Centrifugal Advance vs. Vacuum Advance

[danf_fl]

On my '68 Charger, 383 4bbl, mild cam, Edelbrock 1406 carb.
The vacuum advance is attached to "ported" side (passenger side when looking from front) per instructions of an individual who has worked on Mopar for years.
The "manifold" port is blocked.

A few questions:
1. What RPM does centrifugal advance take over for the vacuum advance?
2. If my vacuum advance is on the "ported" side, then it does not advance timing until there is sufficient air going into the carb (and advances more as RPM increases until it hits a stop, correct?
3. If I recall correctly, when working on cars (back in the day), the vacuum gauge was connected to the port that had vacuum advance attached. If trying to check vacuum from a "ported" point, it will not give me correct vacuum settings at idle, correct?
4. With no smog devices (except PCV), would it be better to connect vacuum advance to the "manifold" side?

It's been years (over 40) since I worked on auto engines, so bear with me and my questions.

 

[oldbee]

You’re right, except centrifugal tip-in is dependent on the spring tension holding the advance weights. Probably starts to do something @1500rpm, so you could have vac/cent advance at the same time.

 

[R/T Boy 67]

The main difference between ported and manifold vacuum, is there is no vacuum at idle with ported, if your engine is running on the idle circuit. Once the throttle is opened a bit, ported vacuum will follow manifold vacuum more or less. It does not increase with airflow like venturi vacuum does(holley secondaries). At wide open throttle, both ported and manifold will be near zero, while venturi vacuum will be high. The ported vs. manifold is more of an idle- off idle tuning issue.

 

[HawkRod]

Here is my understanding of timing. I'm sure others will chip in with more details, etc.
To start with fuel does not explode in the cylinder, it burns (albeit quickly). It takes time for the fuel to burn completely and drive the piston down. So there are three main elements to timing:

1. Base timing:
   This timing is set by rotating the distributor. Base timing never changes. Base timing is there because it will always take some amount of advance to properly burn the fuel.
2. Centrifugal advance:
   This advance increases as the speed of the engine increases. This is set by the springs and weights in the distributor. As your engine speed increases, the rate the fuel burns does not increase, so you need to trigger the spark sooner (assuming a constant air/fuel ratio).
3. Vacuum Advance:
   Vacuum advance is there because when a street car has a high vacuum signal, it means it is operating in a more lean situation with low or no load. A lean fuel mixture burns more slowly than a rich fuel mixture, so you need more timing. This is controlled by the vacuum canister attached to the distributor.

So, for your question #1, centrifugal advance will never "take over" for the vacuum advance. Each performs a different function.
Your question #2 again links vacuum advance to RPM. It is related to vacuum only. You can have a high RPM, high vacuum situation too. One rule of thumb is that vacuum advance triggers more as you take your foot off the gas, regardless of if the engine is going 1000 RPM of 4000 RPM. You are correct that ported vacuum is essentially zero at idle.
Question 3: Correct
Question 4: You will have a lot of people arguing about this one, but I believe it is best to use manifold vacuum.

I hope this helps,

Hawk

 

[khryslerkid]

With a mild cam I would try the ported side first and see how it does.

First thing to do is find out how much mechanical timing your distributor has. Plug off the vacuum advance, start the engine and see where your timing is set. Lets say it's 16* at idle. Then watch the mark as you accelerate until it stops advancing. Let's say it stops at 34*. Having a tach hooked up, take note at what rpm it stops. That's your "all in" mechanical advance at that rpm.

Now hook up the vacuum advance and do the same thing, watching the timing mark as you increase the rpm. Once it gets to the 34* mechanical it will keep advancing. That's the vacuum advance taking over and it shouldn't go past, around 50*. If it does and you have a adjustable vacuum advance can, you can limit it with the adjustment in the nipple in the can. Takes a small allen wrench. An adjustable vacuum can has a six sided housing. A round housing isn't adjustable.

Anyway you go through this with the timing light to see what you have and where you need to adjust.

 

[Ranger16]

Ported was an early form of emission control. I capped the ported side and run my vacuum advance off the manifold side.

 

[Photon440]

A few questions:
1. What RPM does centrifugal advance take over for the vacuum advance?

If it's any help, the 1970 service manual has these specs for the 383 distributor:

 

[383man]

The main difference between ported and manifold vacuum, is there is no vacuum at idle with ported, if your engine is running on the idle circuit. Once the throttle is opened a bit, ported vacuum will follow manifold vacuum more or less. It does not increase with airflow like venturi vacuum does(holley secondaries). At wide open throttle, both ported and manifold will be near zero, while venturi vacuum will be high. The ported vs. manifold is more of an idle- off idle tuning issue.

That's right as ported vacuum is basically manifold vacuum that's above the throttle plates so it does not pull vacuum until the throttle is opened some to uncover the port and then it comes in as manifold vacuum. And it drops to about zero at WOT. Also carb venturi vacuum which is what vacuum secondaries and vacuum 6 pk's work on get stronger as more air flows through the carb and venturi vacuum is strongest at WOT. As for vacuum advance it can be used at idle or come in off idle. It depends on the combo and what it likes as either can work. I done mine both ways. I usually prefer no vacuum advance at idle and bring it in just off idle. Which ever way you do it you have to make sure you work the mech advance to have the right amount to work with how you use the vacuum advance. Course at WOT you never have any vacuum advance. Ron

 

[khryslerkid]

That's right as ported vacuum is basically manifold vacuum that's above the throttle plates so it does not pull vacuum until the throttle is opened some to uncover the port and then it comes in as manifold vacuum. And it drops to about zero at WOT. Also carb venturi vacuum which is what vacuum secondaries and vacuum 6 pk's work on get stronger as more air flows through the carb and venturi vacuum is strongest at WOT. As for vacuum advance it can be used at idle or come in off idle. It depends on the combo and what it likes as either can work. I done mine both ways. I usually prefer no vacuum advance at idle and bring it in just off idle. Which ever way you do it you have to make sure you work the mech advance to have the right amount to work with how you use the vacuum advance. Course at WOT you never have any vacuum advance. Ron

Well said Ron!

 

[Wietse]

A few questions:
1. What RPM does centrifugal advance take over for the vacuum advance?

Centrifugal advance is set by weights and spring, so this is adjustable. You can check your setup by using a timing light and slowly increase engine rpm, you can see the timing mark starts moving as rpm increase.
You can note at what rpm how much advance is added.
At some point it will stop and the mech advance is at the max setting. (Disconnect vacuum advance when doing this.)

2. If my vacuum advance is on the "ported" side, then it does not advance timing until there is sufficient air going into the carb (and advances more as RPM increases until it hits a stop, correct?

When vacuum advance is connected to the ported line, it does not pull any advance at idle, when the throttle is slowly opened it exposes this port (located above the throttle blade) and only then it will start to advance timing as there is a vacuum in the venturi, depending on the setting/setup of the vacuum can.
Vacuum advance only works when there is vacuum, so it does not increase with rpm, at constant cruising speed it will work, if it runs 2000 or 4000 rpm does not matter, just depends on engine load and the opening of the throttle. So at WOT there is near 0 vacuum and vacuum advance is not working at all.

3. If I recall correctly, when working on cars (back in the day), the vacuum gauge was connected to the port that had vacuum advance attached. If trying to check vacuum from a "ported" point, it will not give me correct vacuum settings at idle, correct?

When working on the engine and you want to check manifold vacuum you need to connect it to the manifold vacuum port, otherwise you can not see the idle vacuum.

4. With no smog devices (except PCV), would it be better to connect vacuum advance to the "manifold" side?

It all depends on the engine and setup, what camshaft, carb, etc. I tried mine on both (440 with 750CFM 4 -barrel) and it does not like manifold vacuum at all.
With ported signal attached to the vacuum can it pings at light loads.

You need to play with it.
If you connect it to manifold vacuum, you might have to lower the initial timing.
For example mine, it likes to idle nice and drive well at 20 deg initial timing (without vacuum advance), with the vacuum canister attached on the manifold port it pulls an additional 10 deg and i am idling at 30 deg initial timing, which is way too much and runs like crap.
If i set it at 10 deg initial timing and use the vacuum can to add the 10 more it will have the same initial timing at idle and will run the same.
When connecting the vacuum can on the ported signal, i can leave the initial timing of 20 deg alone because at idle the vacuum can is not doing anything as there is no vacuum signal present.

Off idle or WOT the story continues, i have 18 deg mech advance which is all in at 2500 rpm.
Combined with the 20 deg initial it will run 38 deg total advance at 2500rpm and up.
If i had set the initial timing at 10 deg as above with the vacuum can attached, and keep the mech advance same, i lose 10 deg of timing at high load and WOT.
Because there is no vacuum, which leaves me with 10 deg initial and 18 deg mech advance, so a total of 28 deg (where without i reached 38 deg).
So at WOT it might not run good at all, but the idle remains good.
I could compensate for this by changing the bushing in the distributor to add 10 deg mech advance, but it will be correct for WOT, but maybe not for cruising and light loads as it pulls too much advance at that stage because vacuum is now also present as the throttle is partially open...so there will be 38 deg total advance and 10-15 deg of vacuum advance which results in 48-53 deg total advance which is too much for my car. (some engines will run with this at cruising speeds though)
So here i could play with the springs, installing stiffer springs will make sure that most of the mech advance comes it at higher rpm while at lower rpm the mech advance is being held back a bit by the stronger springs.
What the mech advance cannot give at this time is added by the vacuum can.

Long story, but just write this out and hope it comes clearer of how it works more or less.
It is trial and error at this stage, each engine runs different with vacuum advance setups.

 

[danf_fl]

Update.
I connected to manifold vacuum and drove it around a bit. I still have to check timing, adjust idle air screws, and set idle speed.
It does make a difference on my setup. It runs smoother, idles a little faster in Park, and pickup from stop signs is better.

This car has had problems with rough idle since the engine rebuild (done a few years ago when car was with my step-father). He and another fellow kept adjusting carb, timing, and other things without realizing that maybe the fuel tank was rusted (as well as sender unit).

I've since replaced tank, sender unit, fuel lines, filter, and fixed the leaks in the fuel system. When I saw that vacuum advance was on the "ported" side of carb, something did not seem correct. Thus my questions.

This is a work in progress and I still have a ways to go.

Thanks for the input!

 

[kowalski]

With a mild cam I would try the ported side first and see how it does.

First thing to do is find out how much mechanical timing your distributor has. Plug off the vacuum advance, start the engine and see where your timing is set. Lets say it's 16* at idle. Then watch the mark as you accelerate until it stops advancing. Let's say it stops at 34*. Having a tach hooked up, take note at what rpm it stops. That's your "all in" mechanical advance at that rpm.

Now hook up the vacuum advance and do the same thing, watching the timing mark as you increase the rpm. Once it gets to the 34* mechanical it will keep advancing. That's the vacuum advance taking over and it shouldn't go past, around 50*. If it does and you have a adjustable vacuum advance can, you can limit it with the adjustment in the nipple in the can. Takes a small allen wrench. An adjustable vacuum can has a six sided housing. A round housing isn't adjustable.

Anyway you go through this with the timing light to see what you have and where you need to adjust.

Minor point- when you are adjusting the vacuum can, you are adjusting the Rate it comes in. The Amount available is a set total built into the can (or rather the arm in it- it can be messed with, but it's alot more involved).
Otherwise, great write up!

 

[khryslerkid]

Minor point- when you are adjusting the vacuum can, you are adjusting the Rate it comes in. The Amount available is a set total built into the can (or rather the arm in it- it can be messed with, but it's alot more involved).
Otherwise, great write up!

The vacuum can I have has too much in it. There use to be different ones for these Mallory distributors but they are far and few anymore. I might have to lenthen the arm or find a way to restrict how far it goes. That's Hot Rodding!

In case you haven't seen the inside of one...