Single vs dual diaphragm distributor 72 Q Code

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nbracken

Well-known member
Joined
Aug 13, 2014
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Location
Cologne, Germany
My Car
1972 Mach 1 Cobra Jet
Guys apologies for the basic question.....

I want to order the correct vacuum lines from Marti http://www.martiauto.com/reproducedparts.cfm

There are two sets shown for my car a 72 Q Code. How can I tell if my car has the single or dual diaphragm distributor please? I do not have Ram Air (in fact I don;t believe Ram Air was offered as an option on the 72 Q code.

Thanks

 
Dual diaphragm has a vaccum port both at the front and rear of the advance unit...

4871f7e8-77d4-45de-b076-6f01c8d29980.jpg


Single diaphragm only has one vacuum port at the front...

dva1_2.823.jpg


 
Usually when I see a dual diaphragm it is on a 4 speed car

Not positive that is the rule though

Don

 
You guys are so helpful. Thank you. I'll see if I can check tonight. My car is in the workshop getting the clutch replaced and the flywheel skimmed.

 
you might need to pop off the air cleaner to get a good look making sure there is or isn't a rear vacuum line.

 
.

stock would be a dual and is for smog, but you will get more performance from a single.

 
I've got a dual on my car. I found the coolant actuated vacuum tree which switches the distributor between advance and retard to be worthless. Bypassed the whole thing and just run a vac advance to the distributor. Plugged the rest.

.

 
The coolant actuated vacuum tree to the distributor is designed to aid in cold engine starts. The Vacuum came from the manifold port, since vacuum from the carburetor was very low during higher rpm's.The timing was advanced on the distributor to accommodate the higher RPM's to get the engine warmed up. Once the coolant, carburetor, and engine reached optimal operating temperatures, it switched back to vacuum port on the carburetor, releasing the vacuum on the diaphragm, allowing the vacuum advance to operate on Carburetor vacuum.

Agree on doing away with the dual vacuum.

mustang7173

 
I do not know about the 72 but my 73 came with a dual it is 351, 4-V, 4 speed. One of the first things I changed on my car was to put Ford's conversion to a centrifugal advance dual point distributor on. The kit came with a new guts for the dist. that included several sets of weights and springs to tune the advance. It also required that you change the center cam because the position of the points had to be raised in order to get the mechanical guts in the dist..

I always pulled the dist. out and set the points using a dial indicator on the bench. I did not have any way of tuning the distributor back then but seat of the pants. I played with the springs and weights and looked at the timing at different RPM levels. My brain cells with that info have long since died off since I did that in the spring of 73.

The vacuum advance diaphragm is still on the distributor but the arm going to the plate in the dist. is cut off so it does nothing.

I will have to see if the box with the kit is stuck on a shelf in the garage to get a part number. Might still be some on a Ford shelf somewhere. Anyone have a parts book to look it up?

David

 
Thanks everyone. I checked my car last night that is in the shop and yes it has the dual diaphragm as several of you suspected.

The guy working on the car (Jeff Schrembi at Oval Repairs in Livonia MI) knows his stuff re older Mustangs, and was surprised that my car still had the points step up inside the distributor and wondered why I hadn't changed over to Pertronix. Apparently you don't have to do any non reversible work so can always revert back, just install the new innards in the distributor.

I had a quick read on here and can see several people have changed over, some things to watch out for etc. But what I have works today, and I know will need maintenance in the future. I think I need to do some more reading to form a better opinion. As usual I'm finding the help I get on this forum amazing. Thanks all.

 
The coolant actuated vacuum tree to the distributor is designed to aid in cold engine starts. The Vacuum came from the manifold port, since vacuum from the carburetor was very low during higher rpm's.The timing was advanced on the distributor to accommodate the higher RPM's to get the engine warmed up. Once the coolant, carburetor, and engine reached optimal operating temperatures, it switched back to vacuum port on the carburetor, releasing the vacuum on the diaphragm, allowing the vacuum advance to operate on Carburetor vacuum.

Agree on doing away with the dual vacuum.

mustang7173
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

 
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
 
Last edited by a moderator:
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
I just found a reference to the dual vacuum distributor in my original 1971 Car Shop Manual, Volume 2, engine, page 23-15-01 where it describes the Distributor Vacuum Control valve. I'm not going to quote it all, but in part it reads " During periods of prolonged idle, should the engine temp rise above normal, the valve close the normal source vacuum port and connects the distributor port to the alternate source vacuum port causing an immediate increase in engine speed, which will continue until the engine temperature returns to normal".

Normal source would be the connection at the carburetor and the distributor port (Advance?).

Interesting comments throughout and very helpful, I too have a dual vac. distributor and I also run Pertronix. My issues are with a badly curved reman. distributor that in hind sight should never have been put in the car the way it is. It is likely going to cost me at least a re-hone and a set of pistons. Maybe a replacement block!!

 
Last edited by a moderator:
The guy working on the car (Jeff Schrembi at Oval Repairs in Livonia MI) knows his stuff re older Mustangs,
I am from Lavonia Michigan.

 
@ Stanglover for the DVCV normal source is ported off the carb, and the secondary is full manifold vac from the intake manifold.

it really was a genius way to get the best of both worlds and overlap a problem area with engine overheating.

it was on all A/C equipped cars since the extra load of the AC could send an engine heat over the top on a 90 degree day sitting in traffic not moving.

 
My 72 351 H-code automatic also has a dual vaccum. It is original to the engine. I can testify to that based upon how firm it was siezed in the block. Thankfully the machinist was able to break it loose with little damage.

 
@ Stanglover for the DVCV normal source is ported off the carb, and the secondary is full manifold vac from the intake manifold.

it really was a genius way to get the best of both worlds and overlap a problem area with engine overheating.

it was on all A/C equipped cars since the extra load of the AC could send an engine heat over the top on a 90 degree day sitting in traffic not moving.
Thanks 72Hcode, your comments are most helpful regarding the distributor setting. I think you might have just proved my point to my engine builder re my reman. dist. I have not yet stripped it down, but I think it has the wrong baseplate for the early 71. Very interesting stuff and now I know what to look for.

Thanks,

Stanglover (Geoff)

 
@ Stanglover for the DVCV normal source is ported off the carb, and the secondary is full manifold vac from the intake manifold.

it really was a genius way to get the best of both worlds and overlap a problem area with engine overheating.

it was on all A/C equipped cars since the extra load of the AC could send an engine heat over the top on a 90 degree day sitting in traffic not moving.
At the risk of displaying my ignorance, what does 'L' stand for with respect to the distributor base plate. Also, it is quite obvious that you are very knowledgeable in these matters. A listing I found on Mustang Tek, shows a distributor # as D0OF-12127-V for the 70-71 351C 4V. I have the specs from my manual, but I'm not sure what I'm reading. My question is, what would be the correct baseplate for this dist. I have a rebuilt in at this time, and until it gets pulled and stripped, I have no idea what it actually is. This MAY be the reason why I am having timing issues and the cause of my problems. To clarify, my engine is a D0AE-L, date 1A7, 351C 4V, 4 speed, no A/C, 3.25:1 open dif.

Your advise would be very much appreciated.

Stanglover.

 
.

exactly what is your timing problem?

any ford distributor can be set to whatever spec you want.

if you cam and/or compression is no longer stock, the factory setting no longer applies.

as i mentioned earlier, you can probably get more power by setting your timing curve for your particular engine . . failure to do this and just assuming the factory setting is best is doing a great disservice to yourself.

 
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