TheSamba.com :: View topic

Fri Jul 30, 2010 8:25 pm

I've done a lot of reading on the topic, and it seems nearly unanimous with VW folk that the vac advance can on the distributor (assume SVDA) should always be connected to ported vacuum (zero vac advance at idle). Contradictory to this advice, I've read a very convincing article elsewhere, authored by an old GM engineer, which says that using a ported vacuum source for the vac advance can is really a ham-fisted emissions hack, and that manifold vacuum is really what you want with any vac/mech distributor.

His reasoning took into account the effects of engine speed, richness of the air/fuel mixture, and the time it takes the mixture to burn in order to get the maximum energy out of the burn at the optimal crank position. The whole gory detail can be found here, but to paraphrase:

You need vacuum advance at idle to take into account the slower engine speed (zero mech advance) and the slower burn-rate with the lean mixture. With advanced ignition at idle the peak burn will be more perfectly timed to the power "sweet spot" and crank position. Result: smoother performance at idle.

You do not need vacuum advance when accelerating from idle because the rich mixture burns faster than the lean idle mixture. As the car accelerates the mechanical advance takes over and compensates for the need to start the mixture burning sooner and keeps the peak power from the burn timed correctly with the crank position.

Vacuum advance is engineered to compensate for variable loading on the engine, and the only true measure of how the engine is loaded is via manifold vacuum.

With either ported or manifold vacuum, at full throttle vacuum advance not a factor.

So, these physical characteristics of how spark-timing works and how the mixture (lean or rich) burns wouldn't be any different with a VW than, say, on a big V8 from Detroit, so I am puzzled why the common wisdom here insists on using ported vacuum for the vac advance. From what the engineer says, it seems like it would work a lot better with manifold vacuum, after all ported vacuum (according to him) is an ill-conceived emissions hack.

So that's the first part of my question....Ported or Manifold: Why would a VW be any different than my '63 Thunderbird w/respect to vac advance signal source and spark timing?

Next part: if I'm using dual carbs, and have a crossover tube (a source of manifold vacuum, right?), wouldn't it make sense to hook the vac can to the crossover tube? Seems like a decent option to have fewer opportunities for vacuum leaks on the carb, and I'd be tied into the "better" option for vacuum source.

Discuss. :)

JC

Fri Jul 30, 2010 8:39 pm

You don't want your distributor to advance at idle. Manifold vacuum would do this.

Fri Jul 30, 2010 8:44 pm

Engineer-guy says you do....that's my question. Why wouldn't I want advance at idle? Seems like the only other time you'd use a vac advance (compensating for engine load being the other).

Any theory/facts to help explain? I'm trying to get a handle the conflicting theory (yes you do, no you don't, need advance at idle).

Fri Jul 30, 2010 8:51 pm

V8's and VW's don't necessarily operate using the same principles. V8's normally use huge plenum manifolds so the vacuum dynamics may be different than the relatively small volume VW manifolds. I'm no expert on this and what I said may not even have any important bearing on how things operate. I read the article you refer to also and the guy makes a very convincing arguement but....if manifold vacuum were needed or helpful to the VW engine the VW engineers would have used it. In fact, they may have with the old stock DVDA distributor which had vacuum advance and retard, but don't quote me on this. Simple test would be to hook your distributor up to manifold vacuum and see how difficult it is to tune and make run right.

Fri Jul 30, 2010 10:25 pm

Idle speed will be unstable if you try to use manifold vac.
That's a good reason.

Fri Jul 30, 2010 10:53 pm

Connect it to manifold vacuum and see how long your exhaust valves live.

Contrary to the article you read, even GM connected thier vacuum advance on all thier vehicles to ported vacuum, NOT manifold vacuum.

I would be willing to bet the numb-nuts that wrote whatever you read was not an engineer at all based on the inaccurate driibble coming out of his mouth (or keyboard).

In short, NO manufacturer ever based ignition timing to the distributor from manifold vacuum.

Fri Jul 30, 2010 11:49 pm

The modern crankfire systems seem to be happy to get a engine load signal from ported manifold that the MAP sensor can use.

Would be interesting the see the 2 different signals graphed through the driving range.

Sat Jul 31, 2010 1:29 am

i use manifold vacuum.
the reason is because i like the late adjustment of my saab turbo pressure tin.
the early adjustment at idle is no problem, and my head temps are very low.
idle adjustment is more easy then befor, but i also use a supercharger so i would not say it comes from the timing.

but this is with 8psi negative pressure at idle and semi semi hemi chamber of a 36hp engine. and i use 50%E85 at the moment, this all would make the early timing to no preblem.

i think it is not to compare, but manifold vacuum is usable

Sat Jul 31, 2010 1:49 am

If i remember right, some of the old GM engines did have the distributor diaphram hooked to manifold vacuum. It kind of worked backwards to the way our VW's work. The points plate was spring loaded in the advance position and when the engine was idling, the manifold vacuum caused the diaphram to pull the points plate to the retarded position. When the engine accelerated the decrease of manifold vacuum allowed the points plate to go to the advanced position. As the manifold vacuum increased it pulled the diaphram back to the retarded position, but by then the mechanicial advance weights have moved out providing the advance.

Randy

Sat Jul 31, 2010 4:32 am

If you had a link to the article you referenced, it would be an interesting read. But I think as posted earlier, there is more to it - I suspect (or can pretty much guarantee) that the distributor is designed and curved to work with manifold vacuum - that would make a ton of sense.

Would love to see the full article though.

Sat Jul 31, 2010 8:03 am

Pretty laughable to me. There is no direct correlation between what advance you need and manifold vacuum, approximations with air speed and depression through the carb can do a better job. The majority of people don't even know the difference in models between the early distributors without the advance weights, and the ones that have them, and some of those will happily substitute one for another. Seen it too many times.

Vacuum advance and retard may be used to affect emissions, but they also have a place in the efficiency department as well as the power department. But people just keep using those great 009s, and "never have a problem with them".

Higher strung engines with longer cams and individual runner induction tend to be harder to set up vac with, due to weaker vac signals, and different model carbs provide different signal characteristics, so once you get over about 290-300 degrees of duration and standard LC/cam timing, the signal gets weak enough that it's a coin toss. Not that it can't be done here, but really a programmable advance system would be best, and in fact, would be best in any case. Something that listens to AFR, TP, and MAP can model a good advance setup, to get the most response out of your engine.

Sat Jul 31, 2010 8:22 am

It was in the first post in the thread, but too hard to find, I suppose:

http://www.corvetteactioncenter.com/forums/c3-tech...vance.html

It's worth the read, and I'd love to hear comments on what the article says vs. Common practice with our VW's (stock or otherwise).

JC

Sat Jul 31, 2010 8:37 am

krusher wrote: The modern crankfire systems seem to be happy to get a engine load signal from ported manifold that the MAP sensor can use.

Crank-fired systems are generally programmed to ignore manifold pressure at idle; any vacuum compensation at idle would have a drastically different programmed curve than the vacuum curve at 3500 rpm.

Where the SVDA distributor has only 1 vacuum curve, my crank-fired ignition has the potential for about 1,213 vacuum curves (13 RPM bins with a unique vacuum curve programmed across the RPM bin, plus 100 interpolated curves in between each RPM bin). Granted, some of those curves are redundant, such as the RPM bins around idle, as well as some RPM bins above 5K RPM, but the curve can be anything I want it to be, where I want it to be. The SVDA is going to react to manifold vacuum regardless of the RPM; you don't want that at idle, and the electronic program of a crank-fired ignition system reflects this.

Sat Jul 31, 2010 9:45 am

I run EDIS off manifold vacuum and I have a fixed timing value at idle. The fluctuations in vacuum at idle are too erratic to compensate for any other way. I can't imagine a distributor idling too well off of manifold vacuum.

Sat Jul 31, 2010 9:48 am

the main reason you don't want to connect to MAP is because it will create an unstable condition at idle. If you start increasing elevation, or losing compression, or even changing engine temperature, your vacuum and hence your idle advance will change and so will your idle speed.

For idle you want a STABLE condition, otherwise it will hunt and RPM fluctuate and drive you nuts.

Sat Jul 31, 2010 6:23 pm

interesting read.

I'm kind of weird. I like to run as much advance as I can without ping.

I run manifold vacuum, and like rpm advance.

if I have huge vacuum at higher rpm i want more advance because my cylinder is not fully charged, and it can safely use more advance.

at the same RPM, at WOT, the cylinder is filled, and if I ran a lot of advance, I will get detonation.

Any old chevy guy will tell you a 350 without vacuum advance off the manifold will only get 10 mpg.
Hook up the vacuum advance, and you increase your mileage 20 to 30 percent.

ported vacuum advance I am guessing you would only run if you didn't have rpm advance, and I guess the theory is you get more port vacuum as your RPM increases.

Sun Aug 01, 2010 12:33 am

drmiller100 wrote: interesting read.

I'm kind of weird. I like to run as much advance as I can without ping.

I run manifold vacuum, and like rpm advance.

if I have huge vacuum at higher rpm i want more advance because my cylinder is not fully charged, and it can safely use more advance.

at the same RPM, at WOT, the cylinder is filled, and if I ran a lot of advance, I will get detonation.

Any old chevy guy will tell you a 350 without vacuum advance off the manifold will only get 10 mpg.
Hook up the vacuum advance, and you increase your mileage 20 to 30 percent.

ported vacuum advance I am guessing you would only run if you didn't have rpm advance, and I guess the theory is you get more port vacuum as your RPM increases.

I think you got that all wrong :?

Lest take a 034 sdva distributor. Its has both rpm and vacuum advance

The rpm you know , it advances as rpm increase as to its advance curve usually from 7.5 to about 30 degrees.

Then it has a vac advance, this take its signal from ported carb vacuum, this port is by the butterfly in the carb, its high vacuum when the butterfly is near closed and low vacuum when its fully open (so it doing its best mechanically to sense engine load)
Also the vacuum draw will go up and down depending on air flow.

So driving down the highway at cruise 3000rpm, you have the carb only just open. So were running a lean cruise chamber fill. We have 30 degrees from rpm advance, and about 12 from vacuum, running 42 total to help the weak/lean cruise charge ignite early.

Now we hit a hill,open the throttle fully , the vac signal goes away, so now we just have 30 total from the rpm advance to light the now much denser chamber fill.

Many people seem to think its "more throttle more vacuum", but its just the opposite (although more rpm at the same throttle opening should produce more vacuum)

Sun Aug 01, 2010 5:39 am

krusher you are correct, but mainfold vacuum will do pretty much the exact same thing- except that it will be more reactive to engine idle fluctions, which will fluctuate the vacuum advance, which will creat more fluctuations in idle quality, and so on, and so on.....

I have heard numerous people say the "dont want a vacuum advance distributor" because the should "get a mechanical advance" unit because the are building a "performance motor"- or even the opposite.

Most people dont understand that a vacuum advance distributor (most anyway) have BOTH- vacuum AND mechanical advance.

Sun Aug 01, 2010 6:59 am

sbnova wrote: krusher you are correct, but mainfold vacuum will do pretty much the exact same thing- except that it will be more reactive to engine idle fluctions, which will fluctuate the vacuum advance, which will creat more fluctuations in idle quality, and so on, and so on.....

I have heard numerous people say the "dont want a vacuum advance distributor" because the should "get a mechanical advance" unit because the are building a "performance motor"- or even the opposite.

Most people dont understand that a vacuum advance distributor (most anyway) have BOTH- vacuum AND mechanical advance.
.
We must not also forget that you need a vacuum can that is suitable to go with the vac signal produced by the carb or manifold your using, to strong a vacuum will end up with to much extra advance all the time and to little with no advance at all

Sun Aug 01, 2010 11:38 am

I would be very interested in knowing the credentials of the author of the post that all of this is referring to. Interesting read, but I don't know about you, but I have seen too many internet experts that can make a good case, but have no data to back it up. A "former GM engineer" can mean that they engineered ash trays in Chevettes.

Now, I am not an expert by any mean, and some of what was said seems very correct, but some points do not.