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Mustang Technical Discussion > Pre 1973 > Vacuum advance > Community Forums > Mustang Forum Australia - Mustang Tech

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hybrid
Mustang King

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I can only find a pic of one side of the procomp.



You can see one outlet on the back of the carb, but who knows what it's for.
Can you see any holes that are *just* above the throttle plates when the throttle is completely closed?

If there are none, then it doesn't have a vacuum advance port.


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donoauto
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Awwwww look at the little thing , it's sooo cute !


(PRAYER) Oh thank you Wild Turkey American Honey & Cola in those small thin bottles, for giving me the strength to act like a half wit, and say stupid things at inappropriate times, semi-anonymously, on a public forum.
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potto
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Here is a pic from the other side where the connection should be
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donoauto
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You can run manifold vacuum to the dizzy if you want. Do a bit of googling about "ported vacuum vs manifold vacuum for vac. advance" & you'll find some interesting reading. Pros & cons for both sides although I prefer ported vac. from the carby. Typically , the advance mechanism will offer 12-15 degrees advance (please correct me if I'm wrong on that one fellas!).


(PRAYER) Oh thank you Wild Turkey American Honey & Cola in those small thin bottles, for giving me the strength to act like a half wit, and say stupid things at inappropriate times, semi-anonymously, on a public forum.
Amen.

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glen68
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Hi Guys, I am real sorry if this ruffles any feathers but on pre 70's and any modified engine vacuum is taken from the manifold. Early carbs you will only find a manifold vacuum port. It was not until later that the port vacuum was used a part of the pollution control system. Carbs carry both ports today so as the carbs can be fitted to all engines.

You should disconnect your vacuum line. Block it and set your factory timing at idle. Then reconnect your vacuum line and in most cases wind down idle speed as the idle will increase. You will notice an increase in ease of starting, take off and cruise. You should notice a increase in economy as well. I know I am going to get some flack for saying this because of all the timing confusion out there.

If you think this is incorrect find any pre 70 engine manual Ford, Chev etc. Late model cars such as current Fords Commodores, BMW, Mercs run 50 to 70 degrees advance at idle on cold start and a little less once warm. It will not hurt your engine as that advance is not there under load...

Cheers Glen


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potto
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Thanks glen I ended up finding the vacuum port on the carb( but it did run alright in the manifold vacuum, I just had to adjust the vacuum advance)

Potto


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mert
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"...on pre 70's and any modified engine vacuum is taken from the manifold. Early carbs you will only find a manifold vacuum port...."

Close, but not quite. Ported vacuum was used at least as early as in the late 60's on Mustangs, OEM Holley's for big block 68's had it for sure. And I believe it was used even earlier elsewhere, possibly into the late 50's from some posts I've seen.

However, as you notes it is predominantly an emissions issue (which were a concern far earlier than the 70's, and did drive engineering changes to carbs and other issues such as removal of road draft tubes and development of early PCV systems in the early 60's, etc). With ported vacuum the engine burns hotter at idle, but one off idle the ported and manifold vacuum track pretty much the same.


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Shaunp
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Ok the way 70-80's emissions vac advance worked on local fords and holdens was to only allow advance when the car was in top gear, or over heating. that was ADR 27a? I think which also included a charcoal canister, it was done with solinoids etc to cut or allow vac. In any case on engines with big cams you don't want it anyway, as you have little vac. You build the advance into the mech advance curve, and forget vac. . You'll find K code 289s have no vac advance.
Vac advance is really just for fuel eccon in real terms.


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mert
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Once you floor it the vac is overridden by the mech, there are really few cons to using vacuum advance on a street car.

Vac advance will, in general, provide better mid throttle response, and accomodates load differently than mech.

Mech will, in general, work better on the track or strip, running at WOT (but if tuned properly a vac unit gets pretty close, as under WOT the mech is all-in and overriding the vac, so no real difference).

Most folks would see no significant improvement by tossing a properly tuned vac advance for a pure mech on the street.


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jas24zzk
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You won't see a performance increase with Vac.
You will see an economy improvement down the highway.

Keep in mind the K codes run dual point, zero vac advance.

You can delete the Vac advance and NOT create a problem.


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glen68
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Hey guys, I have done some research and testing and tuning as well as a few chats with some knowledgeable people. This is what I have found.
I have been through dozens and dozens of car manuals (thanks to my Dad) covering Singers, Austins, Humbers, Vauxhalls, Humbers, Wolseleys, Hillmans, Wyverns, Chyrslers, Fords and Holdens etc and I could not find any instance of ported vacuum.
I asked Yoda (Dad) what he knew about it and was told that he never saw it in Australia until the introduction of the emission laws. I asked if he knew of any other instance and was advised that in the State of California in USA “this crap was introduced in the early 60’s and retrofitted to cars back as far as 55” in his words to his knowledge. (So if you were selling cars that were sold in that state you would have to allow for those modifications)
He told me that in the 70’s he knows of mechanic’s who did a roaring trade disconnecting all the pollution crap. Hooking up the vacuum advance (only if the Vac unit was correct and the timing curve for the mechanical was also correct. Grabbing a dizzy out of a pre pollution engine was the simple way.) To give their owners a better car to drive with money in their pockets from the fuel savings. This was verified by numerous old mechanics … none of which ever did it… LOL
Why would you want vacuum advance?Cooler running at idle and at cruise ( so less chance of overheating in traffic or on hot days). More torque at Idle and cruise, particularly important for auto’s going from park into drive. But when driving on the roads 80 to 90 percent of the time is spent on part throttle and you will notice an improvement in pick up and response. Less throttle is required so less fuel. Even at idle, the idle speed screw usually has to be wound down to reduce idle speed once the Vacuum advance has been reconnected.
Why wouldn’t you want vacuum advance. It has one drawback and that is harder for the starter motor to crank the engine over. So on cars with increased compression in the old days the vacuum was removed. ( i.e K code mustang). But with current battery tech and better starter motors this is no longer an issue.
So in the words of my Dad (and I agree). The last thing you would want to remove on a carburetted pre computer street car is the vacuum advance. Why would you want to make your car harder and less pleasurable to drive and also cost you more to do it in fuel costs???

Glen


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glen68
Baby Mustang

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Just wanted to be clear that I was speaking about the benefits of manifold vacuum. Ported vacuum is a disaster. It was never intended to be used for anything but emissions reduction. No manufacturer would ever willingly want their vehicles to have less performance and less economy unless Government mandated.

Glen


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nassi
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From Wikipedia........
Manifold vacuum vs. venturi vacuum

Manifold vacuum is caused by a different effect than venturi vacuum, which is present inside carburetors. Venturi vacuum is caused by the venturi effect and depends on the total mass flow through the carburetor. In engines that use carburetors, venturi vacuum is proportional to the total mass flow through the engine (and hence the total power output).

Manifold vacuum may also be "ported" where the opening is placed so it is normally above the throttle plate at idle, but as the butterfly valve opens, the opening is effectively below it, and the opening sees nearly the full manifold vacuum. Ported vacuum is often used for distributors and emissions items.


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potto
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glen68 wrote
Hey guys, I have done some research and testing and tuning as well as a few chats with some knowledgeable people. This is what I have found.
I have been through dozens and dozens of car manuals (thanks to my Dad) covering Singers, Austins, Humbers, Vauxhalls, Humbers, Wolseleys, Hillmans, Wyverns, Chyrslers, Fords and Holdens etc and I could not find any instance of ported vacuum.
I asked Yoda (Dad) what he knew about it and was told that he never saw it in Australia until the introduction of the emission laws. I asked if he knew of any other instance and was advised that in the State of California in USA “this crap was introduced in the early 60’s and retrofitted to cars back as far as 55” in his words to his knowledge. (So if you were selling cars that were sold in that state you would have to allow for those modifications)
He told me that in the 70’s he knows of mechanic’s who did a roaring trade disconnecting all the pollution crap. Hooking up the vacuum advance (only if the Vac unit was correct and the timing curve for the mechanical was also correct. Grabbing a dizzy out of a pre pollution engine was the simple way.) To give their owners a better car to drive with money in their pockets from the fuel savings. This was verified by numerous old mechanics … none of which ever did it… LOL
Why would you want vacuum advance?Cooler running at idle and at cruise ( so less chance of overheating in traffic or on hot days). More torque at Idle and cruise, particularly important for auto’s going from park into drive. But when driving on the roads 80 to 90 percent of the time is spent on part throttle and you will notice an improvement in pick up and response. Less throttle is required so less fuel. Even at idle, the idle speed screw usually has to be wound down to reduce idle speed once the Vacuum advance has been reconnected.
Why wouldn’t you want vacuum advance. It has one drawback and that is harder for the starter motor to crank the engine over. So on cars with increased compression in the old days the vacuum was removed. ( i.e K code mustang). But with current battery tech and better starter motors this is no longer an issue.
So in the words of my Dad (and I agree). The last thing you would want to remove on a carburetted pre computer street car is the vacuum advance. Why would you want to make your car harder and less pleasurable to drive and also cost you more to do it in fuel costs???

Glen



I have to agree with you Glen, I had the vacuum advance on both the ported and the manifold and it started much easier and ran better on the manifold. Just my opinion Smile


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Jiffy
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Forget trying to connect it to the air cleaner.
If you have a manifold-ported vacuum source, use that, just remember to set the idle advance with the vacuum disconnected and blocked. Vac advance on cars were set using manifold vacuum for years before emissions.

Don't go drilling your carb anywhere for any reason....

Most importantly, Pinto-Pete - can we have a full-size photo of the bird in your avatar with the cute bum?


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