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Old 01-28-2004, 06:55 AM
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Vacuum advance source-ported vs non ported

I have a 65 Riviera with a Edelbrock 1406 carb. It has 2 vacuum ports for the vacuum advance. At idle one is zero vacuum and the other is vacuum all the time. If I have 5 degrees BTC initial and then hook up the vacuum to the constant vacuum, I get a 15 degree advance. So now sitting at idle its 20 BTC. It stays there until the mechanical takes over. I get about 35 total at 3000 rpm.
If I use the other source , I am still at 5 degrees at idle and then the vacuum takes over quickly until the mechanical comes into play. Which is better for performance? which is better for mileage?

If I am tuned to 10 initial then it runs a lot better(better throttle response) but its harder to start. So I run at 5. (the specs say 2 1/2).

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Old 01-28-2004, 07:11 AM
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ALWAYS hook it to the low to no vacuum port,retarding your timing when you stomp the pedal is the worst thing you could do. I have seen so many cars hooked up that way it blows my mind,many hooked up by supposed performance minded mechanics. Its so common that its one of the first things I check for when diagnosing ignition problems. I have even had guys argue about it with me.
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Old 01-28-2004, 08:13 AM
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rifraf, maybe you could answer a question for me.I am re-installing the motor in my 75 vette.I marked all the vacum hoses with tape in ink and very carefully painted them when i redone my engine bay.The best part is , i forgot to take pictures . the
hei distributor needs no vaccum until it revs right?
there's a hose with a round disc in line , is that it.? do I need vacuum for initial start up?
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Old 01-28-2004, 08:24 AM
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Old 01-28-2004, 08:27 AM
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HEI`s use a ported source or one that doesn`t pull vacuum until it`s off idle. there will likely continue to be arguments forever about where they should go, but I never seen the reason why a engine would need vacuum advance at idle, what good is it doing? the vacuum line you have with a disc in it, would likely go to the vent controls or the like, anything to where the vacuum can only be pulled one way and not the other, when you accelerate hard and the vacuum drops, the vent controls would go back to where they were, when a 1 way check valve is used in the line, it holds the vacuum it has when vacuum drops.
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Old 01-28-2004, 09:03 AM
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OK I have 5 degrees initial then you add full time vacuum and it goes to 20 degrees.

now what is the difference between initial 20 degrees?

Well I can tell you the difference. My car wont start if its at 20 degree initial.

I am missing something here. How does the engine know that the advance is from the initial or (initial + vacuum)? The only answer I can think is that the vacuum is real low when the starter is cranking.
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Old 01-28-2004, 09:12 AM
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Arrow

Most Fords run a manifold vacuum for the advance can. The GM cars I have worked on use ported. I believe the mfg.s curve the dist. for what type they use, in a Ford the centrifugal overlaps with the vacuum advance for a transition into centrifugal.
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Old 01-28-2004, 10:44 AM
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You can run ported, manifold vacuum, or strictly mech. advance.

Reason why you would run manifold vacuum from the carb:

If you set the initial timing to 5* BDTC and run vacuum advance at idle for 15-20*, you get two benifits. 1

1) Since you initial timing @ cranking is pretty much 5*, your engine does not have to fight the advance timming to start (less stress on the engine). Then once the engine is idling, your manifold vacuum is kicking in to give 15-20* at idle and low RPM cruise speeds. Higher timing @ idle and cruise will give better gas mileage, better throttle response, and little more power on you low rpm range.

With manifold vacuum advance method, the manifold port vaccum drops as the throttle blades open, but at this point the engine is picking up RPM and as the vacuum advance retards, weighed advance picks up till you reach total timing. You should have the proper weights and springs so you don't have a dip in the advance curve.



You don't have to run it this way, heck I ran only mech. advance before - race engines don't use vaccum advance. Lot of guys run ported vacuum, but if this is a daily driver and you do a lot of cruising (rpm range), then you'll benefit more with manifold vacuum port for better gas mileage.
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Old 01-28-2004, 09:49 PM
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Older non-smog-regulated cars that I'm familiar with used manifold vacuum for their advance cans. The advent of smog rules in the late '60's & early '70's brought the use of ported vacuum. Vacuum advance at idle makes a car idle smoother & with less throttle opening, but it also increases emissions. Later cars with distributors designed for ported vacuum should probably use it. Race engines & hot street engines can benefit from vacuum advance by making them smoother & more civilized to drive at slow speeds appropriate to the pits or residential areas.
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Old 01-28-2004, 10:37 PM
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Vacuum

Justinh! I have been reading all the post's and in less I missed something, here is what I see missing: the distributors the answer to the question. There are Vacuum advance distributor's and then there are rodder advance distorter's. And then there are to deferent place to get the vacuum from. One is from the carburetor above the throttle plates, the other is below the throttle plat's or intake manifold vacuum. Now it has been quite awhile but sense I worked on this problem, but the full vacuum from the intake manifold or from below the throttle plats on the carburetor is for the vacuum advance distributor and the rodder advance distributor vacuum comes from the carburetor above the throttle plat's. They are very deferent in the way they advance the timing on the engine. I know that there are other types of distributors out such as the Rev Pole just to name one. Some time you have a problem when you switch carburetors or distributors. If I'm wrong about this I'm sure there will be someone on here that will set it straight. And that is OK too. Just something to think about! ------------Gene Neal
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Old 01-28-2004, 11:16 PM
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Im back. There are other cars that have a variety of methods to advance and retard the timing for different reasons,as mentioned prior. If yours increases when you plug it into the under the throttle plate port,or manifold vacuum as you stated it did,you need to switch it to the other port. There are a host of different temperature sensors,check valves,etc but if you want it basic and functioning properly take my advice.

Mikewayman,Im not familiar with how yours was set up originally,sorry
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Old 01-29-2004, 03:43 AM
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Quote:
Originally posted by jimfulco
Older non-smog-regulated cars that I'm familiar with used manifold vacuum for their advance cans. The advent of smog rules in the late '60's & early '70's brought the use of ported vacuum. Vacuum advance at idle makes a car idle smoother & with less throttle opening, but it also increases emissions. Later cars with distributors designed for ported vacuum should probably use it. Race engines & hot street engines can benefit from vacuum advance by making them smoother & more civilized to drive at slow speeds appropriate to the pits or residential areas.
This is what I believe to be true also. I run my engine with the FULL vacuum to the advance/retard vacuum can on the HEI. That works best for me. If I use the PORTED vacuum it falls on its face when going to WOT from a stop. Switching to FULL vacuum cures this immediately.

My suggestion is to try both and see what works best for you. All that is involved is switching the vacuum hose to try it out.
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Old 01-29-2004, 06:05 AM
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Quote:
Originally posted by jimfulco
Older non-smog-regulated cars that I'm familiar with used manifold vacuum for their advance cans. The advent of smog rules in the late '60's & early '70's brought the use of ported vacuum. Vacuum advance at idle makes a car idle smoother & with less throttle opening, but it also increases emissions. Later cars with distributors designed for ported vacuum should probably use it. Race engines & hot street engines can benefit from vacuum advance by making them smoother & more civilized to drive at slow speeds appropriate to the pits or residential areas.
I agree with this 100%! The primary purpose of vacuum advance is fuel economy period! This is why race motors don't have them. A vacuum advance canister can pull in as much as 60 degree timing. Under heavier loads timing requirements decrease due to combustion pressure. A ported system only reads the total airflow through the carb, not engine load as a manifold system can. Do you rally want to pull in as much as 60 timing pulling a heavy load up a hill? I don't think so.
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Old 01-29-2004, 09:53 AM
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True, whatever timing curve your setup needs - go with that curve. There is no right or wrong vacuum port - in the end its what the engine needs. I draw all my experience from pre 1970 chevy cars. A stock engine will have different needs than a warmed over street rod or a strip/street car.
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Old 01-29-2004, 07:55 PM
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Let me go into further detail,lets say your distributer advances 10degrees at full vacuum. Lets say your advance weights offer 18. The combination yields 28. Now,when you stomp the gas,the manifold vacuum drops to zero.At an idle it sits,depending on the cam,about 16.If you set it up to manifold vacuum your timing will retard that say 10 degrees under acceleration and only see full advance when your load has diminished or you let up on the gas,a poor recipe for performance and gas mileage.
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