Understanding Distributor Mechanical and Vacuum Advance Springs

[67 GTX]

I am in need of some explaining on distributor mechanical and vacuum advance springs.

On the mechanical springs:
Do the springs limit the actual amount of mechanical advance, or when the advance comes in/how fast or slow it does?

On the vacuum advance (VA) springs:
Does the spring limit the travel of the diapghram, or just affect when the advance comes in?
I attached a pic of the VA that I am referring too. It is the one that has the removable top to access the spring.

Is that the only adjustment on this type of VA?

 

[HALIFAXHOPS]

OK both springs in the mechanicale advance adjust the rate of advance tand he slot on the advance cam limits the total amount of advance.
Now the same on the vacuum side in Vacuum what limits its total is the shims in with the spring and when totally compressed limits the total amout of vacuum or what the arm is marked at is the usual way it is limited, it bottoms out against the outside of the housing. Hope that helps.

 

[67 GTX]

OK both springs in the mechanicale advance adjust the rate of advance tand he slot on the advance cam limits the total amount of advance.
Now the same on the vacuum side in Vacuum what limits its total is the shims in with the spring and when totally compressed limits the total amout of vacuum or what the arm is marked at is the usual way it is limited, it bottoms out against the outside of the housing. Hope that helps.

Thank you. Ok, I am following the mechanical advance side.

So for the vacuum advance, changing the spring (or adding shims to the existing spring) will change the amount of advance?
I was thinking of adding an orifice in the vacuum advance rubber line to try and limit the vacuum that way, not sure if it would work though.

 

[RJRENTON]

Thank you. Ok, I am following the mechanical advance side.

So for the vacuum advance, changing the spring (or adding shims to the existing spring) will change the amount of advance?
I was thinking of adding an orifice in the vacuum advance rubber line to try and limit the vacuum that way, not sure if it would work though.

If you are considering an orifice, look at Ford or Motorcraft (or Standard Motor Products or Dorman), as they used inline vacuum delay valve (a calibrated orifice to slow the rate of vacuum to the down stream device in various delay times....10 sec, 15 sec, similar times) combined with a quick release check valve to relieve the vacuum, as an inline device. They were plastic in nature and could be fit in the vacuum advance hose.....and very inexpensive to buy.....the device limited the RATE OF VACUUM not the amount of vacuum applied. I believe Mopar used a similar device on the EGR system.
BOB RENTON

 

[HALIFAXHOPS]

Thank you. Ok, I am following the mechanical advance side.

So for the vacuum advance, changing the spring (or adding shims to the existing spring) will change the amount of advance?
I was thinking of adding an orifice in the vacuum advance rubber line to try and limit the vacuum that way, not sure if it would work though.

No the only adjustment on the vac side is for the rate of advance not total advance, Here iis a cutaway of a regular one pic might help explain it.

 

[Dragon Slayer]

For a little more detail on stock type. On the mechanical, one spring has an oval loop and is usually heavier spring. The other is a smaller lighter spring. The small spring controls initial rate as the heavy spring larger oval loop is free. Once the larger heavy spring makes contact the rate of advance slows down and this controls high rpm rate. The slot length controls maximum advance. With one caveat. As long as the heavy spring doesn't prevent movement at your rpm limit. Usually they never do, but some springs do not give max mechanical until 4800 rpm.

Vacuum for the Prestolite is similar. The arm number stamp is the max vacuum advance that can be added. The spring length and thickness of coil determine the rate. The shims preload the spring and control at what vacuum the arm starts moving. In most cases for vacuum the combination may prevent reaching max arm extension because your engine may not have sufficient vacuum at cruise to pull the arm fully in against the spring and preload thickness. You can easily test the vacuum required to start to move the arm, and the point it reaches max retraction. Actual degree of motion in between would be a swag unless it was on the car.

If that vacuum can is original to the distributor, the prestolite chart would give you the vacuum curve along with mechanical curve. The Mopar service manual also does in the electrical section.

 

[67 GTX]

No the only adjustment on the vac side is for the rate of advance not total advance, Here iis a cutaway of a regular one pic might help explain it.

Thank you. I don't believe the dual pt. vacuum advance has that type of adjustment correct? Just have to mess with the springs on it?

For a little more detail on stock type. On the mechanical, one spring has an oval loop and is usually heavier spring. The other is a smaller lighter spring. The small spring controls initial rate as the heavy spring larger oval loop is free. Once the larger heavy spring makes contact the rate of advance slows down and this controls high rpm rate. The slot length controls maximum advance. With one caveat. As long as the heavy spring doesn't prevent movement at your rpm limit. Usually they never do, but some springs do not give max mechanical until 4800 rpm.

Vacuum for the Prestolite is similar. The arm number stamp is the max vacuum advance that can be added. The spring length and thickness of coil determine the rate. The shims preload the spring and control at what vacuum the arm starts moving. In most cases for vacuum the combination may prevent reaching max arm extension because your engine may not have sufficient vacuum at cruise to pull the arm fully in against the spring and preload thickness. You can easily test the vacuum required to start to move the arm, and the point it reaches max retraction. Actual degree of motion in between would be a swag unless it was on the car.

If that vacuum can is original to the distributor, the prestolite chart would give you the vacuum curve along with mechanical curve. The Mopar service manual also does in the electrical section.

Thank you. From what I read from the FSM, seems on average, around 16" vacuum will be required to get full advance from the vacuum advance. I think I will try to see what vacuum my engine makes at different rpms, as well as at quick throttle changes.

 

[67 GTX]

Does anyone know if anyone makes or sells the dual pt vacuum advance with different total advances?

 

[67 GTX]

Bump on this.

Also, has anyone been able to limit the advance the dual pt vacuum advance provides?
Maybe somehow limit the arm movement within the distributor itself?

 

[Geoff 2]

What you describe in the above post I have done many times on different distributors: limit the travel of the VA actuator. How it is done on your dist will depend on the dist type. You might have to get 'creative'....

 

[AR67GTX]

On GM distributors I’ve built a stop that prevents the arm on the vacuum advance from pulling all the way in, thus limiting the amount of advance. The same thing can probably be done on a Mopar vacuum advance but I haven’t tried to do so.

 

[threewood]

Bump on this.

Also, has anyone been able to limit the advance the dual pt vacuum advance provides?
Maybe somehow limit the arm movement within the distributor itself?

It's quite common to weld up the slots on the mechanical advance arm to get the desired total advance. The more initial advance you add, the more you need to weld up to keep from going beyond the 36 to 38 degrees.

 

[AR67GTX]

For centrifugal advance I have welded up a points plate but I’m not that good a welder and it took a lot of work for me to get an acceptable plate finished. Then I discovered the adjustable advance plate at FBO and it’s like sliced bread. I’m never going to mess with welding up another advance plate again.

 

[67 GTX]

What you describe in the above post I have done many times on different distributors: limit the travel of the VA actuator. How it is done on your dist will depend on the dist type. You might have to get 'creative'....

Have you tried it on big-block dual pt. ones? I am thinking this is something I might have to do.

On GM distributors I’ve built a stop that prevents the arm on the vacuum advance from pulling all the way in, thus limiting the amount of advance. The same thing can probably be done on a Mopar vacuum advance but I haven’t tried to do so.

Ya I've heard GM ones are a bit easier to do this on.

It's quite common to weld up the slots on the mechanical advance arm to get the desired total advance. The more initial advance you add, the more you need to weld up to keep from going beyond the 36 to 38 degrees.

Even if I get the limiting plate, I'll still have to restrict my vacuum advance a little.

 

[Geoff 2]

No, have not tried it on a dual point. Hence the suggestion to get 'creative'. You might have to drill a hole in the side of the dist to mount a bracket that limits travel. Just thinking out loud...

 

[JoePole]

Subscribed

 

[RJRENTON]

Have you tried it on big-block dual pt. ones? I am thinking this is something I might have to do.


Ya I've heard GM ones are a bit easier to do this on.


Even if I get the limiting plate, I'll still have to restrict my vacuum advance a little.

On my RS23V0A****** GTX, I use a recurved (to provide a total of 35° advance-including ~12°-15° Initial advance all in by 2400 RPM) Prestolite dual point distributor WITHOUT vacuum advance. The RATE OF ADVANCE is fast enough to provide crisp throttle response. The vacuum advance arm remains connected to the breaker plate to provide stability. Because of the engine's cam this combination works for me. BTW the car is a 4 speed with a Dana 60 with 4.10 gears. Just my opinion of course.....
BOB RENTON

 

[JoePole]

Some good info here....

How To Limit and Adjust Chrysler Vacuum Advance Cans

 

[67 GTX]

Some good info here....

How To Limit and Adjust Chrysler Vacuum Advance Cans

That was very helpful, thank you! I think I have an idea for mine. Will post later with some pics when I get around to it!