Ported vaccum v manifold vaccum

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Author Topic: Ported vaccum v manifold vaccum  (Read 1023 times)

Offline heeno

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Ported vaccum v manifold vaccum
« on: September 17, 2019, 11:16:28 AM »
Hello all,

I have a 302 windsor with holley 600 carb vac secondary. Currently I have the vaccum advance hose going to the manifold tube near the bottom of the carby. The other day I dared to switch that hose up to tube up near the top of the carby in the metering block. I think this is called the ported vaccum tube. The idle spead up considerably I noticed which leads me to ponder which port should I be linking into? Any ideas at all are appreciated. Peter

Offline BAC

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Re: Ported vaccum v manifold vaccum
« Reply #1 on: September 17, 2019, 12:44:38 PM »
The search box is your friend!

From one of the gurus here:

there is only one correct school of thought and that is ported vacuum.

if you have a huge cam, you can sometimes benefit from having more advance at idle than your car would typically be able to start with, so one way to cheat to get the needed advance without adversely affecting how it starts is to use manifold vacuum, however, since manifold vac may not be exactly the same all the time, it is better to set the timing where the engine likes it then use a timing retard box for starting the engine.

also, not all vacuum cans are adjustable and some will provide WAY more advance than you need.

for optimum perf, you should set your timing curve so it is optimal for your particular engine then use an adjustable vac can and set it to add around 5 to 6 degrees of advance.

If you want to read the whole discussion, it's here:

https://www.mustang.org.au/forum/index.php/topic,29304.msg289476.html

I assume the port at the bottom of the carby is manifold vacuum and the one near the top is ported.  You want ported.
Cheers,
Brian

Offline barnett468

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Re: Ported vaccum v manifold vaccum
« Reply #2 on: September 18, 2019, 11:25:50 AM »
lol, yup, that pretty much sums it up.  :thumb:

Offline barnett468

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Re: Ported vaccum v manifold vaccum
« Reply #3 on: September 18, 2019, 11:34:54 AM »
ok, as BAC mentioned, the ported vacuum port "should" be on the right side of your particular carb as you sit in the drivers seat. This port should have basically zero vacuum with the engine idling at 700 rpm or less. If it has enough vacuum below 700 rpm to cause the timing to advance when the dist hose is connected, something is probably wrong with the carb.

You should set the dist advance curve so it is optimal for your particular engine if you want to get the most hp, mpl, and best cooling effect from it. If you do this, you do not want the vac advance to add more than around 6 degrees of advance. You will need an adjustable vac advance can to set it to this amount if you want to do that, otherwise, if the advance curve is set to optimal, you can just plug the dist can and not use it.


Offline jiffy

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Re: Ported vaccum v manifold vaccum
« Reply #4 on: September 19, 2019, 11:09:31 PM »
odd - if I understand it there should be more vacuum in the manifold (than ported) at idle - which when connected to manifold vacuum would advance the timing and make the car idle faster.

If you connected the vacuum to ported and the idle speed increased, then that's odd...?

Are you sure you blocked off the vacuum tubes when you weren't using them?

With the dist connected to manifold vacuum, have you blocked off the ported vacuum nipple (and vise-versa when you tested ported vacuum...)?
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Offline BAC

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Re: Ported vaccum v manifold vaccum
« Reply #5 on: September 20, 2019, 08:46:55 AM »
odd - if I understand it there should be more vacuum in the manifold (than ported) at idle - which when connected to manifold vacuum would advance the timing and make the car idle faster.

I was thinking the same thing.  Unless maybe the timing was sooo over advanced with full manifold vacuum that the idle speed had started to drop again?
Cheers,
Brian

Offline barnett468

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Re: Ported vaccum v manifold vaccum
« Reply #6 on: September 20, 2019, 01:16:04 PM »
odd - if I understand it there should be more vacuum in the manifold (than ported) at idle

This is 100% correct, and a typical stock or near sock engine will have at least 15 hg of manifold (not ported) vacuum at idle, and ported should have less than around 2.
« Last Edit: September 20, 2019, 01:18:15 PM by barnett468 »

Offline heeno

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Re: Ported vaccum v manifold vaccum
« Reply #7 on: September 26, 2019, 11:04:46 AM »
Thx for those responses fellas much appreciated. Pete

 

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