can you school me about vacuum dist advance ?

[Nacho-RT74]

when should it work and how much ?

[Chryco Psycho]

It should work with RPM , higher air speed through the carb will create vacuum so there should be no vacuum at idle but it should rise with air flow through the carb .

[Musicman]

Operates off engine vacuum regardless of RPM.
Should be connected directly to manifold vacuum... ported vacuum is an emissions gimmick held over from the 60's.
If your car is driven primarily on the streets and highways, you should have one.

[A383Wing]

this might help also... http://www.setyourtiming.com/

our cars have the dist advance off the ported side of the carb

Bryan

[Musicman]

The whole point of a vacuum advance system is to advance the timing of the engine under light or no load (idle) conditions to improve performance and fuel economy. The lighter the engine load, the greater the advance... the greater the engine load, the less the advance. Not just some of the time... all of the time. Ported vacuum was/is an emissions gimmick from the 60's which is why "our cars" used it.

 

[Chryco Psycho]

Everytime I have hooked it to manifold vacuum I have heating problems , the vacuum is advanced at idle & retards with rpm eliminating the curve & going backwards from the mech advance so I typically leave the vacuum advance disconnected & just create a mechanical curve that works for the engine typically 12-20* at idle with 32-38* total

[A383Wing]

yea..I have tried to connect the dist up to manifold vacuum...it creates more problems than it cures...it won't work on a stock engine with stock dist.

Bryan

[Musicman]

What a load of crap 

It works on both of mine... if it doesn't work on yours then you need to start over and tune it correctly  The only change you should see when swapping from ported vacuum to manifold vacuum is a slight increase in engine speed due to a more efficient burn. In fact Chrysler use to use this trick with A/C cars using a vacuum switch. The result was a slight increase in engine speed which bumped up the speed of the water pump for better cooling, and a more efficient burn which lead to a cooler running engine.

Your old, but I still love you Bryan 

[Paul G]

I have never got manifold vacuum to work right on a Mopar. Seems like it would ping terribly at cruise speeds. I didnt want to recurve the dizzy to try to make it work. I just dont use it at all. I know FBO calibrates there Dizzy's to use manifold vacuum advance on Mopars.

Thinking through it, the mechanical advance would have to be held out longer, and limited sooner to allow for the vacuum advance you would get at part throttle/idle when using manifold vacuum. 

Without any vacuum advance, mechanical advance comes in gradually till 2000 RPM when it is all in for 36° total. Simple.

With ported vacuum, you get vacuum, varying amounts, at any throttle opening, just no vacuum at idle.
With manifold vacuum, you get full vacuum at idle, no vacuum at throttle opening, full vacuum at cruise, no vacuum at wide open throttle. Right or wrong?

To add in some vacuum advance, be it ported or manifold, the mechanical advance would have to be reduced.  Right or wrong? Then, at full throttle you would have no vacuum advance, and not be at 36° total. Just doesnt make sense to use it.

When using a stock distributer, stock built engine, you should use stock type ported vacuum. Thats what the dizzy is set up for. I would say you can not just plug the dizzy in to manifold vacuum without recurving the dizzy to work with it. Right or wrong?

Am I on the right track?  Or all screwed up?

[Musicman]

The ONLY difference between manifold vacuum and ported vacuum is the advance at idle... that's it!
It doesn't matter if you run your vacuum advance off of manifold vac or the ported vac, that's a personal decision. It only matters to the tree hugg'n emissions guy when the car is idling, everything else stays exactly the same. The moment you crack open the throttle and come off idle on a ported vacuum system it sees and operates off manifold vacuum. Criusing down the highway, your operating off manifold vacuum... around town, your operating off manifold vacuum. It doesn't require any special dizzy adjustments to make it work. If your engine has been timed "correctly", and your idle circuit tuning has been set up "correctly", the only difference will be a slight increase in rpm at idle, which means you back off on the curb idle screw (less gas) to compensate.

It's a VACUUM ADVANCE system... it operates on engine vacuum... it's not magic and it's certainly not rocket science. Your listening to 50 years of Dogma rather looking at it for what it actually is and was originally designed to do.

Bottom line... who cares... Mopar used ported vac to help beat emissions, that's the plain and simple truth of it... but it's fine either way.

Rig for silent running...

[willard]

its easy to verify : hook a vac guage to manifold vacuum, make a test drive, switch to ported, test drive. dizzy disconnected. then youll see :-)

[John_Kunkel]

Ported vacuum source was around long before emissions.

[Musicman]

This wouldn't fit on one page so I had to use 2...
Post written by a former GM engineer on another forum... take it or leave it... Part #1

As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

[Musicman]

Part #2

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren't fully-deployed until they see about 15" Hg. Manifold vacuum, so those cans don't work very well on a modified engine; with less than 15" Hg. at a rough idle, the stock can will "dither" in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15" Hg. of vacuum at idle need a vacuum advance can that's fully-deployed at least 1", preferably 2" of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8" of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don't understand it, they're on commission, and they want to sell "race car" parts.

[Musicman]

Typical vacuum switch seen on many Mopars... guess wht it does?

http://automotivemileposts.com/autobrevity/distributorcontrolvalve.html

[A383Wing]

When using a stock distributer, stock built engine, you should use stock type ported vacuum. Thats what the dizzy is set up for. I would say you can not just plug the dizzy in to manifold vacuum without recurving the dizzy to work with it. Right or wrong?

Am I on the right track?   Or all screwed up?

Ported vacuum source was around long before emissions.

agree to both posts...

GM mostly used manifold vacuum, Chrysler used ported vacuum, and Ford used both actually, manifold vacuum when cold, ported vacuum when warm...

I tried experimenting with my Charger and my mid 60's GM cars I had back when...my Charger would never run right by switching over to manifold vacuum. And my GM cars that ran manifold vacuum would never run right by switching over to ported vacuum. Also, running manifold vacuum increases timing at idle and raises HC's out the tailpipe more than running ported vacuum...so it's doing more harm than good

Bryan

[firefighter3931]

From my experience ; trying to tune vacuum advance with anything but a stock engine is useless. The vac advance cannisters are designed for engines that make 15hg (or more) of vacuum at idle. 

Once you install a bigger cam the idle vacuum is reduced and all the associated tuning issues rear their ugly head.   


Ron

[Musicman]

That's correct Ron!... although I wouldn't limit this catagory to stock!

As for the rest, I am reminded of a classical animal psychology experiment by Harry Harlow here .... The tale of the five monkeys and the hanging banana.

In Harlow's experiment, five monkeys were put into a regular monkey cage, with a banana hanging high on a rope from the roof of the cage (outside the reach of the monkeys). The researcher then put a step ladder enabling the monkeys to reach the banana. However, whenever one of the monkeys attempted to climb and reach for the banana, ALL monkeys were sprayed with freezing ice cold water. After few attempts, they all learned the association between reaching for the banana and the group collective punishment of being sprayed with freezing ice cold water. If they want to stay warm and dry, they better not reach for the step ladder. From now on, none of the five monkeys tried to reach for the banana anymore. There was no need for the water treatment from that point on.

At this stage the researcher replaced one of the five monkeys with a new monkey. The new monkey, not aware of the icy water treatment, tried to reach for the banana. Within a fraction of a second the other four monkeys pounced on him and beat the hell out of him – again and again, till he stopped and did not try anymore. Note, that icy water treatment was not used anymore. The same process was repeated, one of the four monkeys who experienced the original icy water treatment was replaced by a new one, and again all the monkeys beat the new monkey to submission. Finally, the cage was populated by five monkeys of whom none have experienced the icy water treatment. The experimenter then introduced a new monkey to the cage. When this monkey tried to reach for the banana, all five monkeys jumped on him and beat the hell out of him. None of these monkeys knew about the collective punishment of icy water, none knew why they are not allowed to get the banana, but somewhere along the way they learned that reaching for the banana is not allowed. They became the guardians of this rule without knowing its purpose.

It's done that way because that's the way it's always been done. Although I'm not really sure why?

[A383Wing]

so stop playing with yer banana!

[Chryco Psycho]

The problem with manifold vacuum is it is highest at idle as you open the throttle the vacuum drops off , @ WOT you should have close to no vacuum advance so it work opposite to mechanical the mechanical system is trying to advance timng to give the burn more lead time while the vacuum advance is retarding timing .
I just leave the vacuum disconnected on performance engine & work with the mechanical advance to get the needed advance * total & the ramp up correct .

[Musicman]

so stop playing with yer banana!

[Musicman]

I just leave the vacuum disconnected on performance engine & work with the mechanical advance to get the needed advance * total & the ramp up correct .

Vacuum Advance is used on builds that produce a good vacuum signal. Certainly not all, but many high performance builds do not generate enough vacuum for a Vacuum Advance system to operate correctly, consequently generating a number of unwanted issues. This was stated somewhere earlier...

My 505-6Pk pulls 20" of vacuum at idle, so vacuum advance is a no brainer.



Excuse me, I have to go look for my Banana now 

[Paul G]

I like the monkey scenario.   They are more like humans than we know.

[firefighter3931]

That's correct Ron!... although I wouldn't limit this catagory to stock!

You are correct Mike....i should have stated any build that makes better than 15in at idle can be made to work with vacuum advance. Your 500in motor with the mild VooDoo cam is a perfect example of a modified engine that fits those parameters 

Now....about those Monkeys 



Ron

[440]

I can see it depending on which way the vac pot pulls the breaker plate whether it would require ported or manifold vaccum.