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Ported versus manifold vaccum.

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N

nomad

Yoda
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Ported vaccum versus manifold vaccum for distributor vaccum advance. Why does it make a difference? Is'nt vaccum going to be the same regardless of where its taken. I have a early 1098 carb setup on my BE 1275 1968 smog engine. The carbs don't have a barb for the ported vaccum so I am running off of a barb fitted to the manifold. Whats the difference going to be if any? Car runs great except for a prodidgous thirst. (high 20's) Carbs have been messed with before I got them.
KA.
 
It's been explained to me this way: Manifold vacuum is constant creating a continually advanced position on your distributor throwing off the timing. Ported: The vacuum increases with acceleration and advances the distributor accordingly. I'm at the point where I'm just not going to use the vacuum and have the car tuned to run without it. I don't have any vacuum fitting on the base of either SU carb. Not sure where else I can port it to.
 
Actually I think it's manifold vacuum varies greatly, at idle it's high, throttle plates closed, all the engine vacuum held behind the carb in the manifold, a vacuum line on the manifold sees a lot of vacuum and pulls on the vac advance to advance the distributor at low idle or deceleration. As soon as the throttle opens up the vacuum in the manifold drops off and the vac advance doesn't do anything. It's not doing anything most of the time.
I read it here article

It was in a thread here not long ago thread
 
Ported vacuum is taken above the throttle plates and manifold vacuum is taken below the throttle plates.
There is no vacuum at the line with ported until the throttle is opened.
Vacuum will vary as throttle is opened. This means less at wide open throttle.
To see how this works, think about the old vacuum wipers. when you stepped hard on the gas, no wipers. Less pedal pressure and the wipers worked well.
Some people prefer to set up the initial advance with vacuum and some without, thus the two means of obtaining vacuum.
I will not debate which is better, (personal preference) but I hope this explains what each does.

Dave :savewave:

P.S. This is true for both carbs and Fuel Injection
 
A manifold vacuum source will supply vacuum at idle, which can help smooth the idle. Manifold vacuum at idle will increase idle speed when applied, so you have to reset the idle speed. If your engine starts to idle irratically for whatever reason, the manifold vacuum will drop and idle speed will be affected more than when it is set up for ported vacuum.

Manifold vacuum drops away quickly when the throttle is opened, which can help prevent detonation during tip-in and when changing engine speed while cruising.

Ported vacuum is not present at idle. Ported vacuum does not drop away quite as quickly or quite as much as manifold vacuum when changing engine speeds.

Use the solution that works best for you. I like to limit total vacuum advance to 10 degrees if I can. Easy to do with a Ford dizz. I have not tried to adjust the Midget dizz yet.
 
Manifold vacuum goes away when you nail it.
Some distributors and advance mechnisms (General Misundertanding, for one) are designed for this, and work backwards of the norm.
To elucidate, when you load the engine and the manifold vacuum goes away, the distributor advances.

I have seen more clueless individuals hook up Fords to manifold and cannot get it to work right.

My Lucas is designed for ported.
That is also the way it is connected.

If your design is for ported, and you connect to manifold, it will ping like heck off the line, and you will need to back off, and off, and off the timing.

Another issue.

If you backfire, and are connected to manifold, there is a good chance you will pop the diaphram in the advance unit, especially if it is old.

BTDT.
 
A pretty good explanation of the differences on Wikipedia as well, https://en.wikipedia.org/wiki/Ignition_timing pretty much matches up with what TOC said. I understood that ported vacuum systems really came about as part of the rather clumsy first attempts at emissions control back in the 70s and was not designed for good performance, rather decreased emissions at the expense of the best running engine. Either way, the distributor needs to match the source being used. Maybe Jeff at Advanced can chime in and give us his take.
 
This thread has been very informative for me. Now I better understand why a pure mechanical advance distributor seemed to clear up my problems.
 
Legal Bill said:
This thread has been very informative for me. Now I better understand why a pure mechanical advance distributor seemed to clear up my problems.
Click to expand...

:iagree:
 
Makes me wish I'd grabbed the Sunnen distributor machine when I had the chance. :madder:
 
I have a Sun Distributor Machine.
I have set up and checked a lot of distributors over the years.

I could give you a dissertation on variations of vacuum advance designs.

From straight mechanical witha vacuum "brake", an adjstable pad to retard advance, to entire distributors that rotate with the advance can, to ported vacuum (Ford, Willys, Dumbcooks, Chryalser, and probably dozens of other names I can't think of at 0830), to manifold vacuum, to vacuum delays (and when one of those got a bit of crud in them, you would bomb down the road, get full advance, come to a stop, and it would not bleed off).

Then the Feds got involved and screwed it all up (look at California.....with their CARB-mandated emmissions controls, they use 10% more fuel for the number of vehicles, by last count), variable-venturi carburettors, computer controls, distributorless ignition......

aaaaaaaaaaarrrrrrrrgggggggggggghhhhhhhhhhhhhhhhhh!!!!!!!!!!!!!!!!!!
 
I can't remember exactly where the vac. adv. line attaches to the SUs I know it's close to the butterfly, is it on the manifold side so it's manifold vac, right? If it were on the carb side of the butterfly it'd be ported. Just curious now. On my old Opel GT I tore the Bosch dizzy apart and rigged up a way to limit the mechanical advance, which was pretty fun, different topic I know. I have a timing light with advance knob, I should try it and see what I'm getting at various rpms.
 
Depends.
Not having an SU in front of me, if the actual port is above the throttle plate, it's ported.

I have seen carbs with the line fitting into a port that goes into the carb, makes a 90, goes up (or down, or sideways) a bit, and then another 90.
You would have to check.

Bottom line is, whatever it was designed to do (advance can, advance curves, ported or manifold), unless you know what you're doing, leave it that way.
 
TOC said:
Manifold vacuum goes away when you nail it.
Some distributors and advance mechnisms (General Misundertanding, for one) are designed for this, and work backwards of the norm.
To elucidate, when you load the engine and the manifold vacuum goes away, the distributor advances.
Click to expand...

Ford dizzies that have a spark retard feature are those that have two hose nipples on the advance can. The inner nipple is for the retard. That's right, I wrote the word nipple. Many Ford engines from 1968 and later were set up so that ported vacuum was the normal source, but a temp valve in the t-stat housing would switch to manifold vacuum when the engine got above normal temps. This had nothing to do with emissions, rather it would increase idle speed and mechanical fan speed thus increasing air flow through the radiator.

Fun with hose diagrams.
 
That corvette article linked by ivandyke was incredibly informative. It's got me a little confused though. My (often faulty) memory seems to indicate that my 1970 B had a vacuum port on the carburetor (ported vacuum?), and I know for sure that my 1971 BGT has manifold vacuum (port is right on top of the manifold). This seems to be the reverse of the article which indicated that later cars had ported vacuum in order to deal with smog requirements (and eff up everything else). Is this wrong? Is the carburetor vacuum port on an earlier B below or above the throttle plate?

Another thought...my BGT has long suffered from overheating when idling for long periods, even with the cooling system in perfect shape. I wonder if perhaps my vacuum advance is inoperative and thus contributing to that problem. What's a quickie way to check?
 
I looked at my HS2 a little bit ago and it sure looks like the fitting is in front of (carb side) of the butterfly making it "ported" vacuum, I guess. It's right on top of the butterfly so hard to tell.
 
davester said:
Another thought...my BGT has long suffered from overheating when idling for long periods, even with the cooling system in perfect shape. I wonder if perhaps my vacuum advance is inoperative and thus contributing to that problem. What's a quickie way to check?
Click to expand...

if you throw a timing light on it and rev the engine, you should see the timing mark move
 
The two-port advance on Fords was ABSOLUTELY emissions related.
I worked for Ford when they came out, and was a state-certified emmissions specialist.

Ported vacuum advances on basically ALL Fords from 1949 until they went to the EEC system.

The retard was off a TCVS (Temperature Controlled Vacuum Switch) which operated diminished performance until warm.

BB's seemed to find their way into those hoses with regularity.
 
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