yup switching to pertonixs takes minutes and you can keep the points and condenser in the trunk to swap back if you have a ignition failure.
the dual advance as mentioned above helped for cold starts on big blocks in 72, the idea was retard the timing when cold to make the exhaust temps rise faster, the oem aircleaner base used the snorkel that went through the exhaust shield on the drivers side, thus the intake air would heat faster and bring the engine up to temperature faster. once the engine warmed up the heat sensor on the block switched to normal advance operation using ported vacuum. then the heat sensor on the air cleaner moved the snorkel from warm up to normal intake operation which brought in colder air from the top of the engine instead of by the exhaust.
the operation is simiar to the Distributor Vaccum Control Valve. except that only comes on during emergency cool down when the engine is overheating.
all this stuff does not effect performance when operating correctly. you can argue it is some emissions control in reducing bad exhaust gases when the engine is cold.
however all it really does is improve street driving performance and makes the engine less bitchy when warming up.
In 73 EGR was introduced and that cause timing issues so the Dual vacuum advance was needed to prevent a retard timing issue caused by some throttle positions with the EGR system there was some type of feedback that also messed up the shift points of the automatic transmissions thus in 73 ford installed dual vacuum modulators on transmissions to solve this feedback issue. EGR (exhaust gas recycle) combined with the new more retarded engine timing and other stuff was emmissions and caused tons of engine running issues as it was basically the first emissions controls put on engines. the EGR clogged everything up with carbon from the exhaust.
you can change a dual system to a single very easy just replace the vaccum can on the distributor and rehose everything for single advance. you will not notice a change other then hey the engine runs crappier when it is cold, but when it warms up it will be fine.
for 73 removing the dual system is a bit more involved because you want to ditch the EGR system and that usually means you need to change the intake manifold to a 71 or 72 version without EGR or go aftermarket and install a aftermarket edelbrock or holley carb.
something harder to deal with is converting from duel points to single points in the distributor, so you should make sure you have a single points distributor.
duel points was pretty cool for back in the day before electronic ignitions it reduced the points bouncing issue which limited top end RPMS, so that was why the higher end engines got dual points, and usually dual vacuum advance. setting up dual points is difficult because you have to set the gap between 2 points plates and sort of balance them to work right. many mechanics hated them and so the first time the car came in for a tune up they would either try to convert to single points or pull the distributor and replace it with a single version.
today finding good points is REALLY hard it is all made in china junk and the Dwell shifts on them insane every 100 miles. the condensers are garbage today also. you have to basically try and buy NOS vintage stock from ebay and it rarely comes up. i still have a vintage 1970s Accel High rpm Points conversion kit that will sit in a box forever unless an EMP knocks out my electronics ignition system.
Ported timing is what you are thinking about.
there was the change from full manifold vacuum to ported.
the only difference between the 2 is at Idle RPMS vacuum for ported is 0HG
that difference kept the engine at initial timing when sitting in traffic.
that kept the exhaust temps high and ran the engines hotter sitting in traffic to burn off extra emissions from the tail pipe. this reduced smog majorly with cars sitting in heavy traffic stop and go.
ford spec'd 4-6 degrees of initial timing on later cars to increase exhaust temps more. they also re-balanced the distributor mechanical advance stopper plates, so you have to watch out for total timing if you start screwing around increasing initial timing to 10-18 degrees for more power. in 71 your plate would of read L12 in 72 it was like L14
it was because they speced like 8-10 degrees in 70-71 for initial so 10+L12 = 10+2X12= 10+24 = 34 total timing, then for 72 you had like 6-8 initial 6+ L14= 6+28 = 34
this is why people talk about the mechanical advance plates needing to be swapped out when increasing initial timing.
so people want like a L6 or L8 plate from a 1969 or 1970 car, because that gives you like a 18 initial timing fudge factor without grenading the engine for more performance.
the change required the Distributor Vaccum Control Valve which acted as an emergency cooling mode for engines over heating. this valve changed the distributor to run full manifold vacuum instead of ported which advanced timing and thereby increased engine rpms at idle which made the fan turn faster which cooled the engine back down.
performance people hated ported because yes in a way it retarded timing.
however for street drivability it was an improvement.
with full manifold vaccum the engine was running advanced timing more often so from a standing start the full mainfold car had advanced timing over the ported mainfold car so the full mainfold car would do a burn out when you stomped on it and the ported car just churped the tires.
later when ford moved to open chamber heads and lower compression ported helped with ping/dentonation under certain conditions. because it held back timing as the engine rpm increased otherwise you needed more fuel to compensate. when they started to look at MPG numbers holding back timing made more public sense then increasing more fuel delivery on an engine getting 4mpg already.
I read somewhere that this was also a California requirement to retard the timing when the car would be in slow traffic and running on the hot side to cool the engine down and thereby cut emissions. Not sure where I got that info. Anyone else read this?
Stanglover
