vacuum advance degree adjustment

[Wrenchero]

hello, i would like to adjust the degrees on my mopar performance distributor on my stock 318. i read that the vacuum advance adjustment only adjusts the speed that the vacuum advance works but doesnt change the degrees. is there a way to do this or do i just leave it alone, i want to try to get a little better mileage. thanks

 

[Cranky]

What is your initial timing set at and your total? When you rev the engine, your timing will change.....

 

[Wrenchero]

my initial is 6 degrees. mechanical is about 20 degrees. and total with vacuum advance is 40 degrees

 

[steve340]

Generally the vacuum advance is mechanically limited in some way - a slot or stop.
Most people want to stop it over advancing, you are the opposite.
If your motor is pretty stock, does not detonate on modern low octane fuel I might be tempted to leave well enough alone.
An extra few degrees of timing may not make much of a difference to fuel economy. However if you increase the initial for a trial it will raise the mech/vac by the same amount.

 

[Don Frelier]

That initial is way low I would say.
If your mechanical is really 20 then try 15 degrees initial advance.

 

[JimCoronette]

A stock 318 would probably do better with 10-15 initial and around 30-32 mechanical all in around 2200.

Use these 2 items to achieve that.

I would try this to limit the mechanical advance
https://www.manciniracing.com/fbomodilipl.html

and this in place of the heavy spring to bring the advance in quicker.
https://www.manciniracing.com/mrdiadkit.html

.
Set up the vacuum advance with this procedure:
How Much Vacuum Advance?

 

[turbine68rt]

Stock 318 would probably do better with 10-15 initial and around 30-32 mechanical all in around 2200.

Use these 2 items to achieve that.

I would try this to limit the mechanical advance
https://www.manciniracing.com/fbomodilipl.html

and this in place of the heavy spring to bring the advance in quicker.
https://www.manciniracing.com/mrdiadkit.html

.
Set up the vacuum advance with this procedure:
How Much Vacuum Advance?

I'm using the FBO plate and the 440 runs great with 18°/18°

 

[beanhead]

hello, i would like to adjust the degrees on my mopar performance distributor on my stock 318. i read that the vacuum advance adjustment only adjusts the speed that the vacuum advance works but doesnt change the degrees. is there a way to do this or do i just leave it alone, i want to try to get a little better mileage. thanks

Yes the vacuum advance works by controlling 'when' it activates the advance mechanism, but in doing so it most certainly does control the amount of timing advance added (within the limit of how much total your particular canister can add). The issue gets confused because it's not as easy as "x turns of the screw adds x degrees".

Your engine only produces so much vacuum, and it changes with load and throttle position/ rpm. The only way to truly know how much and when, is to drive with a vacuum gauge hooked up so you can see it. (this is helpful for carb tuning as well).

Adjusting the diaphragm to start advancing sooner, will give more degrees of timing as vacuum increases. On the flip side, adjusting it for a later start will only add a few degrees before the engine reaches it's maximum vacuum. And, the vacuum advance unit is only capable of adding a certain amount of degrees, some more than others. And! Some aren't adjustable
In other words the mechanism doesn't just slam to full advance when the spring activates.
So let's say your engine makes 10" of vacuum at off-idle cruise. As you increase cruise speed let's pretend it makes a max of 20". Say you've set the advance to activate at 12" of vacuum...that's where it just begins to move the plate/add timing. It adds more timing as it is acted on by more vacuum. So you may pull in an extra 2 or 3 degrees at the lower speed, and up to an extra 10 or 15 at the higher speed. If you were to set it to start at, say, 18" of vacuum, it's only gonna add a couple of degrees because the engine stops pulling on it at the 20" max = it doesn't have the opportunity to pull in any more. Clear as mud, right?

So getcha a long piece of vacuum hose, put the gauge where you can safely watch it (I drop it through my vent window, some guys fasten it to the driver side windshield wiper) and go drive.
If you have a vacuum tester (mity-vac or similar), you can dial it in under the hood in the garage, because you'll know how the engine behaves vacuum-wise. If you don't have a mity-vac you'll have to make adjustments, then start the car and watch the timing with your timing light.
Okay sorry for the long-winded post!

 

[turbine68rt]

Yes the vacuum advance works by controlling 'when' it activates the advance mechanism, but in doing so it most certainly does control the amount of timing advance added (within the limit of how much total your particular canister can add). The issue gets confused because it's not as easy as "x turns of the screw adds x degrees".

Your engine only produces so much vacuum, and it changes with load and throttle position/ rpm. The only way to truly know how much and when, is to drive with a vacuum gauge hooked up so you can see it. (this is helpful for carb tuning as well).

Adjusting the diaphragm to start advancing sooner, will give more degrees of timing as vacuum increases. On the flip side, adjusting it for a later start will only add a few degrees before the engine reaches it's maximum vacuum. And, the vacuum advance unit is only capable of adding a certain amount of degrees, some more than others. And! Some aren't adjustable
In other words the mechanism doesn't just slam to full advance when the spring activates.
So let's say your engine makes 10" of vacuum at off-idle cruise. As you increase cruise speed let's pretend it makes a max of 20". Say you've set the advance to activate at 12" of vacuum...that's where it just begins to move the plate/add timing. It adds more timing as it is acted on by more vacuum. So you may pull in an extra 2 or 3 degrees at the lower speed, and up to an extra 10 or 15 at the higher speed. If you were to set it to start at, say, 18" of vacuum, it's only gonna add a couple of degrees because the engine stops pulling on it at the 20" max = it doesn't have the opportunity to pull in any more. Clear as mud, right?

So getcha a long piece of vacuum hose, put the gauge where you can safely watch it (I drop it through my vent window, some guys fasten it to the driver side windshield wiper) and go drive.
If you have a vacuum tester (mity-vac or similar), you can dial it in under the hood in the garage, because you'll know how the engine behaves vacuum-wise. If you don't have a mity-vac you'll have to make adjustments, then start the car and watch the timing with your timing light.
Okay sorry for the long-winded post!

According to Mr. Ehrenberg the advance is adjusted for the amount of vacuum when full advance happens, not when it just begins to advance.

 

[beanhead]

According to Mr. Ehrenberg the advance is adjusted for the amount of vacuum when full advance happens, not when it just begins to advance.

View attachment 1001380

In the end, yeah you're setting it to give the max total timing you want...simply stating that does nothing for helping the OP understand how they operate.

 

[steve340]

You are adjusting the tension on a spring. By definition (depends on which way you slice it) controls when the unit starts to allow advance or when it reaches full advance.
Same thing different way of saying it.
One thing that adjustment does not do is alter the amount of degrees it will have at full advance.

 

[turbine68rt]

In the end, yeah you're setting it to give the max total timing you want...simply stating that does nothing for helping the OP understand how they operate.

https://www.moparaction.com/2020/08/16/advance-to-go/
Ok, maybe this will.

 

[Wrenchero]

thank you all for help. who sells the canisters with different numbers on the arm?

 

[Wrenchero]

havent gotten my vacuum advance yet so i decided to put my spare emergency distributor in my 318.its a brand new import cheapo from rock auto.i set initial at 10 ,mechanical is 25, and total with vacuum is 48. took it for a shakedown run.seems to run well with no detonation under light load or full throttle

 

[Wrenchero]

forgot to mention that i live at sea level and run this engine on 87 octane