123+ and Idle Vacuum Advance

['76mintgrün'02]

On 5/16/2017 at 10:51 AM, silasmoon said:

@'76mintgrun'02 I will warm her up and double check tonight that the throttle plates aren't open during idle.

According to the Weber set up instructions, you want the idle speed screw turned fewer than two revolutions, once it makes contact with the tab.  More than this and it starts pulling fuel through the transition holes.  Some of your recent symptoms (another thread) may be due to  being on the cusp of this happening.  (?)

 

On 5/16/2017 at 3:41 PM, Simeon said:

What I am trying to think about is how these kind of features can be used where they go beyond the simple mechanics of the capsule and springs. It's not quite as flexible as a real engine management system but we should be able to start thinking beyond emulating stock ignition operation. 

 

 

I sent my latest curve to Toby, for a little input and he sent me this illustration of his e46 advance curve.

 

 

I still need a better understanding of the stock ignition operation, before trying to think beyond it. 

[silasmoon]

I believe I am still under 2 full rotations, but I definitely needed to turn the idle speed screw in more than 1/2 a turn during initial setup. Here Weber notes that "if the speed screw has to be opened 1/2 or more turns then this is also an indication of a lean condition usually requiring greater change...Please understand the need to keep throttle plate as near to closed as possible so as not to prematurely expose the transition holes. This is what causes the visible rich condition, and confirms the need to increase the jet size."

Ordered a few sizes in jets and going to play around with those when they come in. Wish me luck. 

[Simeon]

Are we sure there are no vacuum leaks?

 

Just quickly jumping back to to the ignition, the map Toby gave you looks complicated but it helps to consider it as 2 x 2 dimensional graphs rather than the 3D graph it is. The 'mechanical' advance is on the right and the 'vacuum' on the left (neither is strictly true) but the graph also shows interactions between the two. 

 

This interaction is also present with the 123 as different vacuum signals are present at different engine speeds which represents the load that the engine is under. Also bare in mind that the speed that the advance can go on and off may vary between the engine speed (mechanical) advance and the load (vacuum) so while one may be increasing advance the other may be decreasing advance so the actual advance seen by the engine remains pretty static. 

 

Hope that makes sense, some of this I am trying to get my head around too. 

[Dudeland]

If I am looking at this graph correctly, load increases with engine speed, and hence the vacuum?  

[burndog]

Vacuum decreases with increase of rpm / engine load.

 

 

Sent from my iPhone using Tapatalk

 

['76mintgrün'02]

I installed a second vacuum gauge today and connected it to manifold vacuum.  It is interesting to see how they both move and imagine what they would each do to my advance pod.  They behaved very similarly throughout a lot of the test drive, with the biggest difference being deceleration and idle.  In both of those situations, the ported drops to almost zero, while manifold remains quite high.

 

The photo was taken with the engine warming up and the choke still on.  As soon as I kicked it down, the ported vacuum (left) fell to about zero, thus retarding the ignition by 12*.

I reread some vacuum advance theory I found online.  Some people insist you want full advance at idle and that  only manifold vacuum should be used.  They claim ported vacuum was introduced with emissions concerns in mind.  

 

here's one http://chevellestuff.net/tech/articles/vacuum/port_or_manifold.htm

and another  http://www.hotrod.com/articles/set-ignition-curves-create-optimal-performance/ 

 

Well, the pod I am using gives its advance between 5-11"Hg and was designed/installed as part of an emissions package.  What I am trying to figure out is how to best put it to use for overall performance gains.  Fuel economy and power both interest me, but I do not expect to really feel the difference that vacuum makes.  

 

This pod does not work with JimK's manifold vacuum signal suggestions.  That's where Simeon gets to play outside the box.  If I don't want an additional 12* advance at idle, ported is my only option (with this pod).

 

I realize that this is a 123 thread, but the theory should be the same, whether using weights, springs, diaphragms and things, or pyuter brains and cell phones.

 

Sorry I do not have more insights to share.  I mostly wanted to show off my fancy new console insert.

Tom

EDIT: I noticed that the reason my other gauge was sticking, was because the plastic lens was rubbing on the center of the needle.  I flipped it over and it works fine now.  So, I couldn't resist swapping it into my console, since it goes better with the other white faced gauge... even though it is all temporary.

Edited May 20, 2017 by '76mintgrun'02

[Dudeland]

Both gauges look really nice. I thought I was a little old school for putting one in my car. 

 

The more I am on the forum, the more it feels like home. !

 

 

[silasmoon]

First off - awesome and thanks for all the research being done in this thread. The ported vs. manifold vacuum debate was always sold to me as ported was a poor attempt to burn off emissions, where as manifold was the proper concept designed to assist the engine's timing. That's why it struck me as odd that the 123 forces one to maintain a 29 kPa 0.0 point in the curve when most vacuum pods on cars start around 16 kPa. Indeed when idling mine shows about 16-20 kPa. The advantage therein would be a slightly advanced timing would allow me to lower my idle speed screw and possibly prevent run-ons.  

[silasmoon]

On 5/18/2017 at 0:48 AM, Simeon said:

Are we sure there are no vacuum leaks?

I tightened down the carb on the manifold, as there was a little bit of play still in one of the screws. So far so good, although I think my break booster is about to visit the great landfill in the sky. 

[jimk]

This discussion is mixing what was possible with a carburetor/vacuum diaphragm pot on a distributor and what the pressure sensor is meant to read in the 123 distr.  The 123 distr has a MAP sensor that reads absolute pressure.  The pot only reads gauge pressure.  The 123 vacuum advance plot graph in the initial post is set up for a MAP (Manifold Atmosphere Sensor) sensor reading absolute pressure.

In case readers are not up on what absolute pressure is, 0 kPa is outer space, no pressure, full vacuum.  101.4 kPA (or thereabouts) is std sea level atmosphere pressure, no vacuum.    Absolute pressure read on the MAP sensor is also Atmospheric pressure + gauge pressure.  Keep in mind gauge pressure can be positive + or negative -, (keep your algebra addition correct).

 

So the MAP sensor reads the pressure in the manifold and indicates load as was shown on the above test. However, in the demonstration test, a gauge was used to show it. 

 

Also demonstrated above, is that ported pressure sensed at closed throttle has no reduced pressure (partial vacuum) and therefore no advance over and above the mechanical or static advance might be.  And at partial (or low load) throttle open, the maximum reduced pressure is achieved at the port in the carb (because the velocity is the highest thru the opening between the plate and carb wall) and the total ignition can be greater.  As load is increased the port pressure returns close to atmospheric pressure but not quite.  Yes, manifold pressure is greatest at idle and gradually reduced to very near atmospheric WOT.  (ITBs excepted.  Manifold pressure for ITBs can be quite different).

 

The timing map Toby posted has other activity involved, probably variable cam timing as well as engine speed and load and is a 3 dimensional plot, engine speed, manifold pressure and ignition advance,  The advance plots for the 123 and the mechanical/vacuum advance distr advance are 2d plots (speed vs mechanical advance and vacuum vs pressure)

[ray_]

4 minutes ago, jimk said:

This discussion is mixing what was possible with a carburetor/vacuum diaphragm pot on a distributor and what the pressure sensor is meant to read in the 123 distr.  The 123 distr has a MAP sensor that reads absolute pressure.  The pot only reads gauge pressure.  The 123 vacuum advance plot graph in the initial post is set up for a MAP (Manifold Atmosphere Sensor) sensor reading absolute pressure.

In case readers are not up on what absolute pressure is, 0 kPa is outer space, no pressure, full vacuum.  101.4 kPA (or thereabouts) is std sea level atmosphere pressure, no vacuum.    Absolute pressure read on the MAP sensor is also Atmospheric pressure + gauge pressure.  Keep in mind gauge pressure can be positive + or negative -, (keep your algebra addition correct).

 

So the MAP sensor reads the pressure in the manifold and indicates load as was shown on the above test. However, in the demonstration test, a gauge was used to show it. 

 

Also demonstrated above, is that ported pressure sensed at closed throttle has no reduced pressure (partial vacuum) and therefore no advance over and above the mechanical or static advance might be.  And at partial (or low load) throttle open, the maximum reduced pressure is achieved at the port in the carb (because the velocity is the highest thru the opening between the plate and carb wall) and the total ignition can be greater.  As load is increased the port pressure returns close to atmospheric pressure but not quite.  Yes, manifold pressure is greatest at idle and gradually reduced to very near atmospheric WOT.  (ITBs excepted.  Manifold pressure for ITBs can be quite different).

 

The timing map Toby posted has other activity involved, probably variable cam timing as well as engine speed and load and is a 3 dimensional plot, engine speed, manifold pressure and ignition advance,  The advance plots for the 123 and the mechanical/vacuum advance distr advance are 2d plots (speed vs mechanical advance and vacuum vs pressure)

 

 

Come visit.

 

;-)

[silasmoon]

On 5/13/2017 at 11:05 AM, jimk said:

The map above being in kPa absolute (which is what the world has gone to in EFI and dumped the inHg), the idle will be at about 45kPa unless you have a hot cam.

Set the vacuum advance to zero at about 46kPa and below, full 7degrees vacuum advance at 53kPa and hold it til 75kPa, then straight line it to zero vacuum advance at 85kPa.  You reach high cylinder loads around 85kPa and above manifold pressure.

 

Going back and reading this because of an issue I am having wherein the car dies when I quickly transition from main circuit to idle circuit on my carb, and I think it has to do with some sort of odd vacuum advance setting possibly. My car pulls about 20kPa when idling due to the 292 cam it would seem. Since the 123 won't go lower than 29kPa, should I move the 46kPa point to say 30kPa? My current settings are exactly as you described and are shown below:

 

['76mintgrün'02]

I have a switch on my vacuum advance pod, so I can turn it off while driving, to note the difference.

It is very subtle, despite giving 12*.  I doubt the vacuum feature has much to do with your problem.

In your other post tonight, you said you tried it with and without the vacuum advance and it still did it both ways, correct?

 

I like this thread.  I had no idea you'd be adding atmospheric pressure to the equation.  I thought gauge pressure was all there was to it.  It is a little tricky to all be speaking the same language on this topic.  I hope I am not adding confusion, as I try to sort out my own.

Tom

 

[jimk]

9 hours ago, silasmoon said:

My car pulls about 20kPa when idling due to the 292 cam it would

It appears you are still speaking gauge pressure, while the settings are in absolute pressure.  It is not possible for a car to idle at 20kPa, -20 kP gauge maybe.  The shot of the instrument panel shows -5inHg (vacuum gauge), that equates to -16.9 kp or 101.3kpA - 16.9kP = 84.4 kPa.  That tells me the tap point where the vacuum tube which is supposed to read manifold pressure, is somewhere on the carb, not the manifold.

If the gauge setting could be made to display in absolute pressure, kPa then it would read the same as the settings table and the numbers would more easily understood.  If a vacuum gauge is also being used, it reads gauge pressure, not absolute, i.e. outer space is 0 absolute, sea level is 14.7psia (101.3 kPa) while the gauge siting at sea level with a long sensing tube to outer space would read a minus 14.7 psig (or 29.92 inHg vacuum) and at sea level a gauge reads with the tube open to atmosphere, 0 inHg (or 0 Psig).

Edited May 31, 2017 by jimk

[silasmoon]

Okay that is all starting to make sense now. What a extremely confusing user interface though. Regarding the port on my carb, I had figured it was still a "manifold" port, that is, below the throttle plates. I will check if my manifold has another port, but I think it's just for the brake booster. 

Regarding my initial question though, you made the assertion that my idle would be 45kPa unless I had a hot cam so a normal manifold vacuum gauge reading would be -16inHg (100 kPa - 55kP = 45kPa). So since a hotter cam = less vacuum at idle, would I shift the initial vacuum point up from 45kPa to 50? I can take a reading after work today if I have a port on the manifold itself. 

Thanks for breaking this down for me, I had no idea about the variations in these units.