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vacuum canister
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lonesome mopar
Posted 2015-08-31 7:06 AM (#488394)
Subject: vacuum canister



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Location: Milano, Italy

hello friends,

reading by the references made to the vacuum unit  (or canister, or chamber) in this forum  I see that it is generally assumed that the unit serves to advance the spark.

I am of the opinion that the factory designes the vacuum canister at the same time of the spring advance so that at cruising speed the total maximum  advance is the sum of the two units.

 

At this point the logical question is why adding another mechanism to obtain such advance?  Some wider excursion in the weight assembly (making them opening more)  could give the same total adavance....

 

The reason for the vacuum canister is, in my humble opinion, the need of reducing the advance to avoid pinging at the sudden opening of the throttle  (in example, when one asks

for an extra acceleration  to pass a truck) as the slowing of the flow of gas into the intake manifold demands a little temporary retarding of the spark, compared to what it would be needed  at constant speed.

That is why I assume that the vacuum canister is a " retarding " unit  rather that an advance unit.

 

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Polybun
Posted 2015-08-31 7:54 AM (#488400 - in reply to #488394)
Subject: RE: vacuum canister



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lonesome mopar - 2015-08-31 4:06 AM

hello friends,

reading by the references made to the vacuum unit  (or canister, or chamber) in this forum  I see that it is generally assumed that the unit serves to advance the spark.

I am of the opinion that the factory designes the vacuum canister at the same time of the spring advance so that at cruising speed the total maximum  advance is the sum of the two units.

 

At this point the logical question is why adding another mechanism to obtain such advance?  Some wider excursion in the weight assembly (making them opening more)  could give the same total adavance....

 

The reason for the vacuum canister is, in my humble opinion, the need of reducing the advance to avoid pinging at the sudden opening of the throttle  (in example, when one asks

for an extra acceleration  to pass a truck) as the slowing of the flow of gas into the intake manifold demands a little temporary retarding of the spark, compared to what it would be needed  at constant speed.

That is why I assume that the vacuum canister is a " retarding " unit  rather that an advance unit.

 



See the problem there is, you're dead wrong!

Vaccum advance serves the function of increasing cylinder pressure during partial throttle application. This is accomplished by further advancing ignition timing during partial throttle. I've seen some cars running close to 45 degrees btdc with all of the advance in, and really at 2500rpm cruising down the highway, that's about right! Now for peak power you want a bit less than that, and detonation could occur if the fuel wasn't very nice and the compression high. Sooo, you put a vaccum advance module in there and it's pure win. When you stand on the car the advance can be set to give around 32btdc on the timing and when you shut the throttle blades you get up to around the 40-45btdc timing you want at partial throttle.

BTW, vacuum canister is the wrong word for it. It's a vacuum advance module. Vacuum canisters are another part all together!

It should also be said that some manufacturors did use vacuum retard systems, but they don't work very well because you are now working with a much weaker source of vacuum and you don't get much in the way of change on the ignition timing because of it. You also need a carburator specifically ported to work with vacuum retard.
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lonesome mopar
Posted 2015-09-01 3:32 AM (#488508 - in reply to #488394)
Subject: Re: vacuum canister



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Posts: 397
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Location: Milano, Italy		:-))

I was expecting that, as that is the general opinion on vacuum advance module (or unit')

What keeps one from using a wider advance curve using simply the weight assembly?
We know the weights open with the centrifugal speed of the timer, which proportional to the engine rpm, which increase with the car's speed in direct gear. - and that is independent from the throttle.
Why adding an extra part to obtain the work that another part is already partly making....?

As a matter of fact, any variation in the opening of the throttle -once you are cruising at partial throttle -, demands some degree of retarding of the spark.... there you need the vacuum device..... :-))
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