Vac Advance Port

[BobBrumby]

Could anyone please tell me what the vac advance port on the distrubuter actually does?

I was told I could just hook this up onto a certain port on my carb (i know wich one), so it does not go throught all the emission stuff that I wont to remove.

[GeneralDisorder]

It gives "ported" vacuum to the disty. This is basically a hole into the carb that is ABOVE the throttle plate. Thus it only receives vacuum when the throttle plate is open past the idle position. That was the vacuum advance on the disty doesn't come into play till the engine is running faster than idle. It is just fine to run a line directly from the "ported" vacuum port on the carb directly to the disty. It is NOT ok to run a full time vacuum line (from the manifold, etc) to it, as this will throw off your timing in big way.

 

GD

[subiemech85]

GD, YOU may be wrong "It is NOT ok to run a full time vacuum line (from the manifold, etc) to it, as this will throw off your timing in big way."

 

I chose to run vac line from dist to manifold, idle quality is much improved, runs better, and have had no problems

the vac advance provides about 20*

 

you must choose where to connect the vac line depending on your needs

 

the two options on the carb are: 1. little vac at idle, with vac increasing as does engine speed

or 2. vac decrease as engine speed increase

[BobBrumby]

Which kind of port is the distr. usually plugged into 1. or 2.?

 

What exactly is vacuum advance, does it make it spin faster when vac`ed? or does create less friction cause no air? (I know the second one aint even close but you really go into that in your last post)

[GeneralDisorder]

Beleive whatever you like - it will not run as it was designed without ported vacuum. There is not supposed to be any advance to speak of at idle. If you are running higher octane fuel, then more advance is possible, and having the vac advance unit running at idle might be alright. It could, however, cause pinging and therefore is not reccomended. Go read a book on engine theory if you like. It's nothing new, or Subaru specific - been that way on most cars since shortly after the beggining of time.

 

GD

[BobBrumby]

heh im only 17 so its new to me

[archemitis]

ya, thats about all you will need to keep, its not emissions, its so the car runs right. to understand it, take the disty cap off and suck on the vac line, the rotor will move a few degrees counter clockwise. it just moves the disty rotor so that the timing advances at partial, or wide open throttle. it starts at something like 10 before, and goes up to about 20 degrees when fully advanced. if you were firing at 0degrees you would make very little power.

 

if the spark shoots off before the piston is at the top of the stroke, its kinda like raising the compression ration, because the explosion happens before the piston reaches the top. but too far before, and you slow the piston down with the explosion. thats usually a ping.

[archemitis]

and subimech85, if you hook it up wrong, it works as a vac retard. (not calling you a retard). if its in the wrong spot, it will get tons of vac at idle, and run at full advance, then when you floor it and vac goes away, it will set you back at base timing. instead of the other way around

[All_talk]

it will get tons of vac at idle, and run at full advance, then when you floor it and vac goes away, it will set you back at base timing.

This is exactly what its suppose to do, retard the advance under high load, this way you can run a higher initial advance and avoid detonation at higher cylinder pressures.

 

I’ve seen the debate on where the vac advance hooks up time and time again, the answer is… it depends on how the distributor/system is designed. Manifold pressure is a measure of engine load, the perssure goes up (less vac) as the load goes up. On almost all older engines (pre ‘70s) the advance works from straight manifold vacuum. Once emissions became a concern thing got complicated. If you want to know if your system is running with advance at idle (most have some), disconnect the line to the distributor and plug the line to the vac source. If the idle speed drops its because you have removed advance (this can be confirmed with a timing light).

 

On the Subaru engine specifically the stock system connects both above and below the throttle plate with metering orifices and a check valve (check the emissions diagram in the manual). My Chilton manual also indicates some models use a coolant temp or module controlled vac source valve but I don’t have an early FSM to confirm.

 

For what its worth, on my de-emmissioned ’87 carb wagon I have the vac connect to the manifold and it works quite well.

 

 

Gary

[oddcomp]

i have to go with gd on this one...

if you hook it up to pure vacume source then it will be at full advance at idle and cruise

instead of the intended way

that being ported so when the throttles open it then advances to help increase the tourque ect ect

 

yes there are many ways they are hooked up

the thermo switch kind is for emmsions crap

by pass the thermo and its fine

the dual port ones are meant so it helps retartd the timming when the throttles close thus reducing the chance of a backfire and other funn things

 

it could be that perhaps my 4 foot stack of engine books are wrong.. or i am remembering incorrectly

[Partsman]

I'm also with GD on this one. Look at an old car with vac advance (60's or pre-emissions 70's) and you'll find one vac hose - ported source, going to the advance, and one - manifold source, going to the brake booster (if it has one!) The only difference in signal is when it's at idle - as soon as you crack the throttle, both sources give the same vacuum.

[All_talk]

I'm also with GD on this one. Look at an old car with vac advance (60's or pre-emissions 70's) and you'll find one vac hose - ported source, going to the advance, and one - manifold source, going to the brake booster (if it has one!) The only difference in signal is when it's at idle - as soon as you crack the throttle, both sources give the same vacuum.

I own a bone stock 1965 Pontiac Catalina and it has NO ported Vac on the carb (in fact there are no vac connections on the carb at all), the distributor is connected directly to the manifold.

I’m not one to be augmentative and I don’t like to talk out off my rump roast so I did a little research and found a very informative article on the subject written by former GM engineer (see below, warring: its quite long).

As for the EA-82 (engine in question), as I said be for, it has a connection above and below the throttle plate. I’m not certain if the upper connection is “ported” (at the venture point) or just in the intake horn. Never the less, the way its plumbed its designed to accept the highest vac source which ever it is at the time. If you tee a vac gauge in to the distributor line I’m sure you will see vac at idle, my system is not stock so I can’t do the test myself.

 

 

 

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.

 

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.

[GeneralDisorder]

The engine in question is the EA81, not the EA82. He drives a Brumby - Aus version of the Brat. And the fact remains that the engine WAS designed to use ported vacuum. Be it crude or not.

 

GD

[BobBrumby]

Hmm I think Ill go with the ported vac on the carb, and thanks allot for your replies, I sort of understand it now. Tomorow I shuold start ripping all those vac lines out of my engine bay:brow:

 

B.T.W. I did not think there would be so many replies

[All_talk]

Sorry GD, I saw the year of the rig and assumed EA82, of course if I was on the ball I would have realized that no ’90 EA82s in the US were carbed. In my defense I did say that the proper connection depended on which system/distributor you had. I have seen a lot of miss information and confusion on this general issue and thought I would try to provide some info, in this case, due to my assumption I may have just furthered the confusion, again sorry about that.

 

Gary

[Guest taprackready]

The engine in question is the EA81, not the EA82. He drives a Brumby - Aus version of the Brat. And the fact remains that the engine WAS designed to use ported vacuum. Be it crude or not.

 

GD

No, the engine was not specifically designed to run on ported vaccum. The carb and emissions equipment was. Once the emissions equip is removed you can hook up the vacuum anywhere the engine likes it best. In my case, 1982 Brumby, I changed to manifold vacuum and hands down the vehicle runs better, pulls harder and gets better gas mileage. Same situation with my 1978 chevy pickup, Manifold vacuum is better on these two rigs with the modifications I've done.

 

Bill

[GeneralDisorder]

On the contrary - your information is very informative, and goes against some of what I was tought back in school. I shall look into it further. My point was simply that the systems (including the advance mechanisms inside the disty) were designed to work as a team. The carb provides ported vacuum because the disty accepts ported vacuum. Any change in this configuration needs to be dealt with on both ends. Simply plugging the disty into a source of vacuum for which it was not designed is just as bad as putting an improperly jetted carb on the same engine. It may work quite well, or it may not run at all. It would be interesting to see what effect it has, and also what effect it has on emmissions, but I wouldn't want to sugest to others on the board who are not experienced mechanics to go and experiment with a system that they don't understand in the first place. But I do see your point, and I think I may do a little experimenting myself.

 

I would also like to add that a lot of the disty vacuum advance "cans" are in need of testing.... the one on my 84 wagon was toast when I bought the car. Still ran fine - passed emmissions even. But wouldn't hold vacuum at all. They can be rebuilt by a good distributor shop. They are rubber impregnated cloth on the inside, so they do break down..... and they're all nearly 20 years old here in states....

 

GD

[BobBrumby]

Oi General I blew thru the hardline like u said in previous post and it was full of petrol so i duno whats doing on there.Yer and i definately know ive got the right port on the carbi, ur method simple but effective.

[subiemech85]

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.

 

back to the origional question, the vac advance turns the statour, the 2 prong thing that goes on the outside of the reluctor - the 4 point wheel in the middle that spins, in a clockwise direction\

 

manifold vac is beneficial at low speed when the engine is not running fast enough for the mechanical advance to function, and when the vac decreases at hi speed, the mech advance takes over

[robm]

Not just at high engine speeds. At normal cruising speed (2800 RPM plus, say) there is lots of centrifugal advance (based on engine speed), there is also lots vacuum advance (based on load, which is light, more load = less advance) . One doesn't take over from the other, they work hand in hand.

 

That was a great article on ignition timing. We should get permission from the author to put it in the archives.

[GeneralDisorder]

It's important to note that the article also points out that the disty mechanical advance is designed hand-in-hand with how the vacuum advance will be connected. Application of vacuum advance where it was not designed, could cause an over-advance situation due to the mechanical advance not being calibrated for that application. If I were to change how my vacuum advance was connected, I would have a good look at the mechanical advance, and probably have it re-calibrated for proper running under full-manifold connected vacuum advance.

 

GD

[HillbillyLes]

You can do it either way. (Personally, after reading the above article, I'd lean towards full-time/mainfold vacuum but..) Depends on your liking...

 

1. Connect it to full time vacuum (manifold). Take it for a drive or drive a tank of fuel thru it.

 

2. Connect it to the ported vacuum connector on the Carb. Take it for a drive or drive a tank of fuel thru it.

 

Which feels "better" to you when you are driving it in the environment you usually drive (highway, city, off-road, full-throttle racing, etc). If you have driven thru a tank of fuel each way, does either get better or worse fuel mileage? Just do a comparison each way... and settle for the way that gives you the most satisfaction.

 

BTW, computerized, FI engines usually don't have the vacuum advance but do electronically advance the distributor at idle. Check the timing, sometime, or a 'shop manual'. When you set the base timing, you have to make connections to the harness to effectively retard the timing at idle to the base setting before it can be set, then when the connector is removed, the timing "re-advances" at idle.

 

HbL