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Unread 05-15-2013, 04:08 PM   #1
Bubba Ray
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Manifold vacuum from BBD?

I would like to try using manifold vacuum for the dizzy. I cannot find info about a manifold vacuum port on my non-stepped BBD. I have searched and do not see what port to use. Thanks for any help.

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Unread 05-15-2013, 04:23 PM   #2
Matt1981CJ7
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Any port below the throttle plate on the carb is manifold. If it's got full vacuum at idle it's manifold.

Also, it doesn't have to come from the carb. Any port on the intake manifold will work.

Good luck,

Matt
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Unread 05-15-2013, 04:24 PM   #3
Balvar24
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Have you looked on the manifold?
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Unread 05-16-2013, 07:41 PM   #4
Bubba Ray
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No. I was thinking that it had to come from carb. I guess the clue was manifold vacuum. Will play with it this weekend.
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Unread 05-16-2013, 07:51 PM   #5
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Pick up a vac guage, then do a search on how to use it. You will be amazed on what you can find out about your engine.
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Unread 05-17-2013, 02:51 AM   #6
John Strenk
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All 3 ports on this BBD are manifold vacuum.

One is used for the choke pull down.
One for the is for the PCV valve ,
One is used for a vacuum leak to cause the owner considerable problems and headache.
See the clever cracked rubber cap that is only visible from the bottom.

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Unread 05-17-2013, 06:53 AM   #7
thompsoj22
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why so much advance?

i have watched a few threads and most of you all say manifold vac to give full vac advance effect at idle is what you want. mine is ported and other than a lower idle setting i cant tell any difference. i think i read matt said lower cylinder temps and just plain better for the engine. can you guys explain just one more time?
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Unread 05-19-2013, 08:31 PM   #8
Bubba Ray
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I thought that manifold vacuum came from below the throttle plates. The ports shown by John look to be above the plates or am I looking at this wrong?
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Unread 05-19-2013, 09:48 PM   #9
fourbtgait
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Quote:
Originally Posted by thompsoj22
i have watched a few threads and most of you all say manifold vac to give full vac advance effect at idle is what you want. mine is ported and other than a lower idle setting i cant tell any difference. i think i read matt said lower cylinder temps and just plain better for the engine. can you guys explain just one more time?
The three John shows do produce manifold vacumn.

I noticed when plotting my curve that the ported vacumn on my BBD did not produce any vacumn until the engine rpm's were at 1,700 or such. Meaning that from 650 rpm to 1,700 rpm, the dizzy was only operating on centrifigal advance. I questioned this and Shawn said he had seen/heard the same thing on a BBD.

I have it running on manifold vacumn for the past 6 months or so, notice it runs easier, picks up speed better. Lightening difference? No. But noticable.

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Unread 05-20-2013, 05:05 AM   #10
John Strenk
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Quote:
Originally Posted by Bubba Ray View Post
I thought that manifold vacuum came from below the throttle plates. The ports shown by John look to be above the plates or am I looking at this wrong?
Yes, they are below the throttle plate but not directly below or in the main bore. Kinda like a tributary off the manifold.
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Unread 05-20-2013, 08:59 AM   #11
Matt1981CJ7
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Quote:
Originally Posted by thompsoj22 View Post
i have watched a few threads and most of you all say manifold vac to give full vac advance effect at idle is what you want. mine is ported and other than a lower idle setting i cant tell any difference. i think i read matt said lower cylinder temps and just plain better for the engine. can you guys explain just one more time?
The first thing to remember is the ONLY difference between manifold and ported vac is at idle. Otherwise, both vac sources operate exactly the same.

The second thing to remember is a lean air/fuel mixture takes longer to burn than a rich one. I'll get back to that....

Don't confuse centrifugal advance and vacuum advance. They perform different functions. Centrifugal advance is a function of engine RPMs. As the compression stroke speeds up with higher RPMs, the centrifugal advance speeds up the timing of the spark, so that peak cylinder pressure remains just after TDC. Therefore, centrifugal advance has a direct affect on power and performance under load.

Vacuum advance, on the other hand, doesn't affect the power curve, at all. It is a function of engine vacuum pressure. When your engine is idling, or you are cruising with low load, vacuum pressure is high resulting in lean A/F mixtures. Since a lean mixture takes longer to burn, the vac advance speeds up the timing of the spark for a more complete burn under high vac pressure situations.

Here's where the major difference is. Manifold vac is present at idle, ported isn't. Ported vac advance retards timing at idle, which creates an incomplete burn in the combustion chamber. This does reduce NOX emissions, but it also significantly increases exhaust temps, which in-turn increases coolant and overall engine temps. With increased exhaust temps, engineers found they could inject fresh air (AIR pump) into the exhaust stream and increase the after-burn of hydrocarbons in the exhaust.

So, ported vac advance does reduce emissions, but it's at the expense of idle performance and efficiency. It takes away advance when your engine needs it most....under the high vacuum, lean mixture situation at idle. It increases engine temps, reduces fuel efficiency, and makes a weaker less stable idle.

You don't have to take my word for any of this. I, too, was skeptical until I started experimenting and recording my own results. Once I had a clear understanding of spark timing, the choice between manifold and ported was easy.

Good luck,

Matt
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Unread 05-20-2013, 10:28 AM   #12
John Strenk
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Well the vacuum is not exactly the same above idle. I hooked up several vacuum gauges up to my ported and manifold vacuum at the same time and they were all over the place. I'll look to see if I could find the video I took

I could get higher vacuum on some ported ports at speed than I could get out of the manifold vacuum at any time. This is because were the ported vacuum is located in the air flow.
There were differences between manifold ports on the carb and on the intake, I think size and location, location, location makes all the difference here.
Ported ports on carbs like the YF which has 2 ported vacuum ports also vary in pressure. You would go crazy tying to map them all out.

but all ported vacuum had NO vacuum at idle. It doesn't exactly retard the timing any, unless you were running a ford pinto distributor, but doesn't advance the timing any.

This is most noticeable with the popping exhaust sound you hear when you let off the gas at speed. Lean mixture, no advance, unburnt fuel building up in exhaust....
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Unread 05-20-2013, 04:55 PM   #13
Matt1981CJ7
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John,

I'd love to see that video.

When I hooked up gauges to both manifold and ported (both ports were located on the carb, with similar sized nipples) they acted identical in all driving situations, except at idle. I suppose different port locations and nipple sizes may produce slightly different results. But, in general, I found the two vac sources operate the same, except for at idle.

It should also be noted that post-1982 Jeep distributers were setup to run on manifold vac, only.

Matt
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Unread 05-21-2013, 03:04 AM   #14
John Strenk
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dont know it this will work...
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Unread 05-21-2013, 05:13 AM   #15
Matt1981CJ7
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John,

Thanks for digging that up. I'm assuming the gauge on the left was ported?

I don't recall mine having the wild swings to zero when I shifted, but otherwise you got similar results as I did.

Here's an article that supports my findings. The author hooked up MAP sensors to both vacuum sources and a throttle position sensor to a data logger. As you can see, the vacuum curves followed the same path in relationship to throttle position at all times except under little to no throttle.

Matt
vacuum_chart.jpg  
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