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Old 07-21-2012, 03:12 PM
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Ported vs Unported Vacuum for the Distributor - Why Not Have Both?

Most people into street performance will connect the distributor vacuum advance can to full manifold vacuum, which obviously has a considerable impact on idle. Minor variations in moment-to-moment operation manifest themselves in vacuum and this is amplified through the action of the vacuum can as it pulls on the distributor. In this mode, my vacuum continually varies, but within about 1/4" hg. This make the idle "hunt" just a bit.

On the other hand, some people connect the can above the throttle blades leaving the idle unaffected. When I use this port, I get a rock steady idle as the minor moment-to-moment variations are not magnified through the vacuum. But you have to rev the engine considerably to get to the ported vacuum to finally kick in at the vacuum can, and of course there is this lag.

So what if we "T" the two ports together and send that signal to the vacuum can?

My first impression was that the manifold vacuum should simply be "filled" from air coming in through the vacuum port located above the throttle blades, essentually creating a vacuum "leak," and very little vacuum should be available at the can during idle. Off-idle should be no different than before.

But this is not what I found at all!

When connected in this manner, I found that I only lost about 1" to 1-1/2" of vacuum at idle at the distributor. But the resultant vacuum (and idle) was rock steady with no moment-to-moment variation. And, the vacuum response is instantaneous with none of the prior lag when just using the ported vacuum fitting.

I'm impressed and need to buy a proper size "T" to make this permanent.

This is on a Holley 600 dp. Can other people give this a try and let us know your results?

Thanks guys.

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Old 07-24-2012, 12:53 AM
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Not sure this about this, wouldnt you get double the amount of vacuum at WOT?
Have you tested at WOT? I would think that the dizzy would advance too much at that point....
Just a thought.
Mike
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Old 07-24-2012, 05:10 AM
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off idle, the ported and manifold vacuum is already the same thing.

By using a tee, you just made a manifold vacuum setup.

takes around 8 to 10 inhg to operate a vacuum advance canister.

Ported vacuum is an emissions device.
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Old 07-24-2012, 06:32 AM
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Originally Posted by dub View Post
Not sure this about this, wouldnt you get double the amount of vacuum at WOT?
Have you tested at WOT? I would think that the dizzy would advance too much at that point....
Just a thought.
Mike
No, its just manifold vacuum with a slight leak.
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Old 07-24-2012, 11:38 AM
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No, its just manifold vacuum with a slight leak.
Yes, it's manifold vacuum with a slight leak. But the leak is not to the ambient air, it is to the area above the throttle blades, that essentially becomes the same vacuum source once you open the throttle.

When the distributor was set up, it was set for 34 degrees of mechanical advance and 17 degrees of vacuum advance. But the dyno showed that it liked ~36.5 degrees mechanical better, so it was simply twisted over 2.5 degrees.

Of course this raises the timing at idle by 2.5 degrees, raising the idle speed with it. Turning out the idle screw and adjusting the mixture can reduce the idle to an acceptable level, but it is not as smooth, nor can it idle down as low as when the timing is taken back out. (When the gent who set up the distributor read this post he offered to make-up and send me a 12-degree vacuum can so that should handle the idle without the "double plumbing.")

My theory is that the bit of "vacuum leak" at idle (but still above the throttle blades) keeps the vacuum can from pulling as much timing in at idle and this lets the mechanical timing run up to 36.5 degrees, but keeps the timing at idle to a more acceptable level resuling in a smoother idle and overall vacuum.

As for twice the vacuum at WOT, you can never get more vacuum than the strongest single source. It's just like as if you ran a red and black 16 gauge wire from a 12 volt battery to a fan it will get the same voltage as if you ran three red and three black 16 gauge wires from the battery to the fan. Either way, you get 12 volts at the fan.

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Old 07-24-2012, 11:41 AM
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Originally Posted by toddalin View Post
Yes, it's manifold vacuum with a slight leak. But the leak is not to the ambient air, it is to the area above the throttle blades, that essentially becomes the same vacuum source once you open the throttle.

When the distributor was set up, it was set for 34 degrees of mechanical advance and 17 degrees of vacuum advance. But the dyno showed that it liked ~36.5 degrees mechanical better, so it was simply twisted over 2.5 degrees.

Of course this raises the timing at idle by 2.5 degrees, raising the idle speed with it. Turning out the idle screw and adjusting the mixture can reduce the idle to an acceptable level, but it is not as smooth, nor can it idle down as low as when the timing is taken back out.

My theory is that the bit of "vacuum leak" at idle (but still above the throttle blades) keeps the vacuum can from pulling as much timing in at idle and this lets the mechanical timing run up to 36.5 degrees, but keeps the timing at idle to a more acceptable level resuling in a smoother idle and overall vacuum.

As for twice the vacuum at WOT, you can never get more vacuum than the strongest single source. It's just like as if you ran a red and black 16 gauge wire from a 12 volt battery to a fan it will get the same voltage as if you ran three red and three black 16 gauge wires from the battery to the fan. Either way, you get 12 volts at the fan.
So its a way around recurving your distributor and/or getting a proper vacuum can? Well if it works, then it works I guess.
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Old 07-24-2012, 01:29 PM
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There is no such thing as MANIPORTED vacuum

As we know >>> One of the main reasons for vacum advance is to provide increased part load effieciencies at cruise speed due to the leaner mixture under this type of engine load and throttle position on a carbed engine. When you are moving down the road at part throttle with the two ports connected the vacuum level at the distributor canister should be the same as striaght manifold vac as mentioned by 454.

I do not think it is advisable to use vacuum advance to tailor your idle nor to effect the maximum amount of ignition advance.

The application of vacuum advance is left till after you dial in where your initial and mechanical needs to be. Using a limiter plate to prevent the vacuum advance from adding more than approx 12 deg of timing is the best trick you can implement when it comes to vacuum advance.

A big consideration must be given to the actual vacuum levels that an engine produces at both idle and part throttle conditions. You can tailor an adjustable vacuum advance canister to suit these vacuum levels but in most cases for the common street engine the vacuum level is above the adjustable range of that type of canister especially at idle (most peg at a maximum of 12") so we are chasing our tails trying to adjust the can on manifold vac cause the thing will be pegged at idle.

Quote:
My theory is that the bit of "vacuum leak" at idle (but still above the throttle blades) keeps the vacuum can from pulling as much timing in at idle and this lets the mechanical timing run up to 36.5 degrees, but keeps the timing at idle to a more acceptable level resuling in a smoother idle and overall vacuum.
OP Is that why you are considering connecting to both sides at once to get an adjustable can within range at idle? creative thinking I suppose.

Get a vacuum gage in the car/truck and go for a drive, record all the levels of vacuum the engine is pulling and under what conditions and RPM, this will help you chose and setup the appropriate adjustable vacuum canister.
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Old 07-24-2012, 04:36 PM
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Originally Posted by Custom10 View Post
The application of vacuum advance is left till after you dial in where your initial and mechanical needs to be. Using a limiter plate to prevent the vacuum advance from adding more than approx 12 deg of timing is the best trick you can implement when it comes to vacuum advance.
If you read my posts, this is exactly what is being done by the gentleman who originally set up the distributor and carb, after he read in my post that I advanced the mechanical timing from 34 to 36.5 degrees. The original can was 17 degrees.
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Old 11-20-2012, 03:46 AM
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Good day Gentlemen, i humbly seek your help with my distributor vacuum advance. Below is a photo of the distributor vacuum advancer of my Toyota 2E , a 1.3L SOHC 4-cylinder engine. Could somebody help me identify which line goes to port vacuum and which line goes to manifold, which is for advance and which is for retard.


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Old 11-20-2012, 07:41 AM
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Ported vs Unported Vacuum for the Distributor - Why Not Have Both?

Finding which line is the manifold vacuum is easy. Unplug one of the lines from the distributor if has vacuum at idle it is manifold vacuum. Check both lines. If it is ported vacuum you will not have any vacuum until you rev the engine. To find out which line is the vacuum advance you will need to know which direction the plate turns to advance the timing. IE a maitenance manual. Unplug the vacuum line at the carb 1 at a time and suck on it to see which direction the plate turns.
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Old 11-20-2012, 09:11 AM
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Originally Posted by cdminter59 View Post
Finding which line is the manifold vacuum is easy. Unplug one of the lines from the distributor if has vacuum at idle it is manifold vacuum. Check both lines. If it is ported vacuum you will not have any vacuum until you rev the engine. To find out which line is the vacuum advance you will need to know which direction the plate turns to advance the timing. IE a maitenance manual. Unplug the vacuum line at the carb 1 at a time and suck on it to see which direction the plate turns.
Thank you for your reply. Here's another query, i hope you will not mind me asking, from the illustration below, can you help me analyze/interpret the system.


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Old 11-20-2012, 05:23 PM
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Originally Posted by toddalin View Post
... Minor variations in moment-to-moment operation manifest themselves in vacuum and this is amplified through the action of the vacuum can as it pulls on the distributor. In this mode, my vacuum continually varies, but within about 1/4" hg. This make the idle "hunt" just a bit....
IMO, hooking up the vacuum advance to a full-time manifold port is the best way to go, especially if you are running a big cam. Most of the base engine (low HP) vacuum cans don't behave well hooked up this way (erratic idle, stalling etc)

Fortunately, Chevy made different spec vac cans for the high horse engines. I had a "B22" can on my distributor. Idle was all over the place with my new lumpy cam. Installed a B26 off a 396 vette, all is good. Idle is rock solid. For a really big cam, the B28 vac can off a 327 fuelie vette will work good.
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Old 11-21-2012, 03:13 AM
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Originally Posted by m1911a2 View Post
Thank you for your reply. Here's another query, i hope you will not mind me asking, from the illustration below, can you help me analyze/interpret the system.


image sharing
Your vacuum advance set-up results from the need for specific timing requirements under various conditions.

An example is when the engine is cold, the amount of advance allowed by the system will be different than when the engine is warmed up. That's why the vacuum lines are routed through various vacuum temperature switches, etc.

My advice would be to leave the system as-is. If the vacuum advance unit is faulty, replace it and leave the rest of the system intact unless you have a good grasp of the needs of the engine regarding the vacuum advance.

Should you want to experiment, remove and leave open the rearmost (#1 on the diagram you drew) vacuum port on the vacuum advance. Plug the hose.
Connect port #2 to a manifold vacuum source (full time vacuum, even at idle). If the line on it now is a manifold vacuum, leave it on. If it's not, plug it and find a source of manifold vacuum. Use a "T" if necessary.
See how the engine performs and pay attention to any signs of detonation (pinging).

Next, connect port #2 to a ported source and see how that is. Pick the best one if there's a discernible improvement. But I'd be surprised if there's any difference.

Now, that's not to say there's no way to improve what you have.

Sometimes adding initial timing helps performance, sometimes adding total timing helps, or both. But what you do not want to do is add initial timing w/o verifying the total timing isn't now too high. This is very important because too much total timing can damage the engine with detonation or cause you to have to use premium gas. Tuning it to run on premium is OK, detonation is NOT OK.

Read the section on timing here and see if it makes sense to you. If you grasp it, you might want to use a timing tape and timing light (or a dial back timing light) to plot the way the advance curve is now, and see if adding some initial or total timing helps, or bringing in the mechanical advance sooner helps. This is done by using lighter springs in the advance mechanism.

BTW, the part marked as a filter might be an intake air temp vacuum switch.

Last edited by cobalt327; 11-21-2012 at 03:32 AM.
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Old 11-21-2012, 07:54 PM
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Quote:
Originally Posted by toddalin View Post
Most people into street performance will connect the distributor vacuum advance can to full manifold vacuum, which obviously has a considerable impact on idle. Minor variations in moment-to-moment operation manifest themselves in vacuum and this is amplified through the action of the vacuum can as it pulls on the distributor. In this mode, my vacuum continually varies, but within about 1/4" hg. This make the idle "hunt" just a bit.

On the other hand, some people connect the can above the throttle blades leaving the idle unaffected. When I use this port, I get a rock steady idle as the minor moment-to-moment variations are not magnified through the vacuum. But you have to rev the engine considerably to get to the ported vacuum to finally kick in at the vacuum can, and of course there is this lag.

So what if we "T" the two ports together and send that signal to the vacuum can?

My first impression was that the manifold vacuum should simply be "filled" from air coming in through the vacuum port located above the throttle blades, essentually creating a vacuum "leak," and very little vacuum should be available at the can during idle. Off-idle should be no different than before.

But this is not what I found at all!

When connected in this manner, I found that I only lost about 1" to 1-1/2" of vacuum at idle at the distributor. But the resultant vacuum (and idle) was rock steady with no moment-to-moment variation. And, the vacuum response is instantaneous with none of the prior lag when just using the ported vacuum fitting.

I'm impressed and need to buy a proper size "T" to make this permanent.

This is on a Holley 600 dp. Can other people give this a try and let us know your results?

Thanks guys.
Hey.
Heres a idea of how vacuum advance is intented to work:
Manifold Vacuum
At idle/low throttle your manifold vacuum is HIGH, your canister is all the pulled in. The farther you open the blades the LESS vacuum in the intake manifold, it releases the can and retards the timing.
Example, you rug it, the manifold vacuum drops like a stone, it retards the timing so to avoid detonation at high load/low rpm. At (IE) 3500 your mechanical advance takes over and starts introducing timing when the engine can take it, at a lower load higher rpm.
If you took a stock 350 with 18 degrees initial, and granny driving it on unleaded fuel, it would rattle the heck out of everything with no vac advance. So the engineeers designed the advance system to give the engine less initial timing, so they didnt do 2,000,000 warranty engines.

Ported timing, is slightly opposite. It pulls more timing (Vacuum) as the blades are opened, to a point. You have to start your initial timing higher, to compensate for little vacuum at idle, BUT ( as far as i am concered, and who am i anyways)
You 1, can never achieve a full vacuum signal through the ported circuit, as the venturi effect will never produce as much vacuum as below the blades
2, by nature the signal will "hill" and start at nothing, taper up a bit, and go back down to nothing from idle to wfo (while driving that is)
3, should never use that port, unless it is a quadrajet on a 305.

So this "lag" you are interperating as a problem when you rev it up in park with a light on it, is non existent when you are driving the car, as the engine has no load to produce enough vacuum to pull the can in all the way.

There isnt a single performance engine in the world that makes MORE power with vacuum advance. Slightly better idle? Yes. Slightly better low end driveabilty? Yes.

I believe that the spring is broken in your can, or severly worn out, as 1/4 in vacuum would do nothing to the advance can. The junkers in the 80s moved around 3 or 4 inches at idle, and never "hunted"
If you want to correct your surging idle, and you really feel you need vacuum advance, get an adjustable can, or as stated, a performance OE can.
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Old 11-21-2012, 09:08 PM
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Originally Posted by cdminter59 View Post
To find out which line is the vacuum advance you will need to know which direction the plate turns to advance the timing. IE a maitenance manual. Unplug the vacuum line at the carb 1 at a time and suck on it to see which direction the plate turns.
Here's what i found after i followed your instruction.


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Can you explain what does this mean?
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