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1973 Custom hatchback
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Discussion Starter #1
Ha not a question about ported vs manifold vacuum LOL. not going there

SBC 350 assume stock HEI with vacuum and mechanical advance
Simple ? really: Is vacuum advance added to or incorporated within the mechanical ?
In looking at how the advance works I'm not 100%.
E.G. say I'm at 12 deg no vacuum, base timing ( I am)
and I hook the vacuum advance hose to manifold vacuum, at idle 750, I may have another 15 deg so 12 + 15 = 27
So as rpms increase, high load , WOT, low vacuum the mechanical advance will begin at what RPM? and advance to what degrees?
I'm thinking its not as simple as 12 + 15 + say 15 more from mechanical.
At steady cruizin say 2300 RPM light load, I would think there would be upwards of 25 deg, and no or little mechanical advance.

I cant imagine any scenario where I would have full vacuum and full mechanical.

I'm thinking the vacuum advance exists "inside" the range of the mechanical... if the vacuum is pulling ten degrees the mechanical would have to get up to that ten degrees and past it to effect timing. But as RPMs increase vacuum drops, so the "Passover point" would change?
Is there a scenario where the would fight each other?

Thanks
 

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1973 Custom hatchback
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Discussion Starter #3
The first post in this search may have your answer.

You mean where it says this:

Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

I have read that before, missed that detail but it doesn't anwser my ?
 

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Discussion Starter #4
My issue is TMI. INFO OVERLAOD. I have read so much I cant get it clear in my head... links to read more, much of which I have already read, although appreciated, aren't helping.
My distributer, jegs 555-40005. Jegs site is not letting me ask ?s, the instructions/ tech are no help. I did see in a answered ? that total mechanical advance is 20 deg @ 3500. no mention as to when it starts or the curve.
SO I'm figuring 12 deg + 20 = 32 total. which is safe and normal.
say I'm going 3500 cruizin along, par throttle...lots of vacuum
would there be 32 or 32 plus say ten more from vacuum.
Or would the vacuum not move the timing as the weights already have?
See its a different ?
 

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The 2 are independent of each other. As you started to get at your mechanical will add 20 to whatever your base is. Under high vacuum the vacuum advance can add 10 or more depending on how much you can will move. As far as a curve you’ll have to check that with a light to see what it is at each rpm. If it’s 20 by 3500, you would have to see what rpm it starts advancing. May be 18-2000 rpm. After that it’s kind of linear increase.
 

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Mike - 74 Nova Baltimore, MD
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Rupertflood, In a nutshell, mechanical advance and vacuum advance (VA) are 2 separate timing functions of the distributor.
In your example, you can be cruising at 3500 rpm (light throttle with high vacuum) and have both 32° of mechanical advance (12° initial + 20° mechanical) PLUS 15° of VA... for a total of 47°.
 

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Mike - 74 Nova Baltimore, MD
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Here's a little more technical answer (for a Chevy HEI distributor)...
*The distributor's Mechanical Advance Mechanism controls the "advance movement" of the reluctor (star wheel). As engine's RPMs increases, the distributor's Mechanical Advance Mechanism will rotate the reluctor in a clockwise direction to advance the timing (the amount of advance is controlled by the shape of the weights and centerplate... AND the rate of advance controlled by spring tension on the advance weights).

*The distributor's Vacuum Advance Mechanism controls the "advance movement" of the pick-up. As the engine's vacuum level increases, the VA pin (that is connected to the pick-up plate) will rotate the pick-up in a counterclockwise direction to advance the timing.

As the reluctor and/or the pick-up rotate/move towards one another, you gain increased timing advance.
 

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Discussion Starter #8
Here's a little more technical answer (for a Chevy HEI distributor)...
*The distributor's Mechanical Advance Mechanism controls the "advance movement" of the reluctor (star wheel). As engine's RPMs increases, the distributor's Mechanical Advance Mechanism will rotate the reluctor in a clockwise direction to advance the timing (the amount of advance is controlled by the shape of the weights and centerplate... AND the rate of advance controlled by spring tension on the advance weights).

*The distributor's Vacuum Advance Mechanism controls the "advance movement" of the pick-up. As the engine's vacuum level increases, the VA pin (that is connected to the pick-up plate) will rotate the pick-up in a counterclockwise direction to advance the timing.

As the reluctor and/or the pick-up rotate/move towards one another, you gain increased timing advance.
thanks Rif! PERFECT! I planned on looking at my old distributer to see what I could figure out, IRT what the two advancements move and how, got busy and ran out of time.
thanks again
 
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