Ported Vacuum.......AGAIN!

[Bruce Dorsi]

While doing some reading, I have found two contradictory definitions of ported vacuum.

One defines ported vacuum as being tapped into the venturi.  .....This method will have no vacuum at idle, but the vacuum will increase with load and rpm.

The other school of thought defines ported vacuum as being taken from just above the throttle plate.  ....This method will have no vacuum at idle, but the vacuum will increase as the throttle blades open slightly exposing the port to manifold vacuum, but then diminishes (along with manifold vacuum) as the throttle blades open further.

Is this second method what is referred to as TIMED vacuum?

Do these differing definitions cause the opposing viewpoints in the on-going MANIFOLD vs.PORTED debate?

BTW, I noticed that Edelbrock describes their carbs as being equipped with ports for TIMED and MANIFOLD vacuum.   .....Which port is which on their carbs?

FWIW, Edelbrock states that most engines need 17-25 degrees of advance at IDLE!

[Ed ke6bnl]

there are two small vacuum ports in the front of the carb the lower one is the full vacuum and the higher one from the manifold is the ported vacuum.  Ed ke6bnl

[SKR8PN]

While doing some reading, I have found two contradictory definitions of ported vacuum.

One defines ported vacuum as being tapped into the venturi.  .....This method will have no vacuum at idle, but the vacuum will increase with load and rpm.

The other school of thought defines ported vacuum as being taken from just above the throttle plate.  ....This method will have no vacuum at idle, but the vacuum will increase as the throttle blades open slightly exposing the port to manifold vacuum, but then diminishes (along with manifold vacuum) as the throttle blades open further.

Is this second method what is referred to as TIMED vacuum?

Do these differing definitions cause the opposing viewpoints in the on-going MANIFOLD vs.PORTED debate?

BTW, I noticed that Edelbrock describes their carbs as being equipped with ports for TIMED and MANIFOLD vacuum.   .....Which port is which on their carbs?

FWIW, Edelbrock states that most engines need 17-25 degrees of advance at IDLE!

Bruce,BOTH of those are considered "ported" vacuum. Your FIRST description is what you want your distributor advance connected to. The SECOND description,is used for vacuum actuated appliances such as EGR valves.
You do NOT want EGR functioning at idle,but you do need it from just off idle thru the mid-range power band,but it is not needed at higher RPM's.You also do not need EGR when the engine is cold. That explains why,sometimes, they also use a thermal switch to control the vacuum supply,to the EGR valve as well. A lot of engine designs will require a lot of static timing at idle. I used to run 17 degrees initial,and 36 degrees total timing,with all the advance"all in" by 1800 RPM's,on my 440 Dart. But this was on the track,with a Holly 850 dbl pmpr and #90 jets in all four corners.

[Mikej]

Manifold vaccuum is on the drivers side. Pull the the hose off at idle and it will be the one sucking.

[C9]

BTW, I noticed that Edelbrock describes their carbs as being equipped with ports for TIMED and MANIFOLD vacuum.   .....Which port is which on their carbs?

Utilizing the small bibs at the front bottom of the carb:
Mike's right on the drivers side bib being manifold vacuum.
Passenger side bib is Timed vacuum as you've described it.

I've been running some experiments with a couple of vacuum gauges plugged into the 32's 750 Carter carb.
One to each of the small bibs at the front of the carb.

Drivers side reacts like you'd expect.
IE: 19" or so at idle (with mild cam), 18" or so at 60 mph (light car), 1/2 - 1" vacuum at full throttle with varying levels depending on engine load and throttle setting.

Passenger side follows to a great degree the manifold vacuum although it starts leading (on a downward trend) as the throttle goes down, rpm comes up and engine load is on.
Eventually getting to zero at full throttle.
This last bit a small surprise to me because I expected the passenger side vac bib to show increased vacuum as rpm levels rose.
What we're dealing with in this instance is - in effect - a variable venturi that changes capacity as the throttle blade position is changed.

Got a bit more research to do, but this moving to Arizona stuff is interfering with my life as a - make believe - researcher.... :?

[Bruce Dorsi]

Thanks for your responses so far, guys!

This topic is becoming even more interesting to me, as there seems to be a lot of contradictory info in circulation.

While looking at some cars with Edelbrock carbs, I see that some vacuum advances are connected to the passenger side bib, while others are connected to the driver's side bib.  ....Talking to the owners proved that some don't have a clue --- they thought both bibs were the same.

I found some Edelbrock carb instructions at their site.  ....They specify that the 3/16" bib on the driver's side is for the EGR valve.  .....The 3/16" bib on the passenger side is to get TIMED vacuum for the distributor. ....To get manifold vacuum for the distributor, they say to hook up to the 1/4" NPT port on the base, at the rear of the carb.  

Evidently, these instructions do not apply to all their carb models.  ....Or, do they?

Don't mind me --- I'm still learning!

[C9]

I meant to add to the bit yesterday that I'd found zero vacuum at idle on the vacuum gauge tapped into the right side (timed) bib.
All those interruptions . . . ya know.... :?

Anyway, a little off idle the timed vacuum came up to levels very close to manifold vacuum.

I think one of the big reasons for the confusion is that the instructions for my Carter carbs indicate the passenger side bib is for vacuum advance and no comment is made for the drivers side bib.
Too boot, there are some - quite a few actually - Carters and perhaps Edelbrocks out there that have only one bib and that on the passenger side.

Once I get moved I'm looking forward to running some tests with both vacuum gauges hooked up as mentioned in the above post and noting the readings in a small tape recorder.
One of the tests planned is with a Holley smog carb that has what I believe to be a bib that taps into venturi vacuum.

Lotta folks - and I'm one of them - are impressed with the complexity of a computer controlled engine that sets timing and fuel ratios in milliseconds as you drive.
Regardless, carburetors and simple distributors are very sophisticated devices and not as simple as they first appear.
Toss three different methods of controlling timing in an older engine and things get complex indeed.
Some guys may not think so cuz they can get the carb'd, regular distributor equipped engines running pretty good, but when you start making decisions about timing, how much and when, as well as setting up the fuel ratio's for varying levels depending on what you're trying to do things get complicated pretty fast.

I think the best thing to keep in mind where ignition timing and vacuum sources are concerned is that mechanical (also known as centrifugal) advance timing and vacuum advance timing (regardless of where the vacuum source is selected) are two different things and should be approached as such.

And maybe it would be more understandable if we called the vacuum advance cannister a vacuum retard cannister....