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Old 09-19-2003, 03:10 AM   #1 (permalink)
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Vacuum advance- direct manifold vacuum or timed port?

In a post here about tuning a carb with a vacuum gauge it was stated that distributor vacuum advance should be connected to timed vacuum port( no vacuum at idle ; vacuum increases with rpm).
Am I totally wrong here:I have always thought that in a typical carburetted Ford V8 engine the vacuum advance is there to promote engine efficiency/fuel economy and that the advance should be connected to full manifold vacuum? This way with light throttle / high vacuum / light load the engine will tolerate lots of advance which is beneficial to fuel economy/efficiency/throttle response? Mechanical advance is there to increase advance with rpm , not the vacuum.Why should there be 2 systems to increase advance with rpm?

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Old 09-19-2003, 04:14 AM   #2 (permalink)
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Vacuum advance- direct manifold vacuum or timed port?

I think the same way- especially after reading this artical. I have posted it here a couple times. If you dont recognise the name Navarro- he is an early hot rod pioneer and made all sorts of speed parts in the 40s-50s.

Spark Advance Hop-up Trouble Spot
By Barney Navarro
Motor Life Feb 1954



Among the mistakes made in hopping up engines, few exceed in number the misapplication of spark advancing principles. The chief source of error is the limited information available on the subject of spark lead. That which is distributed, unfortunately, fails to cover some essential factors and very often is no more than a comment to the effect that fuel charges take a certain amount of time to burn so spark must be advanced enough to compensate for the time lapse.
Well informed engineers wish that the problem really was that simple. Most ignition system purchasers overlook every factor except the amount of spark produced. The wrong system can cause plenty of trouble: plug fouling, poor gas mileage (even though the engine has no tendency to misfire), overheating in slow traffic, and other maladies. Basically, engines require some means of advancing spark timing as rpm increases since the pistons, in effect, try to get ahead of the burning speed of fuel charges. Combustion, witch takes a definite length of time, must occur when pistons are at the top dead center before the start of the downward power stroke. If burning finishes too early, energy is wasted because the resultant pressure rise produces a force opposition to rotation. This is readily apparent when starting an engine that has too much spark lead; it will actually kick back against the starter’s efforts. Modern high compression engines, while under full load, audibly indicate spark that is too far advanced by pinging. So the popular method of setting spark timing for maximum horsepower is to set it just below the ping point under full throttle operation. Distributors that employ flyweight governor advance mechanisms use a spark advance curve that conforms to the engine’s requirements under full throttle at any point within the rpm range. At low rpm a lesser spark lead is required so the governor advances a small amount. As speed picks up it advances more and more, always conforming to the full throttle full load requirements. On a drag machine, where full throttle and full load conditions are maintained, the flyweight governor is required. But for ordinary driving, which consists mainly of partial throttle operations with Very light loads, it is not enough. Some other means of compensating for varying loads must be provided. The load compensator is necessary because a light fuel mixture burns more slowly than a heavy charge since the concentration is less and flame takes longer to travel from one fuel particle to the other. If the utmost energy is to be obtained from light charges, their burning should be completed at the same point that the heavy charges finish. So if they take longer, the only way to make them finish at the same point is to start them earlier. Consequently, partial throttle partial load operation requires more spark lead at any given speed than is required at full throttle full load. Load compensation is the most commonly achieved by using intake manifold vacuum to actuate a diaphragm. This diaphragm advances and retards the distributor breaker plate and in some cases the whole distributor case. When the engine is operated with Very light throttle pressure, the manifold vacuum is high, so the diaphragm advances the spark timing to produce the most efficient combustion possible. As the throttle is depressed, the vacuum drops of and the diaphragm produces less advance until it reaches a point of being completely ineffective at wide open throttle. Thus the ideal load compensation is always maintained and results in more power from every drop of fuel.
The second most popular method of obtaining load compensation, though further from perfection is that employed in Ford V-8 distributors from 1932 trough 1948. Instead of a diaphragm, there is piston brake actuated by manifold vacuum, The flyweight governor mechanism is equipped with a breaking disk which cancels five degrees of the governor’s advance when pressure is brought to bear on its edge. At this edge a spring-loaded piston is located in a small cylinder. The spring is on the side of the piston opposite the disc so it causes the piston to be pushed against the disc. Vacuum is introduced on the spring side to oppose its action and lift the piston off the disc. In action, the high vacuum produced by operation with small throttle openings lifts the piston of the disk, allows the full action of governor weights to take effect and gives the Ford engine five degrees more spark advance. By depressing the throttle further, the manifold vacuum drops off and the spring again pushes the piston against the disc to retard the spark. The flaw in the operation of this mechanism lies in the fact that it is either “full on or full off” and permits no gradual compensation like the diaphragm.
Ford’s latest method of controlling spark advance employees an ingenious system utilizing manifold vacuum and venturi vacuum. With this system the flyweight governor is eliminated and in its place is nothing but a diaphragm. This diaphragm not only advances the spark to conform to rpm changes but is also makes load compensation adjustments. All ´49 through´54 Ford and Mercury carburetors have in addition to the conventional manifold vacuum takeoff, such as is found in the throttle body of most passenger car carburetors, a connecting venturi vacuum passage. The manifold vacuum, as usual, is obtained from a small port in the throttle body located slightly above the butterfly’s closed, position, on the side where the butterfly swings upward to open. When the throttle is closed at idling, the vacuum port does not receive vacuum because it is on the opposite side of the butterfly. As the throttle is opened slightly, this port is uncovered and a vacuum is applied to the distributor diaphragm to advance the spark. If the throttle is fully depressed, the manifold vacuum is destroyed and no advance takes place. As speed increases, however, the venturi vacuum increases gradually and advances the spark to conform to the rpm. Letting up on the throttle increases the manifold vacuum (Provided it isn’t let up all the way) and the spark receives load compensation. A balance is always maintained so that the correct amount of spark advance is supplied for all speed and load conditions.
The greatest installation errors center around the misunderstanding of the late Ford distributors. A distressingly large number of mechanics are unaware of the difference between manifold vacuum and venturi vacuum. In fact many attempt to operate Ford and Mercury distributors by connecting the vacuum line to the windshield wiper connection on dual intake manifolds. This sometimes results from a desire to use the old Stromberg carburetors, which are not equipped with vacuum takeoff. So the simple solution seems to connecting the distributor vacuum line to the handiest apparent source of vacuum. Such practice is worse than having no spark control at all for when the engines idles the spark advances fully and retards as throttle is depressed. There is no venturi vacuum available to advance the spark as the speed picks up and it remains retarded until the throttle is let up. So if the old style carburetors are preferred, the stock Ford distributors must be discarded on the late models. However, Stromberg has resumed production of the old “97” and is now fitting it with a venturi vacuum takeoff to make its use feasible.
Four throat carburetion installations also have had their share of improper distributors. Early articles in certain publications gave the impression that no vacuum control whatsoever could be tolerated. It wasn’t pointed out that the only forbidden type is that of the stock `49 through `54 Ford and Mercury distributor. This caused many to purchase distributors and magnetos that were equipped with flyweight governors only. Such installations get very poor gas mileage, so the car owners blame the four-throat carburetor. Even more irritating, is the tendency for spark plugs to foul. Having no load compensation, the spark is never far enough advanced under partial throttle to fire the fuel mixture chargesat the most opportune time. In effect, the engine is being operated with a lower effective compression ratio because burning is completed as the pistons travel down the cylinder bores. And since the plugs never receive a hot flame, soot collects on them. Furthermore, the condition cannot be remedied by using hotter plugs because they will burn up under full throttle operation of flyweight governor distributor with vacuum-operated load compensation device.
In practice, the installation of dual intake manifold on Fords and Mercury’s of the ’49 trough ’54 series should be accompanied by a change in distributors such as prescribed in the preceding paragraph. The addition of two carburetors divides the airflow so only half as much airflows through one carburetor as previously at normal operating speeds. Venturi vacuum is dependent upon the air velocity through the venturi so any reduction in velocity will result in less spark advance. And connecting a line to each venturi vacuum takeoff of a dual set up will not increase the vacuum---such a practice is just a waste of copper tubing. The best advice to keep in mind when purchasing a distributor is not to pinch pennies. An inexpensive unit, if it doesn’t do the job correctly, can prove to be the most costly. The bestway to avoid mistakes is to study the problems involved and learn enough about them so that you can select a distributor that matches your engine requirements.
Cool stuff huh? Did ya get all that? ?
Before seeing that essay by Mr. Navarro, I wrote this, on another message board:
....There are good reasons to use ported vacuum distributor advance and good reasons to use manifold vacuum distributor advance.
If you want to progressively add advance as RPMs increase in addition to the mechanical advance which also does that, then you want ported advance. I can see wanting this kind of setup for drag racing to get an unfluctuating reliable advance throughout the 1/4 mile. his would definitely need to be dialed in on a Dynamometer every tune up. I have an old Ford flat head inline 6 that has to have ported advance because it doesn't have any mechanical advance at all.
If you want to have stop and go traffic driveability and a high advance for cruising at highway speed with good mileage. and enough advance at idle to not be running retarded and over heating, then you want manifold vacuum.
You need about 25º± advance at idle to keep from overheating in traffic. If you set your initial timing that high the engine will be hard to start or wont start at all, so you set it at 12º± with the vacuum disconnected, then hook it back in to manifold vacuum which is high at idle, and the V/advance advances it up to the 25º±.
Then you drive on down the road, your RPM goes up to 2200-2500 RPM and all your mechanical advance is in so that's around 36º-40º advance, plus the vacuum advance brings it up to around 50º± advance.
Why isn't it pinging? it has a light load. If you had 50º advance with ported vacuum you'd have to run racing or aviation fuel to keep from detonating. but with manifold vacuum, if you punch it to pass or whatever, the manifold vacuum drops the advance retards to a safe 38º± and you don't detonate even on regular gas.
Anyone, if you heat up in traffic and are running ported vacuum, switch it to manifold vacuum after setting your initial timing to whatever degree gives you 36º-40º all in mechanical advance, with the vacuum advance not hooked up at all. then hook up to manifold vacuum distributor advance.
I set up my new 262" SBC engine last month by making an extra timing mark at 38º BTDC on the dampener, then with the vacuum disconnected and plugged, I brought the Revs up to around 2500-3000 RPM, and then moved the distributor so that my timing light showed this new timing mark at 0º on the indicator. At idle that gives me around 13º BTDC initial advance then I hooked up the manifold vacuum advance. I can let it sit and idle all day in gear at 181º F, with a 180º thermostat. and a couple of weeks ago, with only 200 miles on the new engine it might have got up to 183º pulling a hill, with no detonation on the cheapest regular. (timing retards pulling hills, so no pinging)
So it's your choice. Driveability or maximum racing power.

Now on that old Cadillac, it may have a vacuum advance/mechanical advance setup designed to work with a specific carburetor. like the Ford Mr. Navarro described. So when you change cams and carbs you throw the system all out of specification. The best thing to do in that case is determine what the distributor actually does, probably on a distributor machine so you can check and adjust/modify the mechanical advance to what you need and the same with the vacuum advance. My Motors repair manual says to disconnect the vacuum advance to set the timing which leads me to believe it used manifold vacuum, or had a dual source setup IN the carburetor similar to Fords in which case it has to work with that one specific carb if it uses the "ported" source.
Another note, I have three different "stock" Chevy vacuum advance units and they each will pull different amounts of degrees, ranging from 12º to 25 degrees! so they aren't all equal either. you have to check what you are dealing with in your application, and after all these years, the "stock" parts you have may not be as stock as you think
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Old 09-19-2003, 04:51 AM   #3 (permalink)
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Vacuum advance- direct manifold vacuum or timed port?

That about covers it doesnt it...Good stuff¨, proving that some essentials just dont change.The article from 49 years back is still valid today.
Hmmm...still wondering whether I should tune my advance curve to maximum possible advance at all rpm, at verge of detonation?
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Old 09-19-2003, 08:02 AM   #4 (permalink)
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Vacuum advance- direct manifold vacuum or timed port?

That's some great information to be so outdated.

For street driving, I like to map out a mechanical advance curve from idle to WOT, that is just on the verge of hard starting or detonation under a pull. A computerized ignition system uses sensors to accomplish this, but you will have to rely on your ears and feel. After I get the mechanical spark lead optimized, I just throw in about 10 degrees more vacuum advance from the manifold port, for a cleaner burn at cruising speeds. You may have to do some distributor mods to accomplish all this. Retuning the carb after tuning the distributor is also a good idea. You will be rewarded with excellent throttle response and a clean buring engine.

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[ This Message was edited by: bluestreek on 9/20/03 10:45am ]
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Old 09-19-2003, 10:49 AM   #5 (permalink)
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Vacuum advance- direct manifold vacuum or timed port?

Bluestreek...shouldnt you get the vacuum signal from manifold vacuum....not timed port...
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Old 09-19-2003, 10:56 AM   #6 (permalink)
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Vacuum advance- direct manifold vacuum or timed port?

Great info! I've preached that for years. As load increases, combustion pressure increases and spark requirements decrease.

Also very important what most people compltely miss is the fact while a mechanical distributer only has about 20 degrees advance built into it. So with 15 degrees at the crank, total timing is 35 degrees.

A vacuum distributer can pull up to 60 degrees total timing in the distruber. This is fine under a light load for fuel ecomony but can you imagine this connected to a ported/timed source that's pulling 60 degrees because the throttle is wide open pulling a heavy load up a hill...

With a manifold vacuum under these conditions, the vacuum advance will fall because of low manifold vacuum and will have about 30-35 degrees dependind on intial timing and rpm.
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Old 09-19-2003, 10:44 PM   #7 (permalink)
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Vacuum advance- direct manifold vacuum or timed port?

Quote:
On 2003-09-19 10:49, ville wrote:
Bluestreek...shouldnt you get the vacuum signal from manifold vacuum....not timed port...
Thanks for seeing my mistake ville. I edited my post.
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Old 09-20-2003, 05:27 PM   #8 (permalink)
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Vacuum advance- direct manifold vacuum or timed port?

A timed vacuum source has the same amount of vacuum as a manifold source. The port is just directly above the throtle blades, so there's no vacuum at idle. With no excessive advance at idle it starts easier and the engine warms up a little quicker. It is not the same as the '54 Ford type that replaces the mechanical advance with a speed dependent vacuum, that gets its signal from the venturi throat (very similar to the vacuum that opens the secondaries on vacuum sec. carb). Either that or I'm all mistaken.

The Navarro story also mentions this:

The manifold vacuum, as usual, is obtained from a small port in the throttle body located slightly above the butterfly’s closed, position, on the side where the butterfly swings upward to open. When the throttle is closed at idling, the vacuum port does not receive vacuum because it is on the opposite side of the butterfly. As the throttle is opened slightly, this port is uncovered and a vacuum is applied to the distributor diaphragm to advance the spark. If the throttle is fully depressed, the manifold vacuum is destroyed and no advance takes place.

The same info can be found on the fordmuscle tech page:

There are two types of vacuum sources that you should be aware of. One type is known as "full" vacuum or "manifold" vacuum. This is a direct connection to the manifold, and if the hose is connected to this port you will get vacuum in the line at idle. The other port is a "timed" port, which only yields a vacuum above a certain rpm. At idle the line will have no vacuum. Most carburetors have both ports. On Holley's the timed is above the throttle blades, and the "full" is below, near the base. On Carter/Edelbrock carbs, the timed port is on the passenger side and the full is on the driver's side. The easiest way to confirm what port you have is to hook up a vacuum a gauge and check for vacuum at idle. The preferred vacuum source is the timed source. This way there is no effect on the initial timing setting.

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Old 09-21-2003, 05:18 AM   #9 (permalink)
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Vacuum advance- direct manifold vacuum or timed port?

Quess we need to test this by running two vacuum gauges ;one to full vacuum and one to ported to see the differences in vacuum under different driving conditions.
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Old 09-21-2003, 12:54 PM   #10 (permalink)
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Vacuum advance- direct manifold vacuum or timed port?

I just checked it. As soon as you open the throtle a little, the vacuum at the timed port is identical as at the manifold.
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Old 09-22-2003, 01:36 AM   #11 (permalink)
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Vacuum advance- direct manifold vacuum or timed port?

So it all boils down to vacuum/advance at idle.I wonder if there actually is any difference in the way engine will react.It was stated somewhere earlier in this post that
lot of advance at idle will make the engine run cooler, perhaps that´s one difference.Starting may be another area , naturally the engine will be easier to start with less advance but I quess rotating the engine by the starter does not produce enough vacuum to affect the advance.
Anyway there must be a reason why there actually is these two different vacuum sources ia a carburetor.I am ashamed to confess my ignorance here but which vacuum source do the factory engines use?
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Old 09-22-2003, 08:34 AM   #12 (permalink)
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Vacuum advance- direct manifold vacuum or timed port?

They take it from the carb. Later (beginning in 196, they made things more complicated with manifold vacuum coupled to the rear of a dual diapragm advance and the "normal" vacuum routed via a thermal switch. The latter probably to keep the timing from advancing when the engine is cold. Later timing is warms up the engine quicker because it's less efficient.

I think the vacuum advance cut off at idle that we where talking about is because ignitability is better at higher cylinder pressures, so the combustion at idle becomes more stable.
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