TheSamba.com :: View topic

Thu Feb 03, 2005 3:02 pm

Thanks Randy. I quite like Bob Hoover's articles (his sermonettes are great reading).

But once again - it's clear that vacuum IS better than centrfugal, but WHY?

Why doesn't advance at low rpms under load cause detonation?

Thu Feb 03, 2005 3:17 pm

The vacuum can detects when there is a sudden change in vacuum by the throttle plate and alters the timing. The change is caused by the sudden opening of the throttle plate (the restriction that gives the vacuum reading), and the timing has to be advanced because the mixture has temporarily changed due to the massive rush of incoming air. Once the engine speeds up to the rpm that corresponds to the throttle plate position and the vacuum level stabilizes the advance goes back to being mostly mechanical.

Thu Feb 03, 2005 4:13 pm

In a nutshell

we want pressure on the piston at a certain crank angle. The important concepts are that combustion is not instantaneous, and combustion speed changes depending on temperature and pressure.

The simplest concept, which most understand, is that we need more advance the faster the engine is, to give the pressure "time" to get to max pressure at the right time. This is accomplished with centrifugal advance.

Combustion speed also changes with temperature. The hotter the air and engine are, the faster combustion speed is. This is why some engines will run fine when cold, but ping/detonate when they are hot. Hotter engines need less ignition timing.

Combustion speed also changes with PRESSURE, because it's related to temperature (air gets hotter the more it's compressed). At idle, the engine is only putting enough air/fuel into the cylinder to run at idle speed, so the dynamic compression of the engine is very low. At more open throttle settings, the engine is actually ingesting more air, and this means the engine actually gets more compression, which means when it's compressed it gets hotter, so it oxidizes faster (burns faster). It should not be too difficult to realize that the more your throttle is open, the more air the engine is taking in, right? This is why you need to do compression tests at WOT, if you leave the throttles closed you could get a false LOW reading. So the more the throttle is open, the less ignition timing it needs. IOW, the engine would run best with more ignition timing at part throttle then it needs at full throttle. Make sense? Lets continue.....

The problem with 009s, and other centrifugal ONLY distributors, is that they only sense engine speed, which only addresses the RPM part of the ignition timing. You must set them for "worst case", which is "full throttle". If you set them to run properly for part throttle the engine will be a dream to drive until you open the throttle beyond 1/2 throttle, then it will ping itself to death in a hurry! This is why vacuum advance is so vital for street cars, they give the engine more ignition advance, but only at times when the engine "needs it". Engines want up to 45 degrees of timing under certain situations, so it should be obvious why a 009 equipped engine will use more fuel, and run hotter, then the same engine with the proper vac advance distributor. Running a 009 timed properly is around 12 degrees too retarded from where it would run best, on the highway. This is why 009 engines suffer MPG and run hotter on the hwy.

This same issue affects throttle response, but I think you get my drift.

John
Aircooled.Net Inc.

Thu Feb 03, 2005 4:17 pm

jolo wrote: I'm pretty sure he only reccomends the stock dizzy for the tyeIVs and anything with FI.

Still very curious about load vs rpm advance, though.
In a nutshell, my understanding is that when you open the throttle (vacuum low) the cylinders fill with more air/fuel mixture and compression goes up. This dense mixture burns quicker, all other things being equal, so you need less advance. And the vacuum advance part of the dist. does exactly that.
Now you're driving along at, say half throttle for example, but driving at highway speed....highr RPM. The density of the air fuel mix in the cylinders is not as high and will not burn as quickly, BUT, the motor is turning over faster and needs a centrifugal action to light the fire sooner to give this slower mixture time to burn before the rapidly moving pistons get too far ahead. (at highway speed, remember?)
Still at highway speed, for example and you hit the throttle hard. Centrifugal component is still advancing for the RPM, but now the vacuum drops again to reduce the total spark advance somewhat because you're mixture density just went up in the cylinders again and now wants less spark advance.
Make sense?
A combination vacuum and centrifugal advance distributor, I think, is probably the best for most cars.

Thu Feb 03, 2005 9:51 pm

>I know John Muir is widely faulted for his praise of the 009, but he makes a point about the "principal is all wrong" for vacuum advance distributors.

Just a quick point, because I cannot add anything to what John and Richard have said on the technical aspects.

Muir talks of "vacuum", "centrifugal" and combination dizzies. Muir didn't like vacuum dizzies, meaning early vacuum only dizzies. He preferred centrifugal dizzies for those early engines over those early vacuum only dizzies. I cannot say that I disagree.

But just as the theory is all wrong with the vacuum only dizzy that Muir disliked so much, the theory is also all wrong with a centrifugal only dizzy. There is no way for a mechanical (009) to sense varying loads. No way for the dizzy to know if you are mashing the accellerator.

When we vw hobbyists (or professionals) today speak of a vacuum dizzy, we are USUALLY referring to a combination vacuum/centrifugal dizzy, truly the best of both worlds.

Muir briefly mentioned combination dizzies. I don't remember when Muir died, but it was in the 70s and I think the early 70s. Combo dizzies were first introduced on VW's in the 70s, long after the first Idiot Manuals were printed.

Anyway, regardless of what Muir thought in the 60s-- whatever whomever wrote the later updates think-- all modern dizzies were combination centrifugal and vacuum, for a reason.

Here's a link to Hoover : http://www.type2.com/library/electrip/advance.htm

Here's another article comparing a 009/010 mechanical dizzies with an SVDA combination dizzy:
http://www.aircooled.net/gnrlsite/resource/articles/distributor.htm

Fri Feb 04, 2005 12:52 am

Excellent fact finding wrt to Muir.

I looked over all of these answers including my own but none of them really addressed the why everyone is asking. In a nutshell, under partial-throttle, the mixture leans out and this requires more advance in order to maximize the pressure (-> torque).

The vacuum advance port is tapped off just upstream of the throttle plate. When the plate is closed there is no vaccum but as you open the throttle the plate obscures the opening in the throttle body and a vacuum condition occurs. As the plate is opened further towards WOT the vacuum level decreases again.

That's all there is too it.

Fri Feb 04, 2005 7:26 am

ratwell wrote: The vacuum advance port is tapped off just upstream of the throttle plate. When the plate is closed there is no vaccum but as you open the throttle the plate obscures the opening in the throttle body and a vacuum condition occurs. As the plate is opened further towards WOT the vacuum level decreases again.

That's all there is too it.
I agree. And I think that most distributors on any vehicles (before all the electronics) used a combination vacuum and centrifugal advance for the reasons I tried to explain, and that this "ported" vacuum ratwell describes is to prevent advance at idle which tends to overheat an aircooled engine after a period of time.

Fri Feb 04, 2005 8:21 am

Thanks a lot, guys. It makes sense now.

Fri Feb 04, 2005 8:30 am

WOW! Lotsa "nutshells" in this thread :lol:

This has to be the best or one of the best threads/discussion on the subject that has been posted on this site...

Good read, thanks for the insight guys. 8)

Fri Feb 04, 2005 9:10 am

Great insight. I'm new to VWs and this forum and the info I'm finding is priceless. Thanks alot for taking the time.

Fri Feb 04, 2005 10:48 am

Just a note. The vacuum retard signal is the one that lurks near the throttle plate and is blocked off the instant the throttle plate opens. The vacuum advance signal is coming from well above the throttle plate where the tail end of the venturi is located. Although the advance unit is nicely sensitive to throttle position, it is picking up its signal based on air speed past the venturi. That means it is not an idiot, it doesn't disappear to 0 every time the throttle is opened. It actually will give you an ever increasing advance based on engine speed as well as drop the advance when you floor the accelerator. The vacuum only distributors were perfectly good distributors for driveability.
John Muir is like my uncle, who worked on his own Ferraris when he raced. (he is in Road & Track this month, celebration of his LeMans wins) Brilliant native talent who could glean the physics from mechanical design. But the minute you put in hidden effects, like transistors and sensors, their brains go haywire with irritation. Give John Muir a centrifugal advance unit and a proper carburetor and his right foot can feel the cause and effect. But give him FI and crossing vacuum signals and he is swimming in unreadable symptoms. I, too, gave up in irritation when my Lexus demanded that the transmission computer tell the engine computer whether or not it was happy before you could test the timing map after reading the fault codes for the anti-knock sensors taken out of the cam timing loop because as you know, you can't fault the antiknock sensors until the cam timing loop has looped you over the head and knocked you faultless.
Colin :D

Fri Feb 04, 2005 11:08 am

That's funny Colin, and true. We are all wired very differently, some of us are in snych with mechanicals, while others can handle more theory. Like artists vs mathmeticians, we all have our own ways of getting around and identifying a problem.
I don't care much for today's modern vehicles, where you can't adjust things, it's all computer controlled, and made of plastic. Who is doing the driving?

Fri Feb 04, 2005 12:57 pm

Amskeptic wrote: Just a note. The vacuum retard signal is the one that lurks near the throttle plate and is blocked off the instant the throttle plate opens. The vacuum advance signal is coming from well above the throttle plate where the tail end of the venturi is located.
This is the layout for carbs.

Fri Feb 04, 2005 6:52 pm

keifernet wrote: WOW! Lotsa "nutshells" in this thread :lol:

This has to be the best or one of the best threads/discussion on the subject that has been posted on this site...

Good read, thanks for the insight guys. 8)
Yeah, I wrote about the same thing as [email protected] about 4 minutes later, and before his post showed up on my computer.
Sorry, John

Sun Feb 06, 2005 9:38 am

I must say, as a most-of-the-time lurker and occasional poster, this is the first thread on the 009 to make sense to me. When Colin was here, we talked about getting rid of my distributor in favor of the "proper" one, but like many things we talked about that day, the theory behind it didn't stick. When spring comes--and it seems right around the corner today, the 009 goes and the new distributor gets ordered.

Thank you all so much!

Sun Feb 06, 2005 11:34 am

WRT throttle response: not to confuse anyone, is that the carb mixes air and fuel. Air is lighter then fuel, so when you crack the throttle and air speed increases in the intake and carb, the fuel takes a little longer to get up to speed because it's denser (inertia). Stated another way to clarify, it takes more energy (time) to get fuel up to speed then it does air. This is why engines lean-out when you crack the throttle, and why most carbs have an accellerator pump, to add the addl fuel the carbs normal metering circuit is not yet dispensing. Additional advance lessens the hesitation, since it reacts much faster then adding fuel. And, it's free (gas isn't). ;-)

the only valid reason to not use vacuum advance is if someone wants a cleaner looking engine (no vac hose nor vac can). But if the vac breaks in an SVDA or Unilite w/Vac, you end up with centrifigual only LOL! You can even buy the SVDA or Unilite w/Vac, and cap off the vac port operating as a centrifugal only, until you have the carb(s) that have the proper port to utilize the benefits of vacuum advance.

Also one last note: Vacuum advance distributors (SVDA = Single Vacuum, Dual Advance, and DVDAs (Dual Vacuum, Dual Advance) The "dual advance" is the centrifugal and vacuum advances)) require ported vacuum. I continually see errors and confusion on exactly what this is.

There are 3 kinds of vacuum.

1) Venturi Vacuum. This is basically what meters the fuel in the main circuit. It's taken from the narrow part in the carb (the venturi).

2) Manifold Vacuum (MAP = Manifold Pressure). This is the intake manifold pressure. It's also a measure of "load" of the engine.

3) Ported Vacuum. This is what vacuum advance needs. Ported Vacuum is simply MAP that is "off" at idle, which is why the location of the port is critical in the carb. Off idle MAP and Ported Vacuum ARE THE SAME.

You can NOT take an SVDA or Unilite w/Vac and tie it into the MAP, it will give too much advance at idle because MAP is not "off" at idle. So ignition timing will be screwy.

John
Aircooled.Net Inc.

Sun Feb 06, 2005 12:21 pm

These vehicles are 30-60 years old so there's hardly anything new to discover about them; information just gets lost in conversion from print to web. Some of these models were built before some of us were even born.

Of the best 009 articles I read about 3 years ago when I began the learning process of how my bus worked was in VWTrends (cough cough) called Ignition Timing 101: Getting Beyond the 009. It was written by Shad Laws who is a really smart guy in VW circles but even that article has a few errors. One such error:

Quote: Leaner mixtures and high temperatures cause the wave to move faster, therefore, they need less advance.
This must be a partial conclusion or editing error because as I know he has a strong engineering background. Leaner air/fuel mixtures during partial throttle are harder to ignite and require more advance hence the need for the vacuum can.

Sun Feb 06, 2005 12:44 pm

ratwell wrote: These vehicles are 30-60 years old so there's hardly anything new to discover about them; information just gets lost in conversion from print to web.

You're right, Ratwell. It's important to consider their (the Volkswagen's)age and we're fortunate to have this forum to pick the brains of the gurus!!!

I'd like to take this time to say thank you to all the helpful men & women on this forum! We must unite and conquer!!!

Sun Feb 06, 2005 12:57 pm

it's not an error. Leaner mixtures are harder to ignite, but are also faster to burn (oxydize) once they do.

But don't confuse yourself by thinking about it too hard. Part throttle operation has less air in the chamber, so the dynamic CR is lower, and therefore slower burning, which needs more advance.

think of WOT: At WOT a 12:1 may be ok while 15:1 will ping, giving the same timing setting.

John
Aircooled.Net Inc.

Sun Feb 06, 2005 1:25 pm

Nice thread. The bay forum seems to have the best threads about engines and their functions. A nice break from lowering, stroking, wheelies and purple paint.