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Old 10-13-2021, 08:14 PM
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Default Vacuum advance connection

I'm trying to learn where the vacuum advance was connected from the factory. Ported or manifold , and where? When I got my car it had a pieced together Q-jet with a ported vacuum fitting on the front of the carb which is where they had the hose connected. Now I have a 7040273 on it which has no ports except for the choke pull off and the power brakes. Right now, the vacuum advance is connected to the port on the manifold that used to go to the air cleaner hot air diaphragm.

Thanks in advance.

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Old 10-17-2021, 09:38 AM
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Mine is a late production 73 and has all the emissions hooked up. If I recall correctly it get manifold vacuum for the first 20 seconds of warm up and then ported vacuum only in 4th gear, or when overheating.

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Old 10-17-2021, 10:55 AM
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Quote:
Originally Posted by Pepi View Post
Mine is a late production 73 and has all the emissions hooked up. If I recall correctly it get manifold vacuum for the first 20 seconds of warm up and then ported vacuum only in 4th gear, or when overheating.
Yeah, this car has a much more simple system with no emission stuff.

BTW, I noticed a error/typo in my original post. The carb I have is a 7040263 not 273. It doesn't really make difference for this question but just to be correct. Unfortunqately it's too late for me to edit the post.

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Old 10-17-2021, 11:26 AM
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On mine, the manifold vacuum comes off the back of the carb tee that supplies the brake booster. The ported comes off the front of the drivers side ported vacuum port.

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Old 10-17-2021, 11:49 AM
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All 1970-74 Pontiac V8Žs, except 1971 455 H.O., uses the Transmission Control Spark system (T.C.S.).

From factory the rear splits to brake booster and the TCS solenoid and from the TCS solenoid to the distributor.
If you no longer have the TCS system working you may bypass the solenoid, then youŽll have 20° added timing from the vacuum advance wich is not the case with the TCS system.
With TCS youŽll have vacuum advance only in high gear or with cold or too hot engine.

I would avoid using a source on a single runner for the ignition vacuum advance, like the one on the #1 runner on a Pontiac intake, as the signal pulsates and are somewhat weaker than any plenum source under the carburetor throttle blades.
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Old 10-17-2021, 01:50 PM
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What I have done when deleting the TCS is to use a adjustable Vacuum advance hooked to the back of the carb tree. In some cases not even use a vacuum advance if weighted, curved correctly for the application.

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Old 10-17-2021, 04:09 PM
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jhein, if you don't mind me hijacking just for a moment I have a related question.
I'm using a 7044266 carb on my 68 400. I have been told that it does not have a true ported vacuum source. If that's the case, and based on what I've just read here, I believe that it is... Should I use full vacuum, no vacuum or what?

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Old 10-18-2021, 08:31 AM
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Retarded ignition timing advance (no vacuum advance at idle) was implemented 1968 to heat up the cylinder heads to reduce emissions.
Most 1967 and earlier used full manifold vacuum to the ignition vacuum advance for highest efficiency and cooling at idle speeds.
While driving at normal speeds there is NO difference between used source for the vacuum advance (load compensator).
Also, there is NO way to compensate lack of vacuum advance by the centrifugal advance alone.
Most Pontiac vacuum advance units starts at 6-8 in/hg and adds max 20° at 15-17 in/hg.
I have found any 400 engine with a 068 or smaller cam, or 455 with a 744 or smaller cam can use full manifold vacuum to the vacuum advance.
With hotter camshafts a ported source will be beneficial.
And, with a properly adjusted/modified carburetor idle/lowspeed circuit there is seldom if ever necessary to alter the factory ignition curve.

FWIW

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Old 10-18-2021, 11:09 AM
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Thanks for all the help. So what I'm going to do is use manifold vacuum off the power brake port. Then I can reconnect the line to my hot air diaphragm. All the parts are available to put the TCS back together but other than the originality aspect that doesn't seem like it will help me very much.

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Old 10-19-2021, 03:09 PM
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You do not want 17 in of vacuum at the advance at idle. If you go this route you will adjusting the carb to the advance. When adjustments are made to spec, the advance is unhooked.
One main purpose of Vacuum advance is to keep the engine from stalling if you GET quick OFF the gas, dropping RPM fast to Idle.
Ported vacuum works best for this as there is Zero pull on the advance at Idle, yet keeps timing ahead enough to keep from stalling.
Leave your initial advance high enough, you will not need the vacuum advance at all.

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Old 10-19-2021, 05:24 PM
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Just for the sake of discussion, if I go from ported vacuum to manifold vacuum the advance will be in effect at idle. If I've already set it up for ported should I adjust that initial or vacuum advance down since it's now much more advanced at idle?... Or just let it have 20+ degrees at idle? Is full manifold vacuum the reason my 1968 engine was originally set at 9°?
I know that the 'all in' is the most important and won't change but just wondering about how much advance idle can take.

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Old 10-19-2021, 06:59 PM
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Quote:
Originally Posted by Greg Reid View Post
Just for the sake of discussion, if I go from ported vacuum to manifold vacuum the advance will be in effect at idle. If I've already set it up for ported should I adjust that initial or vacuum advance down since it's now much more advanced at idle?... Or just let it have 20+ degrees at idle? Is full manifold vacuum the reason my 1968 engine was originally set at 9°?
I know that the 'all in' is the most important and won't change but just wondering about how much advance idle can take.
Keep in mind that The Vacuum advances were Made in MANY DIFFERENT NUMBERS based on application, like the Carbs were.
That said; IF your Vacuum advance has the original specs for when it Starts to pull off ( Vacuum in inches) and has the same total pull, then you can try to set it factory IF the weights and springs are stock.
Most times few if any of these parts are original.
Your '68 engine was set at 9 degrees with the advance DISCONNECTED, I am 100% Positive.
The Vacuum advance gradually looses its pull as rpm increases, but the weights and springs take over from that point in a smooth hand off.
By using manifold vacuum you are really limiting the Vacuum advance as
they are typically all done just past where you are talking having yours at idle.
Seems your Idle would be high, car would be Idling pretty warm too.
Ported is a much better set up, if you even need a vacuum advance in your set up.
Here is what a correct ported set up does.
1. No advance at Idle as the port above the throttle plate doesn't have enough vacuum to pull the advance unit.
2 As throttle is pressed, Vacuum occurs and when enough ( avg like 7-10 inches) happens, the timing advances.
3.More throttle and more vacuum till about 1500 rpm, and then the vacuum advance drops off.
4. As it drops off the mechanical advance is already starting to catch up, and eventually takes over.
5 Higher rpm there is negligible vacuum, and advance is all on Mechanical till its at max ( 32-35 degrees usually)
6. This system works in reverse as well and the vacuum will keep car from stalling if it has a load on it like AC.

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Old 10-20-2021, 04:26 AM
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VACUUM SPARK ADVANCE

Along with the mechanical spark advance, another method is used to control ignition spark under load. This is commonly called the vacuum operated spark advance unit. The vacuum advance unit supplements the mechanical advance, however, it operates differently as it varies spark timing in relation to engine load. Engine load can best be determined by manifold vacuum so consequently, the vacuum advance unit mounted on the distributor is connected directly to engine manifold vacuum.
Although spark timing is not a direct function of the carburetor, it is used on some applications to control the vacuum operated advance unit on the distributor during the idle and off-idle range. On these units a calibrated port is located in the throttle bore just above the throttle valves. It is connected by a vacuum line directly to the distributor vacuum advance unit.
In the curb idle position the throttle valve is below the spark port so consequently, no vacuum is applied to the advance unit and the spark advance remains in the retarded position. As the throttle valve is opened and engine speed increases the throttle valve moves past the spark port so that manifold vacuum below the throttle valve is applied directly to the vacuum advance unit. Timed spark advance is used on applications where engine design demands retarded spark for smooth idle and performance improvements at low speeds.
On other units the vacuum advance line may be connected directly to the manifold or carburetor bore below the throttle valve. In this case the engine will idle with full spark advance. The full advance spark at idle is used on engine applications where it is not detrimental to engine idle, and improves engine cooling. When setting ignition timing, the vacuum advance line should always be removed, especially on engines using full spark advance at idle as initial ignition timing will be set retarded if this is not done. The end result will be very poor fuel economy. To be sure, always disconnect the vacuum advance line to the manifold or carburetor when setting initial engine spark timing.

Vacuum Advance:
The vacuum advance system consists of a vacuum diaphragm mounted on
the distributor body; the diaphragm is spring-loaded in the zero-advance position, and has a rod
which connects to a hole in the breaker plate, which is the movable plate the points are mounted
on. When vacuum is applied to the diaphragm, it pulls on the rod, which in turn pulls on the
breaker plate, rotating it with respect to the 8-sided cam on the distributor shaft which opens and
closes the points. When viewed from the top, the distributor shaft (and the 8-sided cam for the
points) turns clockwise; when the vacuum advance rod pulls on the breaker plate, it rotates the
breaker plate (and the points) counter-clockwise, which “advances” the opening of the points
(which triggers the coil to fire the spark plugs). A typical vacuum advance unit, when fully
deployed, will add about 20 (crankshaft) degrees of spark advance over and above what the
distributor’s centrifugal advance system is providing at the moment, which depends on engine
rpm; they are two independent systems, but they work together to provide the correct amount of
spark advance.
Controlling Vacuum Advance: Let’s look at how the vacuum advance system is controlled.
Referring back again to burn rates, remember that lean mixtures burn slower, and rich mixtures
burn faster. Engine load conditions (idle, steady cruise, acceleration) result in how lean or rich the
air/fuel mixture is (the carburetor handles this), and the best indicator of engine load is intake
manifold vacuum. At idle and steady cruise, engine load is low, and intake manifold vacuum is
high due to the nearly-closed carburetor throttle plates; under acceleration, the throttle plates open
wider, and intake manifold vacuum drops; it is essentially zero at wide-open throttle. As a result,
intake manifold vacuum is a “free” indicator of engine load, which correlates nicely with fuel
mixture being supplied – lean mixture at high vacuum, and rich mixture at low vacuum.
At idle, the engine needs additional spark advance in order to fire the lean (and exhaust-diluted)
idle fuel/air mixture earlier in the cycle in order to develop maximum cylinder pressure at the
proper point after TDC for efficiency, so the vacuum advance unit is activated by the high manifold
vacuum, and adds another 20 degrees of spark advance on top of the fixed initial timing setting.
For example, if your initial timing is set at 10 degrees, at idle it’s actually 30 degrees with the
vacuum advance connected (a properly-calibrated centrifugal advance mechanism will not have
started to move yet at idle rpm).
The same thing occurs under steady highway cruise conditions; the mixture is lean, takes longer
to burn, the load on the engine is low (it only takes about 40 horsepower to cruise at 50mph) and
the manifold vacuum is high, so the vacuum advance unit is again deployed, and adds 15 degrees
of spark advance over and above whatever the distributor centrifugal advance mechanism is
providing at that engine rpm. If you had a timing light connected so you could see it as you cruise
down the highway, you’d see about 45-50 degrees of spark advance; your fixed initial advance of
10 degrees, 20-25 degrees provided by the centrifugal advance mechanism, and the 20 degrees
added by the vacuum advance unit.
4
When you accelerate, the fuel/air mixture is immediately enriched (by the accelerator pump,
power valve, metering rod piston, etc.), and that rich mixture now burns faster, doesn’t need the
additional spark advance any more, and when the throttle plates open, the manifold vacuum
drops, and the vacuum advance unit diaphragm retracts to its zero position, “retarding” the spark
timing back to what is being provided at that moment by the fixed initial timing and the centrifugal
advance mechanism. The vacuum advance doesn’t come back into play until you back off the gas
and manifold vacuum increases again as you return to steady-state cruise, when the mixture
again becomes lean and needs more spark advance for fuel efficiency.
Vacuum Advance Calibration: There are also many different calibrations of vacuum advance
units; some begin to deploy at different vacuum levels than others, and some provide more
degrees of advance when fully deployed than others. The original calibration was selected based
on the intake manifold vacuum characteristics of that particular engine/transmission combination
and how it was expected to perform in daily use. Vacuum advance units were connected to full
manifold vacuum for decades; in the late 60’s and early 70’s, when emissions began to become
an issue, many were instead connected to “ported” or “timed” vacuum sources. We’ll discuss this
aberration a little later.
The Advance Combination: Now we have two different advance systems working independently,
but complementing each other, to manage spark timing – centrifugal, based on engine rpm, and
vacuum, based on engine load and operating conditions. The centrifugal advance system is
purely mechanical and is only rpm-sensitive; nothing changes it except engine rpm. Vacuum
advance, on the other hand, responds instantly to to engine load and rapidly-changing operating
conditions, providing the correct amount of spark advance at any point in time, to deal with both
lean and rich mixture conditions.
By today’s engine management terms, this was a relatively crude mechanical system, but it did a
good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with
absolutely zero negative effect on wide-open throttle performance, as the vacuum advance is
inoperative under that condition. In modern cars with computerized engine controllers, all those
sensor inputs to the computer change both spark timing and fuel/air mixture 50 to 100 times per
second, and we don’t even have a distributor any more – it’s all electronic.
“Ported” Vacuum: Now to the widely-misunderstood manifold-vs.-”ported” vacuum aberration.
After 30-plus years of controlling vacuum advance systems with full manifold vacuum, that “free”
indicator of engine load and fuel mixture, along came early emission control requirements (seven
years before catalytic converter technology was introduced), and all manner of crude band-aid
systems were introduced to try and reduce hydrocarbons and oxides of nitrogen in the exhaust
stream. One of these crude, but effective systems was GM’s Air Injection Reactor (A.I.R.) system,
which pumped fresh air into the exhaust ports to “afterburn” pollutants in the exhaust manifolds.
The key to making this system work at maximum efficiency was retarded spark at idle; with
retarded idle spark timing, the “burn” begins late, and is not complete when the exhaust valve
opens, which does two things which were important for emissions. The incomplete burn reduced
combustion chamber temperatures, which reduced the formation of oxides of nitrogen (NOX), and
the significant increase in exhaust gas temperature ensured rapid light-off and combustion of the
hydrocarbons in the exhaust gas stream when the fresh oxygen-carrying air was introduced from
the air pump.
5
Side Effects: As a result, these engines ran poorly, and an enormous amount of wasted heat
energy was transferred through the exhaust port walls into the coolant, causing them to “run hot”
at idle; cylinder pressure fell off, engine temperatures went up, combustion efficiency went down
the drain, and fuel economy went down with it. “Ported Vacuum” was easy to implement – they
just moved the distributor vacuum port orifice in the carburetor from below the throttle plate (where
it was exposed to full manifold vacuum) to above the throttle plate, where it was only exposed to
manifold vacuum after the throttle plate opened. This meant that the vacuum advance was
inoperative at idle (retarding idle spark timing from its optimum value), and these applications also
had very low initial timing settings; they were usually set at 4 degrees before TDC or less, and
some even had initial timing settings as much as 2 degrees after TDC. The vacuum advance still
worked at highway cruise, but not at idle, which caused all manner of problems. “Ported Vacuum”
was strictly an early pre-converter crude emissions strategy, and nothing more. Don’t believe
anyone who tells you that ported vacuum is a good thing for performance and driveability – it’s
not. Anyone with a street-driven car without manifold-connected vacuum advance is sacrificing
idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don’t
understand what vacuum advance is, how it works, and what it’s for. There are lots of long-time
experienced mechanics who don’t understand the principles and operation of vacuum advance
either, so they’re not alone

Add to this, ALL 1972 and earlier Pontiac V8 ignition vacuum advances adds 20° fully activated.

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Old 10-20-2021, 07:44 AM
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Smile Vacuum advance connection

I currently have my vacuum advance hose hooked up to a connection point on the front near the bottom of my Holley 750. A diagram I have shows it hooked up to the 3 connection plug on the front of the intake manifold. Where is the best place to hook up the vacuum advance to? and kudos to Kenth for this very informative essay.

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Old 10-20-2021, 09:50 AM
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either location can work. if not using the stock TCS, ported is usually best.

i run ported on a mild 400 4 speed that had some "bucking" at some rpms based on cliffs suggestion. also run ported on a 500+hp stroker with an auto based on cliff & others suggestions. both cars run excellent & use the stock initial timing settings.

i vote for ported based on what some others have said about having full adv at idle with mech.

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Old 10-20-2021, 10:45 AM
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Default Vacuum advance connection

Thank You 78w72,

I have a Holley 750 carb with vacuum secondaries and a Summit HEI vacuum advance distributor and since the vacuum connection is on the front of the carb at the base this is manifold vacuum because it is below the throttle valves on the carb? Also I have my vacuum modulator on the auto trans hooked up to a high point connection on the carb. Is this correct since I want all of my vacuum connections hooked up stock OEM?

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Old 10-20-2021, 11:16 AM
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Quote:
Originally Posted by Carousel72TRed View Post
Thank You 78w72,

I have a Holley 750 carb with vacuum secondaries and a Summit HEI vacuum advance distributor and since the vacuum connection is on the front of the carb at the base this is manifold vacuum because it is below the throttle valves on the carb? Also I have my vacuum modulator on the auto trans hooked up to a high point connection on the carb. Is this correct since I want all of my vacuum connections hooked up stock OEM?

Carousel72TRed
yes usually manifold vac is on the base plate of carbs & the ported source is the higher up locations. if vac advance is hooked to the bottom it is manifold & most/many people say ported is best for the vac advance if not using the factory stock set up. i would try switching the vac advance to ported & see how it runs with initial set at the stock number.

for the vac mod on the trans it definitely wants to be connected to full manifold vac for a strong signal, if possible use the factory hard metal line for that & not a rubber hose.

just a suggestion to swap things around, im not saying manifold vac is bad for the advance, just what worked best for my engines & what many others have said including cliff ruggles who builds q-jets for a living.

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Old 10-20-2021, 11:46 AM
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Default Vacuum advance connection

Thank You 78w72,

Next time I have the car out I will switch the vacuum line from the modulator over to a direct manifold vacuum connection. A lot of times when I get on it at a lower speed in 2nd gear it will not kick down into 1rst however going normal in drive it shifts just fine from standstill. When looking at my vacuum advance line I found a split right at the end where it goes into the carb and cut off the split.

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Old 10-20-2021, 01:43 PM
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What I have now appears to be ported...maybe someone swapped a base but I get 0inHg at idle and it jumps up to 15~17 inHg when I crack the throttle to 1000~1200rpm. I have already run it in my garage with straight vacuum and idle is still good either way. I have it set to 650rpm in gear.
I fully understand to do my initial with no vacuum or centrifugal advance involved. Right now it's set to 12*BTDC and I'm adding 14* with the adjustable vacuum. Of course, if I go with straight vacuum, those will be added all the time, not just when I reach 1200rpm.

I haven't had a chance to read everything in the thread yet, a lot was posted since I last viewed it, but I'm pretty sure it will answer my questions.
Seems to me is that the only difference is that I will be at 26*BTDC at idle instead of at off-idle. My original question was whether I needed to dial either vacuum can or initial back now that advance will now be blended at idle. In short, should I leave initial at 12 or dial back to 9? Vacuum at 14 or dial back to 12 or so?
Probably the answer is to drive it and see how it performs, listen for detonation, etc.

I'm really wanting to try straight manifold to see if it would alleviate some high engine temps when was caught a couple of times in complete shutdown weekend summer traffic. Remember, the port I'm using now would give me 0 advance in that situation. I'm talking 30 to 40 minutes to go two miles and nowhere to hide from that blazing sun.

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Old 10-20-2021, 03:24 PM
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Quote:
Originally Posted by Greg Reid View Post
I'm really wanting to try straight manifold to see if it would alleviate some high engine temps when was caught a couple of times in complete shutdown weekend summer traffic. Remember, the port I'm using now would give me 0 advance in that situation. I'm talking 30 to 40 minutes to go two miles and nowhere to hide from that blazing sun.
I don't think this proves anything but, on the carb that came on my car the VA was connected to a ported source on the carb. The only time it ever got hot was once, when it was already at operating temp and I let it sit at idle for 15-20 minutes or so. It went up to 220-ish. Since I've had it on manifold vacuum I've had it idle (fully warmed) for at least that long and it never goes above what I think is 180 on my gauge.

I need to go out and verify this but Davis Unified says my VA at idle, with manifold vacuum of 15 (which is what I have), should be giving me 12 deg of VA. They also said the mechanical advance is 24 deg all in by 3000 RPM. They also recommended an initial timing of 12, which makes sense, giving 36 total when adding the mechanical advance, which is in the ballpark. That makes me wonder about what Formulabruce said about not using VA at all. But, if this distributor's VA is only giving 12 deg then maybe it doesn't matter as much either way. I'll probably try it both ways and see how it runs. Of course, the driving season is rapidly coming to an end here.

One of the things I hate is overheating. I remember that being a problem for me and my Pontiac buddies back in the day. That was in South Florida where it was always hot, all the time, and there was always traffic, all the time. So maybe ported vacuum was part of that problem, don't know.

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