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I have maybe a dumb question. I've got a 1970 302 that is all stock, and it came with the dual vac. advance distributor. This is a daily driver that I installed a Pertronix kit. I currently have the advance hooked up from manifold vacuum to the front of the chamber only, the back of the chamber is plugged. My question is, can I replace the dual vac. chamber for a single chamber on this distributor? I noticed the dual chamber moves the advance plate very little like 1/4 inch, but a single chamber will move the plate quite a bit more.
If you look up part numbers for a vacuum advance it askes what distributor number you have. So is there an internal difference in the distributors? Thanks in advance.
 

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first I'd like to say that 1/4 inch rotation on the dizzy is alot of degrees of advance. Just how much advance are you trying to run? Whats your base timing and how much mechanical advance does your dizzy have? How much vacuum does your motor pull? You might not have enough vaccum to pull the vac advance plate all the way.

I've never seen a dual vac dizzy in person so I cant say if it uses the same vacuum modular mounting bolts, same lenght adjusting arm, and rotating connection on the internals as a single vac dizzy. But I'd bet you should be able to put a single vac can on it, but I also dont think swaping it would allow any more vacuum advance than you already have.
 

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Dual advance was used to temp control the amount of advance, and a thermal switch was usually mounted in the intake behind the stat. It was used to control the amount of advance during cold start and warm up.

Hooking your advance to manifold vacuum is a waste of time and defeats how vacuum advance is supposed to work. You might as well unplug it and just advance the mechanical.

Do a google on vacuum advance for a full tutorial. Many websites explain it fairly well. Most hot rodders just go with a re-curved dizzy and straight mechanical. It's a little easier to figure out.
 

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The reason I ask is, I have a good single port vacuum chamber that I installed and when I tried to start the car, it wouldn't start at all. It acted like the timing was way off. Reinstalled the original and it started right up. The length of the arms that attach to the advance plate are the same length, but when vacuum is applied to both chambers the single port moves alot farther. So I'm thinking the advance plate is moving too far with the single port. Also the ported vacuum switch is not there anymore to control the dual port. Which is the reason I thought I would install the single port chamber. I've never had any dealings with dual chambers before. When I looked up part numbers for vacuum advance for different Fords, they ask for part numbers off of the distributor to get the right vac. advance, which made me think that there where some internal differences in the distributors. Thought somebody here might have run into this before.
Maybe I should run the single chamber to ported vacuum instead of manifold vacuum, then maybe the advance won't come in till after the car starts?
 

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A dual port vaccuum chamber can work just like a single. One port was used to retard the timing and the second port advances it.

All you need to do is eliminate (just remove and plug) the vaccuum hose from (I believe) the inner port. Then hook up the other port directly to a carb vaccuum source. I have always preferred using ported vaccuum.

The vaccuum tree may be required to pass an emissions visual test, but otherwise is not needed.

Next check your total timing at say 3000rpm with both the vaccuum applied and disconnected. This will tell you if you are advancing properly. You can then try the same thing on the opposite port (in case I have my ports backwards.)

I have never tried to convert one to a single port but would assume that timing would change.

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Dennis

<font size=-1>[ This Message was edited by: dennis111 on 2/4/05 1:02am ]</font>
 

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A couple things... first dennis111, if you 'plug' the rear port, would that make the vacuum diaphram unable to move because air is trapped in the rear chamber? I haven't ever taken one apart so I don't know, but it seems like the best thing to do would be to just leave it open to the atmosphere instead of plugging it.

Also, there are two places you can hook the vacuum advance, to the timed vacuum port on the carburetor, which only gets manifold vacuum after the throttle is cracked open off idle, or just hook directly to manifold vacuum. I've heard many arguments for hooking it to manifold vacuum, such as reduced idle temp, reduced emissions at idle, etc... but I think it's easier to tune and get a consistent idle with it hooked to the timed vacuum port on the carb.
 

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On 2005-02-03 14:03, Motorhead wrote:
A couple things... first dennis111, if you 'plug' the rear port, would that make the vacuum diaphram unable to move because air is trapped in the rear chamber? I haven't ever taken one apart so I don't know, but it seems like the best thing to do would be to just leave it open to the atmosphere instead of plugging it.
Perhaps I wasn't clear enough but I will say it again (because I don't know how else to say it.
Plug (or remove) the rubber vacuum hose. You do not need to do anything to the cannister itself-I never did.

Also, there are two places you can hook the vacuum advance, to the timed vacuum port on the carburetor, which only gets manifold vacuum after the throttle is cracked open off idle, or just hook directly to manifold vacuum. I've heard many arguments for hooking it to manifold vacuum, such as reduced idle temp, reduced emissions at idle, etc... but I think it's easier to tune and get a consistent idle with it hooked to the timed vacuum port on the carb.
I prefer using the "timed" vacuum that occurs off idle. I think it gives me a more consistent idle, especially at initial startup.

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Dennis

<font size=-1>[ This Message was edited by: dennis111 on 2/4/05 3:38am ]</font>
 

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Ah sorry, I interpreted what you said as remove the hose and plug the canister, not remove the hose and plug the hose. My mistake.
 

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When I got my '68 302 it got 8 MPG in a '68 Galaxie. It also ran like a dog, hardly moved off the line, and didn't have much oomph up top either. My '67 Gal with a 289 and the same drivetrain got 14-15 MPG typically and was rather quick, blew away the '68.

Just going from the dual vacuum advance/retard unit to an adjustable advance unit got me up to 12 MPG. Not a single other change to the car. It also worked wonders for acceleration. It could actually get out of it's own way! After advancing initial timing a little too and rebuilding the carb and a whole tune-up I managed a high of 17 MPG highway and 14 around town. Timing is not everything, but it's damned close!

The purpose of the advance/retard unit was to control emissions. It also has a side benefit of if the car was overheating it would switch the advance vacuum from ported to manifold to give more timing at idle, raising the engine RPM and thus pulling more air through the radiator helping to cool the engine down. Not sure how much it actually helped but I do know that on the couple occasions I've overheated simply raising the RPM a little cooled it back down pretty quickly. The more timing the cooler an engine runs, the less timing the hotter it runs (since more energy and burn time of the fuel is wasted).
 

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Dual advance was used to temp control the amount of advance, and a thermal switch was usually mounted in the intake behind the stat. It was used to control the amount of advance during cold start and warm up.

Hooking your advance to manifold vacuum is a waste of time and defeats how vacuum advance is supposed to work. You might as well unplug it and just advance the mechanical.

Do a google on vacuum advance for a full tutorial. Many websites explain it fairly well. Most hot rodders just go with a re-curved dizzy and straight mechanical. It's a little easier to figure out.
You say "Hooking your advance to manifold vacuum is a waste of time and defeats how vacuum advance is supposed to work. You might as well unplug it and just advance the mechanical."

Vacuum advance is not a waste of time for street driven vehicles because it was designed to use manifold vacuum to give advance during lean mixtures, which occur at idle and at cruising speeds. For race cars, vacuum advance is a waste of time because engines operate at high RPMs.
 

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Thread is 15 YEARS OLD!!!!
Funny that you said that. I just bought a used 1975 Ford F250 with a Frankenstein 390 (According to the seller it is a 429, but that is a whole different thread) that has this distributor and after reading realized they have the vacuum lines wrong. I am going to yank this thing and put in a decent Distro as soon as I figure out which one would work (I am thinking an HEI style). I will have to find other threads on the cast numbers I have pulled to see what else I need to replace or add.
 

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Surfdaddy65. Back in 1985, I bought a 68 Mustang with a non-original 302 and it had the dual-diaphragm distributor. It originally came with a 289. When I crossed referenced the numbers, it came up as a 1970 part. It is definitely an early Ford emissions-system distributor that uses an integral coolant temperature-sensing valve to regulate timing during cold startups and at normal operating temperatures. This three-port sensing valve is located on the intake manifold right behind the thermostat housing.

According to page 116 of Glenn's Emission Controlled Systems by Harold T. Glenn, 1972, the top port on the valve (marked "1") is connected to the carburetor (ported vacuum source). The middle port (marked "D') is connected to the primary vacuum advance (front) diaphragm on the distributor. The bottom port (marked "2") is connected to the intake manifold and to the vacuum advance secondary (rear) diaphragm using an in-line "tee" fitting.

Back then, I never knew what line went where and more importantly, why and how the system was intended to work. It wasn't 60s looking and I disliked looking at all the vacuum hoses. It is a rather simple system now that I am older and have more experience working on these earlier cars. A lot of Ford engines from 1968 through 1971 used this type of distributor. Ford switched to its Duraspark electronic ignition in 1973 and in 1976 introduced the Duraspark II system, so it is interesting that your 1975 F250 has this distributor.

So, back to my 302. I eventually rebuilt the motor for everyday street use. I junked the dual-diaphragm distributor and all the unnecessary vacuum-related emissions hoses/parts. I installed an early Mallory Comps 9000 electronic unit. It worked well off the shelf and I was pleased with its performance. With no points to mess with and what I believe to be a decent-performing advance curve, my power band was somewhere in the range of 2200-4700 rpms.

AND, no matter how old a thread may be, someone will always be looking for dated information as older cars are still out there and folks like us are always tinkering and learning together. That's what's great about this forum.
 

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i had a dual diaphragm advance unit on my 63 390 only because at the time it was the only unit i could find that was adjustable on the primary side. i simply didn't use the other part
 

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This is an old thread, but basics never die, and will always be relevant. It is often lost in conversations as to what the point and purpose of ignition timing is. The ultimate goal of ignition timing is to control the resulting peak cylinder pressure angle in crankshaft degrees. Spark and fuel are sisters that depend on each other. Fuel cannot do its job best without ignition at the precise moment required for greatest efficiency. That makes best power possible, whether for low track times or peak fuel economy.

Where the peak cylinder pressure (PCP) occurs directly impacts the efficiency of extracting the energy from the burning fuel:air charge. The best PCP for an engine is primarily based on geometry, and typically lies 12-18 degrees after top center. Ignition timing to achieve that varies according to many factors of things like temperature, fuel, charge homogeneity, etc. Igniting the charge well before PCP (typically before top center) allows burn-time to build pressure, eventually peaking at that perfect point in order to efficiently push the piston down the bore while the charge continues to burn and expand.

Igniting too early will cause PCP to occur too early. This "fights" the power flow, increases peak pressure, lowering power, efficiency, and causing other possible effects such as stress and detonation. Igniting too late causes PCP too late, reducing peak pressure, again lowering power, efficiency, and causing other effects such as added heat. The added heat may also manifest as general overheating, valve or seat damage, detonation, pre-ignition, etc. You can see that any timing that results in missing the best PCP angle (early or late) can have some similar and other different effects. Neither too advanced or too retarded is good.

Any changes in conditions can change ignition timing requirements in order to maintain that best PCP angle. Temperatures, fuel mixture (Lambda or AFR), volumetric efficiency (e.g., throttle opening), engine load, rpm, and many others. So, when we speak of vacuum advance (or any equivalent of it such as electronic control), we are speaking of a means to alter PCP angle through ignition timing, based on one factor — manifold pressure (vacuum) as a representation of engine load. A past example I gave is here.

While 50 years ago we still ran race cars without vacuum advance, that mentality was dropped decades ago when those smart people realized they were leaving important factors such as power, response and economy behind. Those are all part of winning races today, and long recognized as a big factor in street use. While there are a few partial exceptions, such as industrial generators, aircraft, boats and drag engines; virtually any engine that varies load, throttle, or runs multiple modes (power/economy) benefits from variable timing. The benefits go far beyond power and economy with many short and long-term side-effects, from engine temperature to valve life.

Don't underestimate the value of well-controlled and well-tuned variable ignition timing, of which the venerable vacuum advance is a part. I have only touched the tip of the iceberg, but I hope that helps with some context.

David

A random example of pressure curve and ignition timing relationship from quadratec:
Peak Cylinder Pressure curve PCP example.png
 
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