vacuum advance was a method for reducing emissions during low throttle and deceleration.

changing to a single advance would actually increase motor performance at the cost of some emissions output from the exhaust.

depending on where you are exhaust testing may or may not be an issue. Some states require all the original emissions equipment on cars working as originally built some do not test at all.

changing to a single advance on a OEM distributor is just a matter of swapping out the dual vacuum diagram unit for a single then hooking the single advance to ported vacuum, or you could use full manifold vacuum with some retuning for more performance.

the difference between ported and full manifold vacuum only occurs at Idle.

some OEM distributors do have some internal changes in regards to dual and single so it depends how old the motor is. i would recommend swapping the entire distributor to be safe.

at idle ported vacuum is shutoff to the port 0 HG" at the ported tube. in this case, no Vacuum advance and everything is driven from the mechanical weights for timing. these should be off as you want to run purely on initial engine timing at idle.

while at idle with full manifold vacuum you can get anywhere from 10 to 25 HG" of vacuum depending on motor internals.

for performance you want as much advance as the motor can tolerate all in before 1500-3000 rpms more advance gives more power but you can cause ping/detonation if not careful.

the idea with a dual vacuum advance was the diaphragm would constantly be fighting between the 2 types to retard or advance timing.

so if you were cruising along and slowed down, the carb would return to idle, ported would shutdown and full manifold would climb sucking the advance the other way and retarding timing. retarded timing increased exhaust manifold temperature and reduced emissions.

depending on year there are usually other vacuum emissions controls like spark delay and temperature sensors.

again you can remove these and simplify the vacuum advance.

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the unit you need is a smp vc31 or bwd v311.

turn the allen screw in the hose fitting in until it just stops then out around 5 turns . . check how much advance it adds at almost 1/2 throttle . . adjust it until it adds around 8.

you will need to change at least one of the springs in your distributor to optimize your advance . . mt gasket sells them for $6.00 on summit.

Another post you may find helpful. http://www.7173mustangs.com/thread-under-the-hood-mysteries-distributor-vaccum-control-valve