Vacum myths BUSTED

[Anonymous]

Having worked on carbs and old Fords 6's for 35 years i was puzzled by all the references to hooking up to old stlye distrib. Having several 2 cylinder motorcycles i have a tune up station with 2 vaccum gauges for balancing carbs. Now all vaccum advace units I have ever messed with always worked the same[ increased vac /advanced timing].Whether it comes with a mechanical advance as well as the vac really doesn't affect that part of the equasion. So this morning i hooked up 1 vac gauge to the intake and one up to the vac adv port of my autolite 2 bbl(center of carb base) It was exactly as i suspected
A---at idle intake was about13#(big cam--stock should be around 17-18 or better)
---at idle ported was 0#
so at idle the intake would be advancing the timing
and the port would have no effect
B --rev it up and intake drops to zero and ported goes to 20-25(fast)
thus with intake vac, timing would retard and port would advance
C--this is the exact same effect that the older 1bbl with "load-o-matic" has.
Therefore when mixing carbs to dizzy's a vaccumn gauge and a piece of hose will allow you to find the 'ported source' for the SCV. The later dizzies have a mechanical(centrifical) advance as well as a vac. so disconnecting the vac completely will work better than useing intake vac. but most any carb designed to work with a dizzy equipped with a vac adv. will have a ported vac source- just because it is not in the top of the venturi doesn't mean it doesn't work off air horn velocity the mechanical is most usefull to overcome low velocity due to cold engine(closed choke) and too large a carb also accelerator pump(lots of gas but reduced venturi velocity in all cases). Proper vac to dizzy will allow much more total advance without running into overadvanced proplems at low rpm--such as hard starting--and raised combustion chamber temp problems such as pinging, run on(dieseling) holes burnt in piston crowns etc. with dual vac dizzies intake vac is used to retard vac quicker when decelerating for a little better emissions result. This is done by simply hooking up to the other side of the vac diaphram.
Just out of curiosity is spell check available when typing on this forum I know carbs --but computers i'm just starting to get a handle on.

 

[wallaka]

Hmm..that's some interesting findings. Glad to see some experimentation with this. So it just would take a vacuum gauge to find which source is which right? Just see when it rises or falls.

I don't believe there is spellcheck in the forum. If it's not too much inconvenience for you, you could write this in Word or whatever you use and paste it over after spellchecking.

 

[Anonymous]

So it just would take a vacuum gauge to find which source is which right? Just see when it rises or falls.

Yep that's it-- 0 vac at idle and vac increasing with rpm, is ported for advance.

you could write this in Word or whatever you use and paste it over after spellchecking.

more work than proof reading

 

[Anonymous]

Having worked on carbs and old Fords 6's for 35 years, I was puzzled by all the references to hooking up to old style distributor. Having several 2-cylinder motorcycles, I have a tune up station with 2 vacuum gauges for balancing carbs. Now all vacuum advance units I have ever messed with always worked the same[increased vac /advanced timing]. Whether it comes with a mechanical advance as well as the vac really doesn't affect that part of the equation.

So this morning I hooked up 1 vac gauge to the intake and one up to the vac adv. port of my autolite 2 bbl (center of carb base) it was exactly as I suspected
A---at idle intake was about13#(big cam--stock should be around 17-18 or better)
---At idle ported was 0#
So at idle the intake would be advancing the timing
And the port would have no effect
B --rev it up and intake drops to zero and ported goes to 20-25(fast)
Thus with intake vac, timing would retard and port would advance
C--this is the exact same effect that the older 1bbl with "load-o-matic" has.
Therefore when mixing carbs to dizzies, a vacuums gauge and a piece of hose will allow you to find the 'ported source' for the SCV. The later dizzies have a mechanical (centrifugal) advance as well as a vac. so disconnecting the vac completely will work better than using intake vac. but most any carb designed to work with a dizzy equipped with a vac adv. will have a ported vac source. Just because it is not in the top of the venturi doesn't mean it doesn't work off air horn velocity. The mechanical is most useful to overcome low velocity due to cold engine (closed choke) and too large a carb also accelerator pump (lots of gas but reduced venturi velocity in all cases). Proper vac to dizzy will allow much more total advance without running into overadvanced problems at low rpm--such as hard starting--and raised combustion chamber temp problems such as pinging, run on (dieseling) holes burnt in piston crowns etc. with dual vac dizzies. Intake vac is used to retard vac quicker when decelerating for a little better emissions result. Simply hooking up to the other side of the vac diaphragm does this.
Just out of curiosity is spell check available when typing on this forum? I know carbs --but computers I’m just starting to get a handle on.

I did the work for you.

 

[Anonymous]

As far as I know all dizzys have a vaccum and mechanical advance, the vaccum advance advances the timing based on engine load, and the mechanical advance works off of RPM.

 

[simon]

Not all of them... read the sticky about the pre- '68 load-o-matic

 

[jamyers]

I must be missing something on Fords...

On every other engine I've ever worked on, the only difference between manifold and ported vacuum is that the ported isn't in effect at idle. Once the throttle is opened enough to expost the ported vacuum port, they are exactly the same.

The only reason GM went to ported vacuum was to "retard" (actually just not advance) the timing at idle - to raise the combustion temps and reduce emissions at idle. Otherwise there's no difference.

Any kind of performance application under those definitions should be using straight manifold vacuum on the distributor advance.

Is Ford's definition of ported or manifold vacuum different? Why do I see so many questions on this topic?

 

[Anonymous]

As you mentioned James, when the throttle plate is open, there is no difference between proted and manifold vac or Ford & GM on the vacuum issue. The confusion comes in on the Load-a-matic distributor and the Spark Control Valve.

Danwagon, If you rev the engine in neutral or park, the manifold vac will increase just as the ported source vacuum does. There engine is under very light load at this condition, so it builds vacuum very rapidly. With the car on the road, it cannot build the vacuum that rapidly and with the throttle openned up, both vacuum and manifold vac will remain much lower for a longer period until the engine rpm builds up to generate more vacuum even with the throttle plates patially open.

Ported vac and venturi vac are not the same. Venturi vac will be low at idle and increase with engine speed. The Load-a-matic is calibrated to operate off a combination of venturi vac and ported vac/manifold vac.
The Spark control valve determines which source is being used under what vacuum load/throttle plate setting.

One important thing to remember when swapping distributors etc is that the vacuum diaphram motors are rated at different settings depending on the OEM application. The vac motors are calibrated to start advancing at a certain threshold of vacuum. It will be fully advanced at another level of vacuum.

 

[Anonymous]

when a carb is opened up mainifold vacumn drops drastically as the vacumn is now sucking gas down the intake, the carb becomes a major vacumn leak. Hook a vac gauge to the manifold, open the throttle and vac drops to zero. This is just a fact. Vacumn windshield wipers slow down everytime you step on it or climb a hill and go WFO everytime you go down a hill with your foot off the gas. Under steady cruising on fairly level ground manifold vac will build back up some, greatly aided by the car's inertia. Best MPG is done with most manifold vac. as this means little air is traveling through the venturi sucking gas. A proper tuned (non progressive individually manifolded)multi carb set up is one where all carbs share a near identical manifold vac. reading at all times ( a large slow turning twin with offset crankpins will actually show the engine strokes and the vac pulses will trade back and forth, here you use a valve to squeese down the vac. in the hoses to the gauges to help adjust things). Idle mix is done to achive highest reading at idle and throttle adjust. will have all dropping to zero together with WOT. Reasonably this means Manifold vac has zero benefit to SCV when you stomp it as there exists no available vac to advance timing. And if you ever had a bad SCV you know how necessary advanceing the timing is to achive acceleration. The venturi source on older1bbl's work just like the pickup tube of a solvent cleaner or the old trick of blowing across a straw with the other end in water blow hard enough and water will come out the end- this is exactly how a jetted carb passage works also.

The Load-a-matic is calibrated to operate off a combination of venturi vac and ported vac/manifold vac.
The Spark control valve determines which source is being used under what vacuum load/throttle plate setting.

The 'Load-a-matic' has 1 vac. line which works off the venturi port. There is no vac from the manifold to the dizzy. The only dizzys that use manifold vac., use it to retard timing under a HIGH vacum situation which is at idle and deceleration. These are the dual vacum SCV's found on many heavily emmision controlled applications. This was done to overcome the mechanical advance uinder deceleration.

Danwagon, If you rev the engine in neutral or park, the manifold vac will increase just as the ported source vacuum does. There engine is under very light load at this condition, so it builds vacuum very rapidly.

Actually Manifold vac. drops to zero immediately if you do this while ported and venturi vac. rises quickly. You cannot have high intake manifold vacum and a wot plate at the same time unless your choke is closed or air cleaner is clogged as the vacum just got used to suck gas into the engine.

 

[Anonymous]

I agree with most of what you said. There is no argument that the manifold vac drops at WOT. In the driveway, I don’t run an engine at WOT. I might blip it open and shut. When you rev an engine in the driveway and take vacuum readings, you typically are not at WOT for any longer than the blip. The point is that Ported Vacuum readings also drop at WOT.
" B --rev it up and intake drops to zero and ported goes to 20-25(fast)
thus with intake vac, timing would retard and port would advance "

If you are getting high ported vacuum readings at WOT, under load, then it is not a vacuum or ported source but rather a venturi source.

The Load-a-matic does just have one line to it, but due to the SCV and the carb’s internal passages, manifold vac is allowed to assist the venturi vacuum. (see the SCV sticky post at the top of this forum).

And just for the record, many older pre-emission cars did use manifold vac to advance the spark. Ported vac sources became more common in the 60’s to lower emissions and have been the standard for the last 40 years. [/quote]

 

[Anonymous]

Okay I am old enough to realise that what seems obvious is not always so. This morning I went out to the garage where i have an extensive collection of carbs. Some too good to chuck. some for spare parts some just because. I have 8-10 Autolite 2bbl's as buying these in a junk condition is probally the easiest way to get spare jets. Took 3 of them of different sizes poured a little ATF down the ported vacumn port added a little compressed air-- the other end of that port is a small hole just below the throttle plate shaft. Took stock carb from '78 200 fairmont --same thing. A pair of Carter RBS's woops no vac port- these came off a twin carb(non progressive) 4 cylinder boat engine--went and looked at boat dizzy--no vac adv. The port is sealed off BUT the hole is still there in the venturi.
--Next step added a TEE to the vac line going to the fuel pump (62 pump) put another one in the vac advance hose layed a vac gauge on the console hooked up to the vac adv, took a spin. 0 vac at idle ,the heavier the load the more vac.(typically 40# cruising, 50# under load, 0 at decel) stopped switched hose to the other TEE (man. Vac) idle-14#, cruising with little throttle 40#, under load decreasing vac with near zero under heavy load 50# under decel.Tees when not in use were capped off..(hey gave me something to do on the way to work)
--This leads me to believe that the main reason for the advent of the centif. adv. is loss of venturi velocity due to larger carbs (especially 4 bbl's, open those back bbl's and velocity in the front 2 bbl's is gonna be way less then needed) The bigger the carb the smaller the velocity. This would also explain why multi carb motor cycle engines never (that I am aware of) used vac in the timing.
--Now i did not visit my GM shed (I keep those parts segregated to advoid contamination of my Ford stuff) this AM to look at Quadrajets, Edelbrock sq. bores or rochester 2bbl's- but will be extremely surprised if they differ.
--Does this mean that any ported source is perfect for a Load-a-matic NO! does this mean that a ported source is much more likely to return favorable results- ABSOLUTELY. Is man. vac. a good source for ign. adv --i wouldn't think so as it will not advance under load but will adv under decel and at idle although idle vac is maybe not high enough to cause full adv.
Unforunately i never put the 144 to this test so I do not have any real time numbers to compare to. Maybe if i get time later i will go out and pull the carb off of it and see if i can see the realitive position of the ports in it's venturi. Can't crank it as it's on a stand with some of it's componets bolted to the 200. If someone with a stock carbed Load-a-matic was to run a vac test like this I would very interested in their findings. A shop type vac gauge is cheap enough and well worth haveing--I was schooled in 1969 as a mechanic and we were taught that a vac gauge was THE tool in achieveing FINE tuneing.

 

[jamyers]

As I'm reading this, it's apparent that we need to clarify some terms. Here are the meanings I'm assigning to the following terms:

Manifold Vacuum: vacuum source taken from any point below the idle position of the throttle blade(s). This is the absolute manifold pressure that the engine is seeing, and when driving this varies with engine load. This will be lowest at WOT (high load), higher at closed throttle idle (no load), and highest at high rpm and closed throttle (negative load).

Ported Vacuum: vacuum source taken from a point just above the idle position of the throttle blade(s). At idle, this will be 0, and at any throttle position above idle will be the same as Manifold Vacuum.

Venturi Vacuum: vacuum source taken from just inside or partially below the venturi restriction above the throttle blade(s). At idle this will be 0, at other throttle positions it will vary (increase) with the amount of air passing through the carb. The overall trend is that it will increase with engine speed.

Given the above, I can see where the SCV / Load-O-Matic system causes confusion, as people swap the term Venturi for Ported or Manifold.

Remember, All systems use Manifold/Ported Vacuum to advance the spark under low load conditions, and as load increases the Manifold Vacuum goes away and with it that advance.

Non-Ford SCV/LoM systems use centrifugal weights in the dizzy to advance the spark as rpm increases.

The Ford SCV / LoM system doesn't use centrifugal weights in the dizzy to advance the spark relative to rpm. Instead, Ford used a SCV to switch from Manifold/Ported Vacuum to Venturi Vacuum, whichever is highest.

That's what makes the SCV/LoM different, the way it senses the engine load (hance the name) relative to engine speed. Probably should have been called the "Speed-O-Matic" instead, it would have been more accurate, because any engine with a Manifold/Ported vacuum advance is by definition a load-sensing device - so they're ALL "Load-o-Matics"!

 

[Anonymous]

Next step added a TEE to the vac line going to the fuel pump (62 pump) put another one in the vac advance hose layed a vac gauge on the console hooked up to the vac adv, took a spin. 0 vac at idle ,the heavier the load the more vac.(typically 40# cruising, 50# under load, 0 at decel) stopped switched hose to the other TEE (man. Vac) idle-14#, cruising with little throttle 40#, under load decreasing vac with near zero under heavy load 50# under decel.

Ported Vacuum: vacuum source taken from a point just above the idle position of the throttle blade(s). At idle, this will be 0, and at any throttle position above idle will be the same as Manifold Vacuum.

This test was done with an autolite 108 2bbl a V-8 carb -Ported Vacum source is directly below the throttle plate effectively cut off from venturi effect at idle -but when throttle is open directly in the heaviest flow area of the carb. now whether you call this a "ported" or a "venturi" effect is of little consequence vac readings going down the road were not exactly the same as manifold readings.
A carb is a venturi with plumbing attached- a port is a passageway.

 

[jamyers]

If you'll look at one of your Quadrajet carbs, you'll see two openings for vacuum passages to draw from, one Ported and the other Manifold. The Manifold vacuum (I think it's the hose fitting on the passenger-side) is just below the throttle plate at idle. The Ported vacuum (hose fitting on the driver-side) is just above the throttle plate at idle. Those are the only two sources of vacuum for that type of spark advance system.

Look at a Ford carb with an SCV, and you'll see a vacuum opening just above or below the throttle plate at idle (Ported or Manifold vacuum source), and another tiny opening in the side of the venturi ring (smallest part of the bore). That's where the system senses Venturi vacuum, which is related to engine speed. More rpm = more flow = more vacuum signal at the venturi.

All the Load-O-Matic system does is use Venturi vacuum to sense engine speed instead of centrifugal weights in the dizzy. The whole goal of any decent ignition advance system is:

* Advance at idle (manifold advance, but not venturi or centrifugal advance)
* No advance at low rpm and WOT (no manifold, venturi, or centrifugal advance)
* Increasing advance as rpm climbs (no manifold, but venturi or centrifugal advance)
* Increasing advance as load decreases (manifold advance, with or without venturi or centrifugal advance depending on rpm)

This is ignoring the static (initial) timing setting, and lumping ported vacuum in with manifold vacuum.

 

[cfmustang]

Man,

Last Summer when I was having all my trouble, I finallay thought i had this figured out but my memory is fading fast.

I could have sworn there was a post on here somwhere where somebody had a graph with the ported, manifold and venturi vacuum plotted on it. Seeing it made it all clear instantly. I did some searching looking for the post and couldn't find it.

Anyone remember the post I am talking about or have I killed too many brain-cells over the years?