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Unread 05-21-2013, 05:22 AM   #16
John Strenk
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Quote:
Originally Posted by Matt1981CJ7 View Post
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
Actually no, that's manifold vacuum, right to the nipple on the intake manifold near the #6 runner.
The one on the right is one of the ported vacuum on the YF.

Like I say, location of the port makes a big big difference. Now I'll have to dust off my data logger and a bunch of MAP sensors and run some experiments also....

Would be interesting to see if there is any difference in manifold vacuums between the one under the carb to the PB booster or on the carb or on the runner.

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Unread 05-21-2013, 05:28 AM   #17
Matt1981CJ7
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John,

If I had the equipment, I'd do some recording, as well. I love learning about this stuff.

Edit: I'm beginning to think you may be on to something about there being differences depending on port location. I currently have a gauge hooked to manifold off the carb. I don't get the swings to zero that your video shows coming from the intake runner.

Matt
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Unread 05-21-2013, 06:13 AM   #18
thompsoj22
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back to manifold vac!

Quote:
Originally Posted by Matt1981CJ7 View Post
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
thanks for the info, the unburnt fuel pop in the exhaust is more pronounced with the way i have it hooked up. so much great experience on this forum. ill go back to manifold vac and come out to 2.5 on idle mixture screws which produced the smoothest idle.
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Unread 05-21-2013, 06:42 AM   #19
swatson454
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I've had the same experience as John. I've done the test (manifold vs ported) on a BG Demon, Edelbrock, BBD and a Weber 38 and they all behaved a little different. The weirdest one for sure was the BBD. IIRC, it would pull up to 7 inches and then hold that for a bit before shooting up to 14 or so. It just had "smog test" written all over it. It also used a slot in the venturi where the other carbs used a regular aperture.

Either way, none of them behaved like manifold vacuum until you were pretty deep in the throttle. IIRC (again) the Demon and the Weber got inline with manifold sooner than the others.


Shawn
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Unread 05-21-2013, 06:56 AM   #20
Matt1981CJ7
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Shawn,

What's your take on John's video that shows manifold vac dropping to zero while shifting?

That's exactly opposite of how my manifold vac works. Mine drops near zero under heavy load, but then spikes back up when I depress the clutch to shift.

Matt
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Unread 05-21-2013, 06:58 AM   #21
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I've only had a half-cup of coffee and it made my eyes cross. I'll look again shortly


Shawn
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Unread 05-21-2013, 07:10 AM   #22
Matt1981CJ7
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Errr...never mind, I haven't had enough java, either.

I just realized John's gauge reads opposite of mine. DOH!

Matt
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Unread 05-21-2013, 07:17 AM   #23
Neuner-9R
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Quote:
Originally Posted by swatson454 View Post
I've had the same experience as John. I've done the test (manifold vs ported) on a BG Demon, Edelbrock, BBD and a Weber 38 and they all behaved a little different. The weirdest one for sure was the BBD. IIRC, it would pull up to 7 inches and then hold that for a bit before shooting up to 14 or so. It just had "smog test" written all over it. It also used a slot in the venturi where the other carbs used a regular aperture.

Either way, none of them behaved like manifold vacuum until you were pretty deep in the throttle. IIRC (again) the Demon and the Weber got inline with manifold sooner than the others.


Shawn
x2 for the weber. I charted out my ported but not the manifold which I'd still like to do if I get the time.
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Unread 05-21-2013, 07:28 AM   #24
John Strenk
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Quote:
Originally Posted by swatson454 View Post
I've only had a half-cup of coffee and it made my eyes cross. I'll look again shortly


Shawn
You should of been the one driving and filming.

This was before iPhones....

i get a head ache watching it....

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