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123+ and Idle Vacuum Advance


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As of late I have been pondering vacuum advance at idle and its effects on idle speed adjustment, run-on and responsiveness off the line. My first question is, "How much vacuum should an engine pull at idle?" I have my 123+ vacuum advance connected to my 38/38 ported vacuum port. Idling at 900 - 1100 RPM I pull about 10-13 kPa / 4-6 in/Hg of vacuum. A lot of people on the forum have their vacuum kick in at around 1,400 RPMs, but I was curious if it would be better to set it at 500 RPMs? That way you could get some vacuum advance at idle, which would increase RPM and let you back off your throttle idle speed screw - possibly preventing a mechanically induced run-on situation. 

The issue is the 123+ requires a 29 kPa 0.0 advance point. I also figured it might be dangerous to run vacuum advance at a lower vacuum value, since at WOT the vacuum advance wouldn't let off soon enough and could dynamite your engine with +7 degrees of advance. 

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Edited by silasmoon
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5 minutes ago, silasmoon said:

"How much vacuum should an engine pull at idle?" I have my 123+ vacuum advance connected to my 38/38 ported vacuum port.

 

I have a vacuum gauge (and my distributor) connected to ported vacuum on my 32-36.

The gauge sits nicely on the ashtray and the little clip on the back locks onto the cigarette holder  : )

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It is simply T-ed into the vacuum line that feeds the pod.

 

I have been watching the needle and have learned that the vacuum pod is giving all of its 6* (12* crank) during >90% of my driving.

 

At idle, it sees zer0 vacuum advance.

 

fwiw, my current distributor is set at 42* total mechanical, with 6* at 850 rpm idle and the BB at 2150 rpm.  

(weights, springs, diaphragm, points, condenser)

(Curve drawn at distributor speed/advance = 38* total (crank))

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The 6 degrees is just the additional advance added by the vacuum. Little bit busy right now but want to try and share my thoughts and learnings on this. Give me a day or two. 

rtheriaque wrote:

Carbs: They're necessary and barely controlled fuel leaks that sometimes match the air passing through them.

My build blog:http://www.bmw2002faq.com/blog/163-simeons-blog/

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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.

A radiator shop is a good place to take a leak.

 

I have no idea what I'm doing but I know I'm really good at it.

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53kPa puts is above the idle manifold pressure.  The vacuum advance should not be in effect at idle. 

75kPa is about what the manifold pressure will be when there is enough load that you would want to be operating on the advance curve map.

Yes a cam with more overlap would raise the idle manifold pressure.

I would suggest you use a timing light to find the idle manifold pressure by shooting the crank pulley while using the values I gave above.  With the engine idling, lower the point 2 number (the 46kPa point) until the timing shows an advance per the light on the pulley.

If the starting number of 46kPa was too low as shown by the above light test, then raise the point 2 number to 47 and so on until the timing doesn't change.  It will tell that you have hit on the idle manifold pressure you have.

If you need to raise the point 2 number from 46 then to give some margin on point 3 raise that point the same amount that you had to raise the point 2 number.

A radiator shop is a good place to take a leak.

 

I have no idea what I'm doing but I know I'm really good at it.

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Interesting, I appreciate the explanation. The 123+ Bluetooth doesn't let you touch the curve while the engine is on, so this will be a bit back and forth. Couldn't I merely observe the vacuum gauge on the dashboard of the 123+ app while it's idling to determine the idle vacuum? When I check it it shows about 17kPa at 900 rpms / 34kPa at 2,000 rpms. 

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Edited by silasmoon
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On 5/12/2017 at 5:34 PM, silasmoon said:

I have my 123+ vacuum advance connected to my 38/38 ported vacuum port. Idling at 900 - 1100 RPM I pull about 10-13 kPa / 4-6 in/Hg of vacuum.

 

I believe JimK is referring to manifold vacuum, as opposed to ported.

 

I may be mistaken, but I am pretty sure you should see no vacuum signal on the ported nipple at idle, unless the idle speed screw is opening the throttle plates.

When the choke is on, I see about 11 inches and that drops to zero, once the choke is off.

(I have my pod connected to ported vacuum).

 

A little research told me that 11 inches Hg = 37.2503 Kilopascal.  (1"Hg = 3.38639 kPa)

 

My vacuum pod gives up to 6* of pull (12* crank) between 5 - 11"Hg (~17 - 37kPa)

 

People who argue that the vacuum pod should be connected to manifold vacuum confuse me.  That brings in full vac advance at idle.

With the 123, you can control what the signal does and set the parameters, so manifold vacuum is an option; such as Jim is describing. 

 

Based on Jim's kPa suggestions for manifold vacuum, if I was going to make a vacuum pod for my distributor, which was going to be connected to manifold vacuum, I would want it to move between 15.5"Hg (53kPa) and 22"Hg (75kPa).  That would require a much stronger spring.  

 

Do people ever set up the 123 vac signal to ported vacuum?

If so, would they be trying to achieve something similar to the stock pod, in terms of signal strength/degrees of advance?

 

On 5/12/2017 at 7:00 PM, silasmoon said:

 

BTDC at idle seems low?

 

seems low to me too.  It was at 11* with my other distributor.  I had to lean out (turn in) the mixture screw about a third of a turn for the new setting.  It idles pretty well though.

 

Our engines are a bit different, so what works for me is not what you're after.  I just think there are some interesting comparisons here.

     

 

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1. It appears that the vacuum advance curve presented are for a manifold connected system and don't apply to a carburetor ported vacuum.

2. It's hard to read the units on the pics of the dashboard, but aren't they showing gauge pressure and in psi not kPa, absolute pressure in kiloPascals, while the graph is in kPa?

Set the same units on the gaugeboard display as the graph and watch look for the normal reading at idle, then set the values in the table.

A radiator shop is a good place to take a leak.

 

I have no idea what I'm doing but I know I'm really good at it.

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1. Interesting - I will email 123+ and see what they say regarding which is the *proper* port. I had it attached to ported, but perhaps that's wrong?

2. The gauge has a poor UI for sure. Above 0 is measures it in PSI (boost) and below 0 it measure it in inHg (vacuum). You can't actually configure the curve to be anything matching the gauge. I've filed a bug to have that fixed. 

@'76mintgrun'02 I will warm her up and double check tonight that the throttle plates aren't open during idle. I was always told ported vacuum, but I emailed them to ask. 

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33 minutes ago, silasmoon said:

2. The gauge has a poor UI for sure. Above 0 is measures it in PSI (boost) and below 0 it measure it in inHg (vacuum). You can't actually configure the curve to be anything matching the gauge. I've filed a bug to have that fixed. 
 

Thank you for filing the bug report on that--it annoys me, too.  The vacuum display shouldn't convert to US measurements since the value is useless in the application.

 

It would be nice if they made the whole page less busy while they're at it.  And made it so that the application would remember the current settings so you didn't have to be connected to look at them.  And made a way of exporting the settings other than screenshots.  Etc...

Matthew Cervi
'71 Bavaria

'18 M2

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They're churning out hot-fixes daily. They worked with me a lot to fix the MAP function on Android to - you know - actually work. They are cranking out a new iOS version soon as well. I think after that they are going to entertain some more features, but as far as I can tell it's two people. 

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3 hours ago, MatthewCervi said:

Thank you for filing the bug report on that--it annoys me, too.  The vacuum display shouldn't convert to US measurements since the value is useless in the application.

 

It would be nice if they made the whole page less busy while they're at it.  And made it so that the application would remember the current settings so you didn't have to be connected to look at them.  And made a way of exporting the settings other than screenshots.  Etc...

 

Funnily enough, I asked for for all of these things too. 

 

Regarding the ported / straight manifold. If you note there is a setting related to what engine speed the vac curve becomes effective at. This simulates ported vacuum but then allows you to differentiate high vacuum at high revs (like letting off the throttle on the overrun). Here you might like to actually retard the ignition if you wanted. Changing this value allows you to simulate moving the point the vacuum advance comes in. This might help with a carb jet transition and is something you can't do with a real carb (apart from maybe filing throttle  plates). 

 

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. 

rtheriaque wrote:

Carbs: They're necessary and barely controlled fuel leaks that sometimes match the air passing through them.

My build blog:http://www.bmw2002faq.com/blog/163-simeons-blog/

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I totally goofed. I'm not using ported vacuum, I'm using manifold. The 38/38 port is below the throttle plate butterflies. At 1,800 RPMs its showing about 33 kPa. 

 

 

Edited by silasmoon
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