Ported or manifold distributor vacume
Thread Starter
5th Gear
Joined: Feb 2011
Posts: 5
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From: Sterling Virginia
Ported or manifold distributor vacume
I am in the process of installing a new eddelbrock carberator on my 1975 c3. I am not sure if I am to connect the vacume to the ported or manifold connection on the carberator. I already removed the old carberator. Anyone know what the stock distributor is?
Melting Slicks
Joined: Aug 2010
Posts: 2,487
Likes: 10
From: Palm Beach Florida
St. Jude Donor '12
The vac. source is suppose to be ported if car was equiped with an air pump. But it is better to have it manifold. I will be doing this to mine tuesday. From manifold the car should run a bit cooler and just a tad better on pedal reaction. Ported allowed the air pump to pump air into the exhaust to make it burn more completly. This in turn caused the motor to heat up more.
Melting Slicks
Joined: May 2009
Posts: 3,079
Likes: 314
2024 C3 of the Year Finalist - Unmodified
With a ported vacuum source you should have no distributor vacuum advance at idle, only as you begin to come off idle. This was done for emissions. Everyone says these cars will idle better, not stumble and run cooler on full manifold vacuum. I'm getting ready to switch mine...as soon as I find a decent tee fitting for my intake manifold.
Team Owner
Joined: Aug 1999
Posts: 30,995
Likes: 98
From: Ontario
With a ported vacuum source you should have no distributor vacuum advance at idle, only as you begin to come off idle. This was done for emissions. Everyone says these cars will idle better, not stumble and run cooler on full manifold vacuum. I'm getting ready to switch mine...as soon as I find a decent tee fitting for my intake manifold.
Last edited by Paul L; Apr 24, 2011 at 07:40 PM.
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
I agree with everything above. Yes the photo above shows the best source for manifold vacuum on Q-Jet equipped cars.
There are several good reasons to run manifold vacuum; cooler running engine in stop-&-go traffic is a good one,..better off-idle performance is another.
Paul74,..do you struggle with inconsistent choke settings (choke valve opening too slowly) and fast-idle settings? If so, I have a fix. See that thin wire spring behind the pull-off diaphragm? Remove it. With a pair of needle nose, simply grab an end of the spring then pull and it will unravel. That spring fights the divorced choke thermal spring (attached to the intake). If your choke works fine, leave it.
There are several good reasons to run manifold vacuum; cooler running engine in stop-&-go traffic is a good one,..better off-idle performance is another.
Paul74,..do you struggle with inconsistent choke settings (choke valve opening too slowly) and fast-idle settings? If so, I have a fix. See that thin wire spring behind the pull-off diaphragm? Remove it. With a pair of needle nose, simply grab an end of the spring then pull and it will unravel. That spring fights the divorced choke thermal spring (attached to the intake). If your choke works fine, leave it.
Last edited by 73, Dark Blue 454; Apr 25, 2011 at 12:30 AM.
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Melting Slicks
Joined: Aug 2010
Posts: 2,487
Likes: 10
From: Palm Beach Florida
St. Jude Donor '12
[QUOTE=paul 74;1577432816]Take manifold vacuum off the carb. Usually the port under the choke. Use a vacuum gauge to check it at idle.
Originally Posted by pws69
My point is that MANIFOLD VACUUM and PORTED VACUUM come from the SAME SOURCE - and that is the MANIFOLD and it is BELOW the throttle plates - NOT above the throttle plates. - but the actual channel routes BELOW the throttle plates - with a "valve" if you will, that opens immediately "off idle".
Reply from Lars back in 2006.
That's not correct. Ported vacuum is pulled from a hole located just above the throttle plate when the throttle is closed - flip any carb with ported vacuum upside-down, and you can see the ported vacuum hole drilled in the throttle bore just above the throttle plate. The ported vacuum hole is not exposed to manifold vacuum when the throttle is closed. As the throttle is opened, the ported vacuum hole is exposed to manifold vacuum - the throttle plate itself acts as the switch - there is no separate "switch."
Originally Posted by pws69
My point is that MANIFOLD VACUUM and PORTED VACUUM come from the SAME SOURCE - and that is the MANIFOLD and it is BELOW the throttle plates - NOT above the throttle plates. - but the actual channel routes BELOW the throttle plates - with a "valve" if you will, that opens immediately "off idle".
Reply from Lars back in 2006.
That's not correct. Ported vacuum is pulled from a hole located just above the throttle plate when the throttle is closed - flip any carb with ported vacuum upside-down, and you can see the ported vacuum hole drilled in the throttle bore just above the throttle plate. The ported vacuum hole is not exposed to manifold vacuum when the throttle is closed. As the throttle is opened, the ported vacuum hole is exposed to manifold vacuum - the throttle plate itself acts as the switch - there is no separate "switch."
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
[QUOTE=daanbc;1577434460]
Not sure what Lars is referring to above.
Yes, vacuum pulled from below the throttle blades is manifold vacuum, and ported vacuum is sourced from above.
The port (tube) shown above doesn't penetrate straight into the venturi. There's a passage to a source below the TB's in the throttle plate.
You can use a vacuum gauge to confirm the above.
There is another source for vacuum on the front of the q-jet toward the driver's side. This is the usual source for ported vacuum. Block that one and use the manifold source.
Take manifold vacuum off the carb. Usually the port under the choke. Use a vacuum gauge to check it at idle.
Originally Posted by pws69
My point is that MANIFOLD VACUUM and PORTED VACUUM come from the SAME SOURCE - and that is the MANIFOLD and it is BELOW the throttle plates - NOT above the throttle plates. - but the actual channel routes BELOW the throttle plates - with a "valve" if you will, that opens immediately "off idle".
Reply from Lars back in 2006.
That's not correct. Ported vacuum is pulled from a hole located just above the throttle plate when the throttle is closed - flip any carb with ported vacuum upside-down, and you can see the ported vacuum hole drilled in the throttle bore just above the throttle plate. The ported vacuum hole is not exposed to manifold vacuum when the throttle is closed. As the throttle is opened, the ported vacuum hole is exposed to manifold vacuum - the throttle plate itself acts as the switch - there is no separate "switch."
Originally Posted by pws69
My point is that MANIFOLD VACUUM and PORTED VACUUM come from the SAME SOURCE - and that is the MANIFOLD and it is BELOW the throttle plates - NOT above the throttle plates. - but the actual channel routes BELOW the throttle plates - with a "valve" if you will, that opens immediately "off idle".
Reply from Lars back in 2006.
That's not correct. Ported vacuum is pulled from a hole located just above the throttle plate when the throttle is closed - flip any carb with ported vacuum upside-down, and you can see the ported vacuum hole drilled in the throttle bore just above the throttle plate. The ported vacuum hole is not exposed to manifold vacuum when the throttle is closed. As the throttle is opened, the ported vacuum hole is exposed to manifold vacuum - the throttle plate itself acts as the switch - there is no separate "switch."
Yes, vacuum pulled from below the throttle blades is manifold vacuum, and ported vacuum is sourced from above.
The port (tube) shown above doesn't penetrate straight into the venturi. There's a passage to a source below the TB's in the throttle plate.
You can use a vacuum gauge to confirm the above.
There is another source for vacuum on the front of the q-jet toward the driver's side. This is the usual source for ported vacuum. Block that one and use the manifold source.
Last edited by 73, Dark Blue 454; Apr 25, 2011 at 10:21 AM.
Melting Slicks
Joined: Aug 2010
Posts: 2,487
Likes: 10
From: Palm Beach Florida
St. Jude Donor '12
[QUOTE=73, Dark Blue 454;1577435600]
Not sure what Lars is referring to above.
Yes, vacuum pulled from below the throttle blades is manifold vacuum, and ported vacuum is sourced from above.
The port (tube) shown doesn't penetrate straight into the venturi. There's a passage to a source below the TB's in the throttle plate.
You can use a vacuum gauge to confirm the above.
There is another source for vacuum on the front of the q-jet toward the driver's side. This is the usual source for ported vacuum. Block that one and use the manifold source.
Thats what he is refering about, and what I was talking about. Some folks look at that port and think it is manifold vac. when in reality that one in the front is ported.
Not sure what Lars is referring to above.
Yes, vacuum pulled from below the throttle blades is manifold vacuum, and ported vacuum is sourced from above.
The port (tube) shown doesn't penetrate straight into the venturi. There's a passage to a source below the TB's in the throttle plate.
You can use a vacuum gauge to confirm the above.
There is another source for vacuum on the front of the q-jet toward the driver's side. This is the usual source for ported vacuum. Block that one and use the manifold source.
Thats what he is refering about, and what I was talking about. Some folks look at that port and think it is manifold vac. when in reality that one in the front is ported.
Team Owner
Joined: Aug 1999
Posts: 30,995
Likes: 98
From: Ontario
I agree with everything above. Yes the photo above shows the best source for manifold vacuum on Q-Jet equipped cars.
There are several good reasons to run manifold vacuum; cooler running engine in stop-&-go traffic is a good one,..better off-idle performance is another.
Paul74,..do you struggle with inconsistent choke settings (choke valve opening too slowly) and fast-idle settings? If so, I have a fix. See that thin wire spring behind the pull-off diaphragm? Remove it. With a pair of needle nose, simply grab an end of the spring then pull and it will unravel. That spring fights the divorced choke thermal spring (attached to the intake). If your choke works fine, leave it.
There are several good reasons to run manifold vacuum; cooler running engine in stop-&-go traffic is a good one,..better off-idle performance is another.
Paul74,..do you struggle with inconsistent choke settings (choke valve opening too slowly) and fast-idle settings? If so, I have a fix. See that thin wire spring behind the pull-off diaphragm? Remove it. With a pair of needle nose, simply grab an end of the spring then pull and it will unravel. That spring fights the divorced choke thermal spring (attached to the intake). If your choke works fine, leave it.
To bring up an old subject, off-idle stumble mentioned above, I have a Viton accelerator pump not subject to ethanol or gasoline additive rot. This matter was studied at length by Lars and Cliff Ruggles about a year ago. The old-style rubber accelerator pump diaphragms do not last long with the gasolines of today. Sometimes ported vacuum gets the blame when in fact it can be just one factor of several.
http://forums.corvetteforum.com/c3-t...t-quality.html
Team Owner
Joined: Aug 1999
Posts: 30,995
Likes: 98
From: Ontario
Here is another pic. For reference that is a #7044206 Q-Jet on a #340261 intake, correct for a 1974 L-48 with TH400. The paint is POR-15 engine black (my NOM engine is black).
Last edited by Paul L; Apr 25, 2011 at 08:18 AM.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Ported may be specified, but manifold often yields better performance results. Again, to the timing sticky...
http://forums.corvetteforum.com/c3-t...long-post.html
...tho you'll need to navigate back to the OP.
http://forums.corvetteforum.com/c3-t...long-post.html
...tho you'll need to navigate back to the OP.
Last edited by TheSkunkWorks; Apr 25, 2011 at 12:16 PM.
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
Get your vacuum gauge and find a manifold source and plug the ported source. Your curb idle will double so adjust it at the carb. You'll enjoy better idle quality and better off-idle performance.
Safety Car
Joined: Apr 2008
Posts: 3,908
Likes: 10
From: Charleston SC
Hmmm, I think I'll try it. I'm also guessing that I'll have to re-time the engine after that? Right now I get the feeling that my vacuum advance canister isn't doing anything. The manual that came with it said to unscrew the internal regulator all the way out for most applications, unless you get spark knock and then to screw it in 2 turns..
Safety Car
Joined: Apr 2008
Posts: 3,908
Likes: 10
From: Charleston SC
From Summit's HEI manual:
When I rev the engine, it doesn't seem to make any audible or noticeable difference if the vacuum line is connected or disconnected...does this sound right?
11. Connect the vacuum hose to the vacuum advance canister. The vacuum line for
the vacuum advance should be connected to a ported/timed vacuum source. To
properly identify this port, you will need to have a vacuum gauge in hand and test
any small (3/16) vacuum ports on the carburetor. The port that does not have
vacuum at idle is the ported/timed source and is the correct port for the advance
of the distributor to be connected to.
the vacuum advance should be connected to a ported/timed vacuum source. To
properly identify this port, you will need to have a vacuum gauge in hand and test
any small (3/16) vacuum ports on the carburetor. The port that does not have
vacuum at idle is the ported/timed source and is the correct port for the advance
of the distributor to be connected to.
