Technical Ported vacuum or manifold vacuum to Dist?

I have always connected the vacuum advance on GM distributors to manifold vacuum ( below the throttle plates), but in helping a friend, I ran into a situation where it is connected to a ported source on the metering block (Holley). I ran this by some people I have known for a long time and that deal with carbs and engines for a living ; they said they always use ported vacuum.
Something seems wrong here. What am I missing?

 

Manifold vacuum is correct in my opinion. I just installed a MSD timing box on my flathead and had to add a manifold vacuum port (below the carb's butterfly's) to send the correct signal to the timing box.

I do have an OT Trans Am (thus the moniker BanditBilly) that had the advance on a ported outlet on the quadrajet. Those cars were running a lot of pollution control equipment, exhaust gas recirculation, etc. There must have been 100 feet of vacuum line under the hood. I said were cause there isn't now and the advance is plumbed to the manifold.

I don't run advances on my 396 car, the Olds or my afore mentioned flathead.

I'm sure squirrel will have more accurate information soon as he logs on. I'd trust what he says over what I think.

 

Yep, manifold vacuum. 'Ported' vacuum is almost always used for emissions reasons.

 

I'd say that on Flat Heads and early OHV V8s manifold vacuum is the correct way but if I'm remembering right somewhere around 1963 or so some manufacturers went to ported vacuum. As usual my memory isn't what I think it is. Joe

 

My (copyright 1950) Motors manual, in discussing carburetor theory says ported vacuum (above throttle plates) is used to allow for a steady idle.

So obviously it isn't some sort of an emissions atrocity as alleged. It's likely been around as long as carburetors have.

 

Ported is not "just" used for emissions. While it was originally used that way, it can be a very good tuning aid.
Best to give all three vacuum ways a try.
1. Ported - Provides no vacuum at idle. Adds vacuum as the throttle is opened. Ignition timing is power and milage. When you open the throttle, you get all the timing you adjust the system to. This is good for milage and power.

2. Unported (or full vacuum) - The old way... You get full vacuum/timing at idle. This is somewhat usless...unless you sit at a LOT of red lights. As the throttle is opened, the timing goes "down" along with the milage and power...again, mostly usless.

3. No vacuum - Just as it says, no vacuum. However you set the crank timing, along with the mechanical timing, this is what you have. A little easier to control, no vacuum canister to go bad and cause a vacuum leak....

For all three of these methods, you need to experiment with the crank timing to see what the engine likes best. DO NOT go by what "you think" the engine needs, or what someone told you. Adjust to what the "engine" wants. If you pay attention, it will tell you in the seat of the pants feel, and the milage. You will have to experiment a bit. A trip to a chassis dyno will get this done much faster than driving around town. This way will take you weeks..!

My choices are #2, then #3, then #1. Again...this depends on what the engine wants to run it's best.

Mike

 

I am not sure when Buick started it, but my 1950 inline eight uses ported vacuum. I believe all inline eights , at least back to late '30s , did.

Ben

 

Not quite right.... As stated, ported vacuum increases with throttle opening, but doesn't take into consideration engine load. Too much timing, and the engine will knock. At 'cruising' speed (small throttle opening), you won't have as much timing as you need for efficient operation, and you can have too much timing at WOT.

Unported/full/manifold vacuum is the opposite. It has the most vacuum at small throttle openings, going down with throttle opening when accelerating or under load. This prevents knocking under load, but allows more timing under part-throttle conditions. If you have a motor that likes a maximum of say 34 degrees timing at WOT (where the vacuum is adding basically nothing to the timing), the vacuum can add up to 16 degrees of additional timing (for a total of 50 degrees) under light load/small throttle opening conditions. This can improve mileage up to 50% over a purely mechanical advance set-up (no vacuum) as well as making the engine run cleaner.

Now, ported vacuum has been used on non-emission ignition systems (the Ford 'Load-a-matic' for example) but these generally used both types of vacuum and had no mechanical advance. How/where it was ported made a difference too. By the mid-fifties, these types were pretty much gone. Ported vacuum came back when emissions started to be an issue...

 

Wouldn't consider manifold vacuum useless. Always how I connect it. Does wonders for idle quality not the opposite. Especially nice if a little overcammed for your converter with an automatic.

 

Ok, if ported, the vacuum isn't present at idle but increases as more air flows through the carb, then the timing added by the vacuum advance adds to the mechanical advance; right.
I just got back from messing with the problem. As always, you guys have some good input. With the advance hooked to manifold vac. it adds about 17 deg to the total timing of about 34 deg. the vac advance unit is marked for 15 deg. For sure we need either a weaker vac signal or a vac advance unit with less travel. Checking with a vacuum guage at the ported connection, the signal begins at about 2400 rpm and the mech advance is all in at about 3300rpm ( 34 deg total, initial plus mechcanical).

 

C9 did Avery good thread on this.. the thread title was A reprise on "Got Time"

 

Remember, if you're checking total advance NOT under load, you'll see all of the vacuum advance. Try driving the car; if it knocks under partial-throttle conditions, then address the vacuum advance. 17 degrees vacuum advance at no-load is well within the possible advance curve.

 

That's how I feel ; this engine is not overcammed, but the compression is shaved slightly to run pump premium with no worries. 8 deg initial + 20 deg mech + 10 deg vac (max) might do the trick. BTW this is a restored Correct 425 hp 396. We want to use all of the pieces we have correctly, if possible.

 

Blowers seem happy with it too. Never tried it until recently, ran it to intake with power valve and only bitch I have is that the idle is dead smooth. Gets 15 M.P.G. even with the carb a little on the rich side. Cooler at lights as well.

 

agree 98% but you may only get 10-20% increase in economy..... other than that changing things means trying and checking, one modified engine is not the same as another, try ported try manifold see what works, try an adjustable vac canister, fit a Vacuum gauge so you know when you have high or no vac. Seems many really don't understand what's going on inside their engines. Vac advance is good , no reason not to run it on a street car. Remember vac advance is ON TOP of max centrifugal advance its ONLY FOR economy (but I read it can make idle nicer on some engines, I'm excluding stupid pollution engines and how their timing works )
SO......the above example , 8 +20 plus 10 vac gives you 28 at WOT which don't sound any good to me , plus you'll only have 38 at cruise, you could start at 8 + 25 ish plus the 10 vac , then try a bit more static, then try something else , till you get to where its best, or pay for a dyno tune by someone who knows what they are doing.

 

As soon as the throttle is opened a little bit, ported and un-ported vacuum are the same (manifold vacuum).
Here is a description from the 1958 Edsel factory shop manual.

 

From 8 mpg to 12 is a 50% increase... LOL. That's what I saw on a FE when I pulled the mechanical-only dual-point distributor out and replaced it with a conventional unit.

 

That can be true, but isn't always true; it depends on where the port is located in the carb. Manifold vacuum is always manifold vacuum, but 'ported' vacuum can be many different things. In the example you're giving, the port is on the side of the venturi opening where the throttle plate goes up/away from the port, effectively putting the port 'below' the throttle blade when open. Move the port 180 degrees so the blade goes down/away and the port stays 'above' the blade, now it's 'ported' vacuum and will react opposite.

 

As I read the post by Dan, I thought about the load thing and re- read the whole thread. I'm wondering if having a vacuum gauge inside the car while driving would give definitive results or just seat of the pants is the best indicator. For this case, the ported connection would match the factory setup.
Thanks for all the input; I'll post results.

 

Some brands offered factory vacuum gauges.

The old man kept a pair hanging from the bottom of his (then current) Studebaker, and drove by them like a frikkin religion for economy. But then, he'd still go ahead and tailgate, curse everybody but his mother and drag race at the drop of a hat.

Ah, the memories!
Say, you do know that some distributors were designed to use ported vacuum - others manifold vac - right?

Below is a factory Pontiac gauge from the 60s.

 

Agree with Falcongeorge--vacuum gauge is an essential tool for tuning.

 

This ^

 

http://www.jalopyjournal.com/forum/threads/a-reprise-on-got-time.54221/

 

Use ported vacuum for the advance, manifold vacuum will give you advance at an idle and you don't want that

 

I agree about the vacuum gauge and tuning; in this case, it has to be temprary inside the car.. no permanent install. I haven't been able to check vac. under load (driving). Sitting still I get about 16 deg. advance @18" vac. from both manifold and the port on the metering block; the difference is the vac. doesn't start until about 2400 rpm. I have been thinking the ported should be a smaller amount. Something amiss inside the carb?

 

George, I won't disagree with you, but I will state that IMO manifold vacuum is usually the 'best' choice for a 'default' vacuum advance source. If using 'ported' vacuum, you really need to know how it works; how much vacuum and under what load/rpm you're getting it. I'm helping a neighbor kid replace his rebuilt-to-death factory Holley on his '85 351W Bronco, and that carb has 3 'ported' vacuum nipples on the carb, one on the carb spacer, and one at the manifold, all interconnected through a maze of hoses, valves, and vacuum switches. Even the new 'generic' Holley replacement has multiple connections, so you just can't connect to one at random.

 

The general answer to the basic question "Ported vacuum or manifold vacuum to Dist?" is "yes". Try both ways, see which works best with your car.

You can also play with the amount of advance in the mechanical advance mechanism, and the amount of vacuum advance.

Every car is different, that's why there are so many different distributors that were made by carmakers for each engine.

 

Actually, because of the lean mixture at idle, which takes longer to combust than a rich mixture, it is helpful to have the timing advanced at idle. As the throttle is opened and the accelerator pump adds fuel to the mixture the timing can be decreased. The intent is to provide max combustion pressure at the proper time in relation to the position of the piston, for best efficiency.

 

This ^^^^^^^^^^

 

What Dan Timberlake posted and what Falcongeorge has written is the way...Vacuum gage a must..