ALWAYS hook it to the low to no vacuum port,retarding your timing when you stomp the pedal is the worst thing you could do. I have seen so many cars hooked up that way it blows my mind,many hooked up by supposed performance minded mechanics. Its so common that its one of the first things I check for when diagnosing ignition problems. I have even had guys argue about it with me.

 

rifraf, maybe you could answer a question for me.I am re-installing the motor in my 75 vette.I marked all the vacum hoses with tape in ink and very carefully painted them when i redone my engine bay.The best part is , i forgot to take pictures . the
hei distributor needs no vaccum until it revs right?
there's a hose with a round disc in line , is that it.? do I need vacuum for initial start up?

 

http://hotrodders.com/showthread.php?s=&threadid=23169&highlight=vacuum+advance

 

HEI`s use a ported source or one that doesn`t pull vacuum until it`s off idle. there will likely continue to be arguments forever about where they should go, but I never seen the reason why a engine would need vacuum advance at idle, what good is it doing? the vacuum line you have with a disc in it, would likely go to the vent controls or the like, anything to where the vacuum can only be pulled one way and not the other, when you accelerate hard and the vacuum drops, the vent controls would go back to where they were, when a 1 way check valve is used in the line, it holds the vacuum it has when vacuum drops.

 

OK I have 5 degrees initial then you add full time vacuum and it goes to 20 degrees.

now what is the difference between initial 20 degrees?

Well I can tell you the difference. My car wont start if its at 20 degree initial.

I am missing something here. How does the engine know that the advance is from the initial or (initial + vacuum)? The only answer I can think is that the vacuum is real low when the starter is cranking.

 

Most Fords run a manifold vacuum for the advance can. The GM cars I have worked on use ported. I believe the mfg.s curve the dist. for what type they use, in a Ford the centrifugal overlaps with the vacuum advance for a transition into centrifugal.

 

You can run ported, manifold vacuum, or strictly mech. advance.

Reason why you would run manifold vacuum from the carb:

If you set the initial timing to 5* BDTC and run vacuum advance at idle for 15-20*, you get two benifits. 1

1) Since you initial timing @ cranking is pretty much 5*, your engine does not have to fight the advance timming to start (less stress on the engine). Then once the engine is idling, your manifold vacuum is kicking in to give 15-20* at idle and low RPM cruise speeds. Higher timing @ idle and cruise will give better gas mileage, better throttle response, and little more power on you low rpm range.

With manifold vacuum advance method, the manifold port vaccum drops as the throttle blades open, but at this point the engine is picking up RPM and as the vacuum advance retards, weighed advance picks up till you reach total timing. You should have the proper weights and springs so you don't have a dip in the advance curve.



You don't have to run it this way, heck I ran only mech. advance before - race engines don't use vaccum advance. Lot of guys run ported vacuum, but if this is a daily driver and you do a lot of cruising (rpm range), then you'll benefit more with manifold vacuum port for better gas mileage.

 

Older non-smog-regulated cars that I'm familiar with used manifold vacuum for their advance cans. The advent of smog rules in the late '60's & early '70's brought the use of ported vacuum. Vacuum advance at idle makes a car idle smoother & with less throttle opening, but it also increases emissions. Later cars with distributors designed for ported vacuum should probably use it. Race engines & hot street engines can benefit from vacuum advance by making them smoother & more civilized to drive at slow speeds appropriate to the pits or residential areas.

 

Vacuum

Justinh! I have been reading all the post's and in less I missed something, here is what I see missing: the distributors the answer to the question. There are Vacuum advance distributor's and then there are rodder advance distorter's. And then there are to deferent place to get the vacuum from. One is from the carburetor above the throttle plates, the other is below the throttle plat's or intake manifold vacuum. Now it has been quite awhile but sense I worked on this problem, but the full vacuum from the intake manifold or from below the throttle plats on the carburetor is for the vacuum advance distributor and the rodder advance distributor vacuum comes from the carburetor above the throttle plat's. They are very deferent in the way they advance the timing on the engine. I know that there are other types of distributors out such as the Rev Pole just to name one. Some time you have a problem when you switch carburetors or distributors. If I'm wrong about this I'm sure there will be someone on here that will set it straight. And that is OK too. Just something to think about! ------------Gene Neal

 

Im back. There are other cars that have a variety of methods to advance and retard the timing for different reasons,as mentioned prior. If yours increases when you plug it into the under the throttle plate port,or manifold vacuum as you stated it did,you need to switch it to the other port. There are a host of different temperature sensors,check valves,etc but if you want it basic and functioning properly take my advice.

Mikewayman,Im not familiar with how yours was set up originally,sorry

 

Older non-smog-regulated cars that I'm familiar with used manifold vacuum for their advance cans. The advent of smog rules in the late '60's & early '70's brought the use of ported vacuum. Vacuum advance at idle makes a car idle smoother & with less throttle opening, but it also increases emissions. Later cars with distributors designed for ported vacuum should probably use it. Race engines & hot street engines can benefit from vacuum advance by making them smoother & more civilized to drive at slow speeds appropriate to the pits or residential areas.

This is what I believe to be true also. I run my engine with the FULL vacuum to the advance/retard vacuum can on the HEI. That works best for me. If I use the PORTED vacuum it falls on its face when going to WOT from a stop. Switching to FULL vacuum cures this immediately.

My suggestion is to try both and see what works best for you. All that is involved is switching the vacuum hose to try it out.

 

Older non-smog-regulated cars that I'm familiar with used manifold vacuum for their advance cans. The advent of smog rules in the late '60's & early '70's brought the use of ported vacuum. Vacuum advance at idle makes a car idle smoother & with less throttle opening, but it also increases emissions. Later cars with distributors designed for ported vacuum should probably use it. Race engines & hot street engines can benefit from vacuum advance by making them smoother & more civilized to drive at slow speeds appropriate to the pits or residential areas.

I agree with this 100%! The primary purpose of vacuum advance is fuel economy period! This is why race motors don't have them. A vacuum advance canister can pull in as much as 60 degree timing. Under heavier loads timing requirements decrease due to combustion pressure. A ported system only reads the total airflow through the carb, not engine load as a manifold system can. Do you rally want to pull in as much as 60 timing pulling a heavy load up a hill? I don't think so.

 

True, whatever timing curve your setup needs - go with that curve. There is no right or wrong vacuum port - in the end its what the engine needs. I draw all my experience from pre 1970 chevy cars. A stock engine will have different needs than a warmed over street rod or a strip/street car.

 

Let me go into further detail,lets say your distributer advances 10degrees at full vacuum. Lets say your advance weights offer 18. The combination yields 28. Now,when you stomp the gas,the manifold vacuum drops to zero.At an idle it sits,depending on the cam,about 16.If you set it up to manifold vacuum your timing will retard that say 10 degrees under acceleration and only see full advance when your load has diminished or you let up on the gas,a poor recipe for performance and gas mileage.

 

Actually, I set my timing to 10* without the vacuum advance. With the vacuum advance hooked up its around 18-20* @ idle and cruise RPMs. Also, you have to have the correct combo of springs, weights, and cuts in the weights for your curve to work with your vacuum manifold setup. My gas mileage and power response from cruise have both increased for my stock 283.

Check out this clipping:

<<It was written by a GM engineer.

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.

 

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.

That whizbang you are writing about is BS. I have been running a whizbang non-vacuum advance dis. for over 30 years on my 307 and 350 and never had any problem at all. I had excellent throttle response, I like smoking up my tires when I hammer the gas pedal, and never overheated, and gas mileage, who gives a dam when I'm running 4:10 gears and a 3500 stall converter. I never did like vacuum advance. You're worry about gas, get a rice burner. Before safety inspection was introduced it passed emission. Still running that whizbang on my 406sbc, today.:evil:

 

Thanx for the article malducci, that makes total sense. In my experience, manifold vacuum on a properly set up car gives absolutely the best performance- reduces knock, improves cooling, improves acceleration, etc.

 

I don't know who this guy is,and because hes a gm engineer don't mean squat to me,I know engineers that I wouldnt let chang my oil. In my opinion he is confused,but thats just my opinion. I agree with a lot of his statements because they are fact but some are off, the math don't work out on my calculator. I realize you can tweak your distributer to make it work properly,Ive done that to start very high compression motors,but Im speaking of stock mainstream gm distributors in relatively stock compression motors using pump gasoline.

 

What's makes sense to me is that in idle and cruise - the carb is still using its idle circuit which almost always lean. If you step on the gas, the accel pump squarts gas into the air/fuel mixture - richening it out. Also as the throttle blades open, the carb switches out of idle circuit mode - richening the mixture further. And to top it off if you have a holly carb, the power valve opens up as the thottle blades open up - richening the mixture as well. Richer air/fuel mixture requires less advance spark, so it makes sense that the timing back off as the fuel mixture richens. Higher RPMs need more advance spark and that is were the mechnical advance mechanizm comes in (weights and springs).

 

Excellent article. Most important thing to remember is that it's the mechanical advance that is going to will races, and the addition of vacuum advance to increase fuel economy. Both should be set to a point where any additional will cause a loss in performance. Every engine will be different, and there is no formula for magically setting it. Setting up the distributor on a distributor machine will get you in the ball park, but the final tuning will have to be on the road. Leave the vacuum off while figuring out how much mechanical advance the engine will handle, and then after you get this set, then dial in the vacuum advance to improve your fuel economy. I also cannot see why anything other than a manifold vacuum source would be used unless you are subject to smog inspections. Manifold vacuum does the best job in filling in the additional advance during idle and light throttle conditions.

 

As I recall, Doc Jacobs makes the same points as malducci's article does about the benefits of manifold vacuum in his very informative and in-depth book on ignition systems. Port vacuum is for smog motors, manifold vacuum is for performance and economy.

 

Vacuum advance was created to aid in the low end acceloration,those whiz bang mechanical distributors work awsome on marine aplications and cars with bigger stall converters where the load is not applied until the rpms have increased. If you choose to go mechanical,curve it to match,but if you choose a vacuum distributer,your timing being lessened under acceleration is not a good choice,never,if your optimal advance is 34,and you tromp the throttle and get 24,your killing your performance. Efficiency is power,if your power valve needs to pick up the slack,all your doing is making is a big soot pump. Your mechanical advance weights work just like a mechanical only distributor,your vacume only helps get you going,because of the speed and nature of low rpm burning.

A lot of this has to do with the port velocity also.

If Im wrong,ask yourself,why would the major performance carburetor manufacturers provide such a ridiculous option,surly not to pass a smog test.

 

Asked myself, and the answer back was Yep, only because of smog and the desire to subdue emissions somewhat at idle. Another fallacy is that you would have vacuum anywhere in the carburetor with the venturis slammed full open. Hook a vacuum gauge up and check out ported vacuum. No vacuum at closed throttle, good vacuum at partial throttle, and no more than manifold vacuum with everything dumped fully open - and this won't be much.

I set my distributors up on the distributor machine, and I set them up for maximum usable advance curve regardless of whether they are straight mechanical, or will be used for the street with a vacuum advance. After the mechanical curve is set for optimum performance, I then adjust the vacuum curve to match the individual engine I'm working on. This might mean either limiting the total advance the vacuum motor can input, or even altering how much vacuum it takes to move the plate. My engines will produce maximum power with a correctly set up distributor regardless of whether there is a vacuum module, the only difference is about 2 MPG on a trip with the vacuum unit functioning correctly.

 

"those whiz bang mechanical distributors work awsome on marine aplications and cars with bigger stall converters where the load is not applied until the rpms have increased."

True. Mechnical advance "only" distributor is meant for race engines that live 3500-8500. You can run mechnical advance "only" distributor on the street, but your not benefiting from anything and actually gas mileage/cruise to WOT throttle response.

"but if you choose a vacuum distributer,your timing being lessened under acceleration is not a good choice,never,if your optimal advance is 34,and you tromp the throttle and get 24,your killing your performance"

I think your forgeting that a vacuum advance distributor has mechnical weighted advance mechanism that is RPM dependent. In other words, if your total timing is 34* @ 2500rpm (without manifold vacuum advance) , then that is the total timing - not 24*. If you were cruising at 2500rpms and had +6* vacuum advance, then your timing is 40* - when you punch the throttle to WOT, you loose all vacuum advance and only have mechnical advance which is 34*.



Actually, were are talking about stock to mild setups, not most strip or race engines.

Ported vacuum has its place, but for a different kind of timing curve - depending on the engine setup and desired RPM use, its mostly for more hi-performance or extreme engine setups.

Ported vacuum was used on older setups too ( pre 1971 I believe ), were you had higher compression and too much advance cruise rpms resulted in engine nock - in that setup mechnical advance + ported vacuum = total timing. Also if the base timing is 4* and the engine had higher compression, it would be easier to start at a lower timing, then when you had acceleration the ported vacuum would put the timing were it needed to be for performance (maybe 10* more than cruise/idle) and as RPM increased the mechnical weights would work out the rest of the timing curve.

You also have to remember that engine setups ran richer back then and emission weren't as strict if at all in effect. Newer engine setups with lower compression and leaner idle/cruise carbs circuits switched over to manifold vacuum. Newer computer controlled timing is even more efficient than manifold vacuum setup.

Like all good engine setups, even manifold vacuum setup needs to be tuned to the specific engines needs.

 

I give up,I haven't forgot,I do understand,and Ive spent more time on distributer machines and wheel dynos than I care to remember,you do it your way and Ill do it mine. Lust4speed,try your vacuum theory under load,see what you get between the two,I think you may find it eye opening.

 

rifraf, didn't mean to affend you. Just trying to explain that manifold vacuum port is there for a reason and most daily drivers would be better off using that kind of setup. Most people think manifold is utterly useless and its not. And usually those people that have done some mild upgrades to there engine have been told to run ported vaccum since it is a performance timing setup. Its like those guys that run over cam'd engines or single plane hirise on a engine whose valve train is limited to 5200rpm.

It's the fact that you said, when you see a car that has the distributor hooked up to manifold vacuum port - you change it to ported vacuum. To me that is crazy unless you know the engine setup from the get go( cam, carb, power band, car usage, etc) and ported would be more/better suited for it.

I'm not saying every engine setup should use manifold vacuum port. I'm in the process of building a strip/barely streetable 327 engine. 300* solid roller cam with .672 lift - victor jr. heads - tunnel ram - 11.5 static CR. Guess what, I'm sure as hell not going to use manifold vacuum port on it. Matter of fact I'm probably going to take out the vaccum advance and put in a mechnical pull switch so I can drive around town on pump gas to avoid engine knock - retarding the timing by 10-12*. And when I get to the track and fill up on some race gas, I can manually activate the pull switch setting the distributor up by 10-12* were is would be optimal for that engine + race gas.