Oneiros

I recently had a vac gauge hooked up to both the front nipples on the spacer, you guys may be interested in the results.

On the front nipple, 0 vac at idle, increasing to about 10mmHg when the throttle opens And gradually moving back to 0 with more throttle. I looked inside the carb and this nipple gets it's signal from a small hole above the throttle plates which would explain the above signal.

On the second nipple, this is sourced from the spacer (below throttle plates) and sees around 20mmHg at idle moving towards 0 with more throttle, standard behaviour.

purple82

So the front nipple is highest just off idle when the throttle is cracked open, correct? Then dropping from there?

Are you sure about your units? 10mmHg is only 0.06psi, which is nearly nothing. Should that be mm of h2o?

Kentetsu

Yes, that will smooth out once you reset the timing.

If you set the timing while hooked up to manifold vacuum, then you will be setting your timing in a more "retarded" position. This will also affect the performance at higher rpms as a result.

When you switch to ported vacuum, you lose that vacuum advance at idle. So you will need to advance your timing a bit to make up for that loss. Then, as a result of that, you get a bit more advance at higher rpms too, which will increase performance in that range as well.

Once you get your timing set correctly for ported vacuum, I'm sure you will like the results...

Kentetsu

What I'm saying is, with the stock rotary (including rat's nest) there are components that basically convert the manifold vacuum and fool it into operating as if it were ported vacuum. That is the function of those solenoids.

But without the rat's nest, you can get the same results by running ported vacuum directly.

Why on earth would you want to lose your vacuum advance as soon as you touch the throttle? It just doesn't make sense, as it will decrease your overall performance.

And that is without taking into consideration the fact that you are retarding the entire ignition system if you set the timing while connected to manifold vacuum.

Is this making sense to anybody?

purple82

If you read the article, it will tell you the reason for vacuum advance in as clear a description as I've ever read. It's meant to complete the combustion process more fully at low loads and high AFRs. The most advance at the lowest load, ie highest vacuum. It's all a matter of flame speed and where TDC is to optimize the pressure built during combustion.

The spinning weights in the distributor handle advance with RPM to cover the reduced time that the charge has to burn in the combustion chamber. The two aren't meant to work at the same time necessarily. They cover different operating conditions to fill out the picture.

You can see from the experiment that Oneiros ran, vacuum is highest at the smallest throttle opening on either port, which is as it should be. One port has it's opening on the plenum side of the throttle, the other on the atmospheric side, which will effect if idle sees advance or not, but both give similar signals once the throttle is opened.

arghx

iTrader: (3)

and this is why Mazda chucked the dizzy and switched to fully computer controlled ignition in 86... full 3 dimensional timing maps based on rpm and load (airflow divided by rpm) for the FC or rpm and manifold pressure for the FD

Oneiros

Sorry yeah I must be wrong on the units, Guess it's inches Hg? (can't check ATM)

As Kentetsu said ported vacuum would be what you want so that timing begins to advance off idle, otherwise it will see the higest vac/advance at idle then retard with more throttle, regardless of load

senator

Unless I missed somthing, I though vacuum advance was used because from spark beginning, the whole propogation thing, to end is going to take the same amount of time, so higher rpm/ throttle opening ( less manifold vacuum) you need to start ignition sooner.

The higher rpm, the sooner ignition is needed to complete burn before the cycle is complete. Ported vacuum advances distributor to acomplish this. So even if a lean idle or cruise mixture takes longer to burn you still would need more advance at higher rpm to complete the burn within a given duration before cycle completion.

A power cycle completion will change in milliseconds with any rpm change, rpm goes up time to complete goes down so you need to start ignition sooner because the burn will be the same in m/s.

I feel like ive repeated myself a bunch, but it makes sense to me. Maby ill do the vacuum source swap/ adjust carb and timing with my old school v8 this weekend.

purple82

The purpose of vacuum advance is strictly to compensate for the mixture differences. Engine speed is handled by mechanical advance. They're meant to work independently. Check out the description in the link.

arghx

iTrader: (3)

vacuum advance changes timing according to load, centrifugal advance changes it according to rpm

Kentetsu

My understanding of this is: vacuum advance handles low rpm, while centrifugal advance handles mid to high rpm.

purple82

Nope. Vacuum advance functions at all RPM, accounting for mixture at low loads. The link describes it very clearly.

Kentetsu

How can vacuum advance work at all rpms, especially if using manifold vacuum? Once you go off idle, you start losing vacuum in the manifold, right?

And take a look at this article: http://www.nationaltbucketalliance.c...uum/vacuum.asp

According to this guy, GM dizzys have the vacuum advance working backwards from us (more vacuum = less advance) while Ford dizzys work, well, just like ours does. Here's a couple of snippets:

The vacuum port on the carb is an active element, meaning it draws vacuum directly from the intake manifold. So, if you were to put a vacuum gauge on this port, you would see a vacuum signal. This vacuum signal actually "RETARDS" the timing on your distributor. Now when you open the throttle, the vacuum signal drops and the distributor "ADVANCES" until the vacuum signal rises again. Meanwhile, as the engine revs up the mechanical advance starts advancing proportional to the engine RPM. Under full throttle both the vacuum and mechanical advances are functioning. However while cruising, the vacuum advance will back off (RETARD) proportional to the vacuum signal and the mechanical advance will be advanced proportional to the RPM. This makes a good general use distributor especially for street driving.

and:

RULE 5: (Holley Carbs) Never use the vacuum nipple located on the metering block for GM distributors! That port operates BACKWARDS to what your distributor needs. This is reserved for selected OEM Ford distributors ONLY.


Mechanical distributors are designed for 3000RPM and up launches off the line. While they work reasonably well on the street, that isn't their design intention. From a dead idle, mechanical advances can only do what the engine RPM tells it to do. Whereas a vacuum advance will go to full advance as soon as you open the throttle. This spontanous vacuum responce is very desirable for off-idle street hotrodding. A vacuum advance is much more forgiving while cruising as well. While cruising, the vacuum advance will retard to it's zero position leaving the advancing to the mechanical part. This makes for a slightly cooler running engine during cruise mode. You can think of a vacuum advance as a "power valve" for your ignition! Its function is IDENTICAL to that of a power valve in a carb. Both provide more "juice" when needed, and "conserve" when cruising. A full mechanical advance will tend to be more advanced than needed for cruising, and usually runs your engine a little hotter. This isn't a problem by any means, but it's not running as efficiently as the vacuum advance.



This is the problem when making comparisons that are not "apples to apples", and I think it may be the source of our disagreement.


Also (I'm editing now, so I can see your next response already ), ported vacuum is created by airflow, so the greater the airflow through the carb, the greater the vacuum created, contrary to what you are stating.

purple82

Manifold vacuum (and ported vacuum for that matter) is a function of throttle position. The more you open the throttle, the lower the vacuum. You can have high vacuum at high RPM if the throttle plate isn't opened very far. If you're driving at a high engine speed but not trying to accelerate on a flat road or on a downhill, for example.

The difference between ported and manifold vacuum is the position of the port allows for a vacuum signal at idle, or if the vacuum signal starts just off idle. Once the throttle plate is opened, they both see essentially the same signal (albeit at different vacuum levels).

Kentetsu

This guy states things pretty clearly (if a bit wordy) too:

http://www.moparchat.com/forums/arch...p?t-64031.html

Excerpt:

Dart05-29-2002, 04:47 PM
Here is some of the post PRO did on this topic a wile ago. Read it...

YOU GUYS ARE TWEAKING! BACK TO THE ORIGINAL QUESTION PORT OR MANIFOLD? THERE IS ONLY ONE ANSWER ..PORTED NEVER NEVER MANIFOLD READ YOUR DIAGRAMS CORRECTLY AND YOUR UNDERHOOD STICKERS, I OWN 50 OR SO FACTORY CHRYSLER SERVICE MANUALS AND NONE I REPEAT NONE HAVE MANIFOLD EVER. WHAT GOOD WOULD IT DO TO FULLY OPERATE THE VACCUM ADVANCE AT IDLE? WHY NOT JUST INCREASE INITIAL TIMING? DAH!! I'M REPRINTING WORD FOR WORD FROM A 1974 SERVICE MANUAL;OSAC VALVE . A TINY ORIFICE IS INCORPORATED IN THE OSAC VALVE WHICH DELAYS THE CHANGE IN PORTED VACCUM TO THE DISTRIBUTOR BY ABOUT 17 SECONDS WHEN GOING FROM IDLE TO PART THROTTLE.PERIOD. I HAVE BEEN A MECHANIC FOR 22 YRS. CURRENTLY OWN MY OWN AUTO REPAIR AND HIGH PERFORMANCE SHOP AND ONE OF THE MOST COMMON MISTAKES EVER IS NOT CONNECTING THE ADVANCE HOSE TO PORTED VACCUM.NOW PROBABLY WHAT YOU THINK IS INITIAL TIMING IS INCORRECT, ALL I REPEAT ALL HARMONIC BALANCERS ARE OFF, TDC IS NOT WHERE YOU THINK IT IS , IVE REBUILT THOUSANDS OF MOTORS AND MANY OF THOSE HP AND DRAG MOTORS AND YOU HAVE TO ACCURATELY KNOW WHERE TDC IS TO (1) SET AN ADVANCE CURVE (2) DEGREE A CAM , MOST MOPRS ARE OFF 2-3 DEGREES(SOME GM ARE 8-9) BUY A TDC TOOL (12 BUCKS) SCREW IT IN NO.1 SPARK PLUG HOLE ROTATE IN ONE DIRECTION UNTILL THE PISTON TOUCHES IT AND MARK YOUR BALANCER THEN ROTATE THE OPPOSITE DIRECTION UNTIL IT TOUCHES AGAIN MAKE A MARK AND SPLIT THE DIFFE RENCE AND YOU WILL HAVE TRUE TDC, A DEGREE WHEEL IS NESSCESARY TO ACCURATELY SPLIT THE DIFFERENCE.TOTAL TIMING FOR A MILD SB SHOULD BE 36-38 DEGREES ,THE ONLY EXCEPTION WOULD BE ADD 3 DEGREES ABOVE 4000FT.INITIAL SHOULD BE NO MORE THAN 12 ON A SB.VACCUM ADVANCE FUNCTIONS ARE (1)TO HELP THE ENGINE SPEED UP UNDER A LOADFROM A LOW RPM. (2) RELIEVE HESSITATION DUE TO INEXACT MIXTURES. AT W.O.T. THE VAC. ADV. IS NON FUNCTIONING AS MOST DONT DELIVER ADV. UNTIL 8-9 INCHES OF VAC. AND YOU PROBABLY DONT MAKE THAT MUCH AT WOT, SO DONT ADD IT TO YOUR TOTAL ADVANCE.BUY A TIMING LIGHT WITH AN ADVANCE **** SO YOU CANSET IT TO YOUR DESIRED TOTAL ADV. AND SPEED THE MOTOR UP TO WHATEVER RPM GIVES YOU MAX ADV. THEN YOU TURN THE DIST UNTILL THE MARK ON YOUR BALANCER IS LINED UP WITH YOUR STATIONARY POINTER(ON TIMING COVER FOR SB). TRY IT I GUARANTEE IT WILL BE MORE DRIVEABLE!!AND FASTER!! IF YOUR RUNNING ELECTRONIC IGN. YOUR DIST WILL HAVE TOO MUCH MECHANICAL ADV. BUILT INTO IT. ITS ACTUALLY AN EMISSIONS PIECE REMEMBER THE 70S?SHORTEN IT UP BY WELD UP THE SLOTS THEN ADD LIGHT SPRINGS AND YOU'LL ROCK...PRO..


YOU GUYS HAVE ALOT OF THEORIES BUT WHERE ARE YOUR FACTS?!!!!Mopar nerd(actually there is no such thing!)your 18* of initial is telling you something..are you listening? such as low compression,sloppy timing chain,too big of carb,too rich of a fuel mix? restricted exhaust?your "FIX"isnt one at all!!Do you know your cyl.s cranking compression?You have to start with the basics.Thoroughness is what wins races,HIDING THE REAL PROBLEM JUST CAUSES MORE PROBLEMS!and your "seat of the pants " form of faster is too close to something else to be reliable.(get a G-TECH).Who said a lean burn carb has no vaccum port? your right!! BECAUSE LEAN BURN IGNITIONS HAVE NO VACCUM ADVANCE ON THE DIST!!!!Dave you say my manual says this and that somewhat mockingly if you would go read one you might learn a little more about your MOPAR!Do yourself a big favor call EDELBROCK,HOLLEY,CARTER,BG,DEMON,PREDATOR and ask them this ridiculous question!You say there is novaccum at the ported vaccum port at low rpmS you say its higher at high rpms, now i know YOU'VE NEVER SEEN A VACCUM GAUGE ON A RUNNING MOTOR!!!ITS JUST THE OPPOSITE!!!!!!!!!!!!ITS HIGHEST AT LOW RPMS!!AT W.O.T. ITS "0"!!!!Do you know what a power valve is on a HOLLEY ? It allows extra fuel enrichment at wot, if its stamped 3.5 that means when the vaccum drops below 3.5" of vac. it opens to add extra fuel!So do you see that it drops at wot?Now My chrysler service manual states (all of them)that at 9" of vac it will provide 10* of advance!!Most stock V-8s have between 14-18" of vac at idle,most 4cyl.s have 12" at idle,anything with a big cam can have as little as 6",all drop to zero when floored, the only exception is if the carb is too small it might only go to 2 or 3"..Port vac. passage has a spring loaded check ball in it that wont allow it to open as vac drops,only as it rises.Yes it is possible to get debris in this port which makes it have vac. all the time.Now look I dont mean to be aggressive here but Ive had hundreds of motors set up on an excessive amount of equipment(gas analyzer,scan tools,DRBs,fuel pressure gauge, vaccum gauge,adjustable timing light,exhaust gas temp gauge,compression gauge,boost gauge,)and Ive built turbo motors and run nitrous oxide , and all these motors had true tdc established,degreed cams etc. and monitored all aspects of engine output. What is the opinion of all carb manufacturers?"PORTED ONLY"this is my opinion too.(and everybody is entitled to their opinion,but mine is accompanied with trophies)Ive even used a vac. gauge to adjust my valves while the engine is running ,I adj. to max vac. at idle if i want more torque,then i go back and check lifter preload,now what Ive determined is what(depth of engagement)lifter preload is more suitable for lower rpm motors, a vac gauge is an invaluble tool ,you can set idle air/fuel mixture with it,you can determine if your catalytic is plugged ,w/it,if your carb is too small, everybody who wants performance out of their car should have one,there inexpensive too....PRO..

Alright,we all have to be willing to admit we dont know everything,its like thinking my car is the fastest one out there,(67 Coronet,440,4spd.runs 11.88 thru the exhaust on 85 oct.) guess what theres always someone faster! Look I am typing with my caps lock off as someone informed me its like shouting,I had something to learn about typing(that it sucks).Having spent the last 22 Yrs under the hoods of performance cars doesnt mean I know it all either,but the minute I decide I do know it all I just quit learning. 1.Manifold vaccum:The vaccum that is present inside the intake manifold.(high at idle,0 at wot) 2.Ported vaccum:manifold vaccum that is plumbed thru a valve(port)so as to change when and how much vac is present.(0 at idle,9-10" at part throttle) You may be right when you said that we were saying the same thing with different terms. Edelbrock pamphlet that accompanies new carbs does not say to connect dist to whichever port works best!!!!(Call Them). Hot Rod magazines article on spark timing does not say manifold vac for dist but ported!!Details Details,thats what makes cars go fast.We are currently modifying a PROWLER(see Rumors and Gossip), we have a best ET of 12.87@109mph as of yesterday,so you see I cant throw wrenches)Mopar people are the best people on the planet,(including you guys)its just Ive heard this attitude on vac source so many times,and Ive been to the track so many times and have convinced everybody that doubted (ask DART).PORTED VAC IS THE ONLY SOURCE FOR DIST ADV.!!! ...PRO...

--------------------------------------------------------------------------------

Kentetsu

This guy explains it pretty well too, finally stating that there is no correct answer.

It seems that a lot of the piston guys that are using big cams will use manifold vacuum to increase the advance at idle, simply to get the motor to idle better.

And the difference between manifold and ported vacuum sources is simply that ported shows no vacuum at idle, but behaves the same as manifold otherwise.

I was getting confused between ported vacuum and venturi vacuum in my explanations though, so you were correct on that point. Once the throttle is open, they will both behave the same way.

http://www.lbfun.com/warehouse/tech_..._explained.pdf

Carburetor Vacuum Ports:
Manifold, Ported and Venturi Vacuum Explained
by Lars Grimsrud
Colorado Corvette Crazies
The Ultimate Corvette Tuning and Beer Drinking Fraternity
Lafayette, CO Rev. New 9-10-03
This tech paper will discuss the concepts of 3 different types of vacuum sources, and will briefly discuss
their potential uses and applications in tuning GM V8 engines.
Overview
The airflow through a carburetor and through an engine’s intake system creates various pressure regions
due to a variety of effects. The low pressure regions are sources for “vacuum” used for signal sources and
power sources for operating accessories.
“Vacuum” in an engine is not truly a “vacuum.” Rather it defines a lower-than-atmospheric-pressure area
in the engine or in the carburetor. Lower-than-atmospheric pressure is measured in “inches of Mercury.”
Mercury has the chemical symbol “Hg,” so the terminology for the measurement becomes “in. Hg.” To
visualize how this measurement works, imagine a U-shaped glass tube partially filled with Mercury. The
one end of the tube is exposed to the atmosphere. The other end of the tube is attached to your low
pressure source. The low pressure on the one end of the tube will cause the Mercury to rise up the tube.
The amount that it rises is “in. Hg.”
A funny terminology issue arises with this: Most people would describe that the low pressure area is
“sucking” the Mercury up the U-shaped tube. To be technically correct, there is no such thing as “suction.”
What is actually happening is that the low-pressure area is allowing the high pressure on the other end to
“push” the Mercury up the tube. Thus, when you “suck” on a straw in a milkshake, you are not sucking:
You are creating a low pressure region in your mouth, and atmospheric pressure is “pushing” the milkshake
up the straw and into your mouth.
How Vacuum is Produced
In an engine and carburetor there are two different mechanisms (processes) for producing vacuum. The
two are interesting in their differences.
The most commonly recognized vacuum source in an engine is “manifold vacuum.” Manifold vacuum is
created in the intake manifold of an engine due to the pistons’ intake downstroke on one end, and the
restriction created by the partially open carburetor throttle plates on the other end. If the throttle plates are
closed tightly and the pistons are moving quickly, a very high vacuum is created. If the throttle plates are
opened more, creating a larger “leak path,” less vacuum is created. If the pistons are moving slowly and
the throttle plates are wide open, the pressure in the manifold will be very close to atmospheric pressure –
no intake vacuum. Thus, intake vacuum can be used as a signal source to determine how hard an engine is
working.
Less recognized, and more frequently misunderstood, is the term “venturi vacuum.” Venturi vacuum is
produced in an entirely different manner, and it behaves completely independently of manifold vacuum.
To understand venturi vacuum, you have to think back to your high school physics class and the Bernoulli
Effect: As a fluid (liquid or gas) moves through a tube, the areas of low velocity produce high pressure,
and the areas of high velocity produce low pressure. Thus, as the tube necks down and becomes narrow,
the fluid flowing through the narrow section has to move faster, and pressure in the narrow section is lower
than the pressure in the larger section. Venturi vacuum, and the Bernoulli Effect, occurs in the venturi of
the carburetor: As air enters the carb, it passes through the “necked-down” area of the venturi. As it passes
through the venturi, the air accelerates. This fast-moving air creates a low pressure area right in the middle
of the venturi, and this low pressure area is used to discharge fuel from the float bowl of the carb into the
air stream. Remembering our discussing in the Overview of this article, the fuel is not “sucked” out into
the venturi: The low pressure area in the venturi allows atmospheric pressure on top of the fuel in the float
bowl to “push” the fuel up and out of the venturi discharge nozzles. The venturi vacuum varies only by the
total amount (“mass”) of airflow passing through the venturi and its velocity: The faster the air is passing
through (more air), the higher the venturi vacuum will be. This is completely independent of the manifold
vacuum.
How Vacuum is Used
Manifold vacuum is very easy to tap and utilize: Simply drill a hole in the intake manifold and stick a hose
in it. This vacuum can then be used to power accessories (headlight doors and heater controls), or it can be
used as a signal source based on how hard the engine is working. One such signal source application is
distributor vacuum advance: When the engine is working lightly (high vacuum produced), the ignition is
advanced, and as the engine loses vacuum due to the throttle being mashed to the floor, vacuum is lost and
ignition is retarded. Pretty simple.
But what if you want to “switch” the vacuum on and off based on whether the engine is just idling or in
cruise mode? Enter the term “Ported Vacuum.”
When emissions became a priority to vehicle manufacturers, a method had to be found to reduce emissions
at idle. The amount of Hydrocarbons emitted out of the tailpipe can be drastically altered by changing the
timing: Retarding the timing reduces Hydrocarbon emissions. But retarded timing adversely affects gas
mileage at cruise. So a method was needed to retard timing at idle, yet maintain it at normal levels for
cruise. The solution was seen to “turn off” the vacuum advance at idle, yet have it operate normally under
all other operating conditions. To do this, a small hole was drilled in the carburetor throttle body just above
the position of the throttle plate at idle (NOT in the venturi area), and this hole was connected to a vacuum
nipple on the carb. When the throttle plates are closed at idle, they act as an “off” switch to block the
drilled hole from manifold vacuum. As the throttle plates are opened up, the hole becomes fully exposed to
manifold vacuum, and normal manifold vacuum is realized at the nipple. Thus, you have a manifold
vacuum “on-off” switch, turning manifold vacuum “off” at idle, and restoring it to normal operation once
the throttle plate is cracked open. Vacuum advance can be eliminated at idle for good emissions, and
instantly restored to normal operation at cruise. At both cruise and Wide Open Throttle (WOT), manifold
vacuum and ported vacuum are exactly the same: There is high vacuum at cruise, and virtually no vacuum
at WOT. The difference in vacuum occurs only at idle.
So if there is no manifold vacuum or ported vacuum at WOT, how can vacuum open the secondaries on a
vacuum secondary carb at WOT? Manifold or ported vacuum is not used to open the secondaries: The
secondaries are opened by venturi vacuum. On a true vacuum secondary carb (Holley and BG are the only
real vacuum secondary carbs – Q-Jets and AFBs are not vacuum secondary carbs), there is a small hole
drilled right into the middle of the venturi wall on the passenger side primary venturi. This passage runs
over to the vacuum diaphragm, and the low pressure created in the primary venturi from very high airflow
through the venturi is used to open the secondary throttle shaft. The more air that passes through the
primary side, the more the secondary diaphragm will open. It is strictly a function of airflow through the
primary venturi and the low pressure that this creates in the venturi. Manifold vacuum can be non-existent,
yet if the airflow through the carb is high, the secondaries will be pulled open by the venturi vacuum. Note,
however, that there is no external nipple on any carb for venturi vacuum: The only source is the small
drilled hole in the venturi, and this hole only runs to the secondary diaphragm through an internal passage.
Thus, we see that we have 3 types of vacuum in the engine: Manifold Vacuum, Ported Vacuum (simply a
“switched” manifold vacuum source), and Venturi Vacuum. Of these, only Manifold Vacuum and Ported
Vacuum can be utilized for tuning purposes.
Tuning with Vacuum
I see a lot of discussion and confusion regarding the use of Ported Vacuum versus Manifold Vacuum for
distributor vacuum advance. Once understood, the tuner can effectively utilize one source or the other,
depending on the tuning requirements of the vehicle. There is no right or wrong answer on which source to
use, but using the correct source for the tuning requirements of a given engine can have a big effect on offidle
and near-idle performance characteristics.
First, be sure to locate and read my paper titled “Distributor Vacuum Advance Control Units.” This
contains a lot of technical information related to this issue that I won’t be repeating under this paper
heading.
The timing advance curve requirements for an engine will vary a bit from one engine to another depending
on cam, compression ratio and other efficiency factors. But in general terms, most GM V8s will produce
peak power at WOT with 36-38 degrees of ignition timing. Peak fuel economy and drivability at cruise is
achieved with about 52-54 degrees of advance. Best idle quality has a much wider range depending on cam
& engine, but tends to be in the 12-24 degree range. Lowest emissions usually occur with timing in the 4-8
degree range.
When tuning, it is important to realize that the upper limits on timing are determining factors for how to set
things up: You want the total WOT timing (the maximum timing the engine will see with vac advance
disconnected and with the centrifugal advance fully deployed) to be not over 38 degrees. 36 is the best
setting for most applications. Once this has been set, it automatically determines what your initial advance
ends up being unless you physically alter the length of the advance curve. In most cases, once total
advance has been set to 36, the initial advance will end up being about 12 degrees-or-so. And, since most
vacuum advance control units pull in about 16 degrees of vacuum advance at cruise speed (where the full
centrifugal advance will also be deployed), the 36-degree setting will produce 52 degrees of total combined
advance at cruise with the vac advance fully deployed.
But what if your engine/cam combination idles best at 26 degrees advance? Radical cams often require
quite a bit of advance at idle. If you simply bump the initial timing up from 16 to 26, your total WOT
advance will go from 36 to 46. The total combined timing at cruise will go from 52 to 62. This is not
acceptable, and can result in severe engine damage from detonation at WOT, and the car will chug and jerk
at cruise from the over-advanced condition. An appropriately selected vacuum advance unit, plugged into
manifold vacuum, can provide the needed extra timing at idle to allow a fair idle, while maintaining
maximum mechanical timing at 36. A tuning note on this: If you choose to run straight manifold vacuum
to your vacuum advance in order to gain the additional timing advance at idle, you must select a vacuum
advance control unit that pulls in all of the advance at a vacuum level 2 in. Hg below (numerically less
than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle,
it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling.
This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing
the timing at idle can assist in lowering engine temperatures. If you have an overheating problem at idle,
and you have verified proper operation of your cooling system components, you can try running manifold
vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps,
but it will also increase hydrocarbon emissions on emission-controlled vehicles.
If, however, your engine idles best in the 12-16 degree range due to a mild cam, plug the vacuum advance
control unit to a ported vacuum source to eliminate the vacuum signal at idle. You will still obtain the 36-
degree WOT total, and you’ll still have 52 at cruise. Also, if you need to pass an emissions test, use the
ported source to reduce your hydrocarbons.
By playing with the total length of your centrifugal advance curve, selecting between ported or manifold
vacuum, and carefully selecting a matched vacuum advance control unit that meets your specification
requirements, you can achieve an optimum idle, excellent off-idle throttle response, and the best fuel
economy possible.
Questions, Comments & Technical Assistance
If you have questions or comments regarding this article, or if you notice any errors that need to be
corrected (which is quite possible since I’m writing this from memory…), please feel free to drop me an email.
Also, if you need any technical assistance or advice regarding this process, or other maintenance
issues, feel free to contact me:
V8FastCars@msn.com

Or here, they seem to clearly agree with me:

http://www.camaros.net/forums/showthread.php?t=40614

A couple of weeks ago, someone asked in a post titled "timing vacuum/holley 600cfm" about where to connect a vacuum advance line.

The consensus was that it whould be connected to a manifold vacuum source, rather than a ported vacuum source. I have heard both ways, but manifold vacuum was the usual answer.

I was rereading the August 2000 Super Chevy and this was written on page 82:

"For good street driveability, your distributor's vacuum advance should be connected to "ported vacuum", which is drawn from a source in the carburetor above the throttle plates. This results in increasing vacuum as the throttle is applied. the function of vacuum advance is to provide timing advance in low-rpm acceleration conditions for smoothness and improved driveability. The alternative, manifold vacuum, is taken from a source below the throttle plates, resulting in decreasing vacuum as the throttle is applied. If your distributor is connected to manifold vacuum, full advance may be applied at idle. Then as throttle is applied, vacuum decreases and timing advance is decreased, causing poor engine performance and acceleration at low speeds."

Can either be used depending on application? Were the Team Camaro techies incorrect? Can Ignitionman save the day and give us the correct answer?


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#2 Aug 8th, 00, 08:34 AM
sudolg
Tech Team

Join Date: Apr 2000
Location: wallington, NJ
Posts: 71



--------------------------------------------------------------------------------

I am always surprized to hear people using manifold vacuum for advance. I was always taught that you want more advance when you hit the gas and that is when ported vacuum kicks in. Otherwise you would have too much advance at idle. No??? I just saw an article in one of the other magazines and it said the same thing that you quoted in Super Chevy.

Kentetsu

And finally, from Wikipedia:

http://en.wikipedia.org/wiki/Ignition_timing

Vacuum timing advance
The second method used to advance the ignition timing is called vacuum timing advance. This method is almost always used in addition to mechanical timing advance. It generally increases fuel economy and driveability, particularly at lean mixtures. Vacuum advance works by using a manifold vacuum source to advance the timing at low to mid engine load conditions by rotating the position sensor (contact points, hall effect or optical sensor, reluctor stator, etc) mounting plate in the distributor with respect to the distributor shaft. Vacuum advance is diminished at wide open throttle (WOT), causing the timing advance to return to the base advance in addition to the mechanical advance.

One source for vacuum advance is a small opening located in the wall of the throttle body or carburetor adjacent to but slightly upstream of the edge of the throttle plate. This is called a ported vacuum. The effect of having the opening here is that there is little or no vacuum at idle. Other vehicles use vacuum directly from the intake manifold. This provides full engine vacuum (and hence, full vacuum advance) at idle.

On some vehicles, a temperature sensing switch will apply manifold vacuum to the vacuum advance system when the engine is hot or cold, and ported vacuum at normal operating temperature. This is a version of emissions control; the ported vacuum allowed carburetor adjustments for a leaner idle mixture. At high engine temperature, the increased advance raised engine speed to allow the cooling system to operate more efficiently. At low temperature the advance allowed the enriched warm-up mixture to burn more completely, providing better cold-engine running.

Electrical or mechanical switches may be used to prevent or alter vacuum advance under certain conditions. Early emissions electronics would engage some in relation to oxygen sensor signals or activation of emissions related equipment. It was also common to prevent some or all of the vacuum advance in certain gears to prevent detonation due to lean-burning engines.

Most vehicle manufacturers specify that the vacuum line for the vacuum advance (if equipped) should be disconnected and plugged when adjusting the initial advance setting, which is usually specified in a workshop manual. Be careful when turning the distributor while the engine is running because deteriorated spark plug wires can deliver a painful shock.

Kentetsu

I think that a lot of the confusion comes from two things; setting this up for emissions reasons, and setting it up to smooth out a big cam. You could spend a week reading everyone's opinions on this topic, there is just so much out there.

Anyway, I would suggest that anybody who's curious about this whole discussion should just try it both ways and see what your car likes best. Mine showed definite improvement using the ported vacuum.

The only thing to keep in mind is that when you do switch between the two, you will also need to reset the timing and (probably) idle speed. And if switching to manifold vacuum, be sure to set the timing with the vacuum hose disconnected and plugged, otherwise you will be setting your static timing with vacuum advance "all in".

So, Purple83, I think that all we've proved so far is that there are widely varying opinions and theories about this topic scattered all over the interweb.

But having said that, I'm inclined to trust my own trial and error findings, since I have driven the car extensively using both methods. I know first hand which one works the best, at least for my car.

Sgt Fox

iTrader: (3)

As does mine. 12A with dual weber 36DCD and stock vac solenoid system.

Carl

iTrader: (2)

Now that you guys got that settled, I have another wrinkle. I just went down and checked the vacuum at the front two ports of my carb. On the farthest one forward, I have >50 cm Hg at idle. Opening the throttle, it drops off immediately to about 45, then slowly drops as the throttle is opened wider. At the second port from the front I have ~0 cm Hg at idle and then it hops up to about 45 as soon as the throttle is opened then continues to drop with additional throttle opening.

This seems to be the reverse of what others have described. I have an '84, but just replaced the carb and don't know the year. Maybe the ports are different on different years. Either way, I don't really think it makes too much different which way it is done, because the vacuum appears to be the same as soon as the throttle is opened slightly. Did I miss something here?

Interesting that this came up just now, as I was autocrossing yesterday and had lots of problems with part throttle. I was thinking that this might be because of having the vacuum advance totally disconnected. I had done that for road racing, since it doesn't come into play really in that case. I will find out in a couple of weekends if that assumption is correct. Autocrossing may be a different animal, since there are is a lot more time at part throttle. I'll try it hooked up next time.

Carl

Carl

iTrader: (2)

An addition. I just went down and pulled the original carb from my 84 out of storage and checked the ports on the carb itself, not on the block below the carb. There are two ports just to the rear of the idle screw. The front most is manifold vacuum (connected to the manifold with the throttle plate closed. The rear most is ported (connected to a small hole just above the throttle plate, which sees the manifold vaccum as soon as the throttle plate opens.

Carl

purple82

The first one is manifold, the second is port, they're working exactly as they should. Try them both, but you should be able to run more lean at idle off of manifold than port. Otherwise they should work exactly the same, as I said, they are essentially the same signal once the throttle opens.

arghx

iTrader: (3)

Have you guys seen this 12A ignition advance map out of the service manual? I don't remember what year 12A it's from, I found that page somewhere and dumped it to an image file a while back.



As I've been saying, timing advances with rpm. It also advances with vacuum. It advances with vacuum because vacuum is inversely related to engine load--the more vacuum you have, the less load the engine is under. For all the other situations Mazda had to install some solenoids to affect vacuum-based timing for improved emissions or driveability. The vacuum advance solenoids are just a bandaid because there is no true electronic spark advance. Electronic spark advance, which even the GSL-SE didn't have, did away with all that complicated crap. The ECU can just command the exact ignition timing needed for the driving condition--including knock retard, warm up retard, retard when an AT is shifting, etc.

j9fd3s

iTrader: (3)

yep, exactly. they turn vacuum off to the advances at zero throttle (i tested an 83, i think with 1 solenoid for both advances).

so at idle timing is TDC (5ATDC for 13B)

and on decel its max retard, like the rx8

mazda was nice enough to print all the distributor curves in the FSMs, it does change slightly over the years.

Carl

iTrader: (2)

The guy writing for National T-Bucket Alliance has essentially no clue. Every GM distributor I have worked on functions the same as ours. In addition, his comments about vacuum advance retarding the timing is incorrect (at least phrased in a confusing manner).

Vacuum advance is active (meaning it is advancing the timing) at closed or partly opened throttle. As someone showed later in this thread, the advance curves for our distributors show increasing vacuum advance with increasing vacuum. The vacuum is maximum when the throttle is closed for the manifold port and when the throttle is cracked open for the ported vacuum port. After the throttle is cracked open the vacuum gradually decreases with larger throttle opening. At some point (depending on the specific vacuum cannister), the vacuum advance will be zero, because the vacuum is insufficient to move the diapragm in the cannister.

Vacuum advance can be active at high rpm as long as the throttle valves are not wide open and there is enough manifold vacuum to move the cannister diapragm.

I don't believe there is any performance difference between ported or manifold vacuum. It may have an effect on idle emissions or idle smoothness, but as soon as the throttle valves are cracked the vacuum (and thus advance) is exactly the same for each port. Look at the location of the ports in the carb and you will see that has to be the situation. Hook up a gauge and measure the vacuum and you will see the same results.

Based on input from various people (including me) it appears that the ports are different for different year carbs. As such, if you care whether or not you are using ported or manifold vacuum, it makes sense to actually measure the vacuum at the ports to see which one you want to use.

Carl