Thread: 12 degrees baseline-37 with vacuum???

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Ported vacuum and timed vacuum are the same thing.
Obtained from above the throttle blades.

Full time and manifold vacuum are the same thing.
Obtained from below the throttle blades.

Lots of advance at idle is good because it takes time to burn the lean idle mixture.
The great majority of engines will run hot in traffic or at idle on hot days - and cold ones as well - if the vacuum advance source vacuum is selected to ported vacuum.

Ported vacuum and manifold vacuum both retard the timing when the throttle is depressed.
It may help to view the vacuum advance cannister as a vacuum retard device.

Rpm controls the mechanical - or centrifugal timing - nothing else affects it.

Load controls the vacuum advance due to varying vacuum levels of the engine.
Except at idle when ported vacuum shows zero and manifold vacuum shows full vacuum - 17" - 19" with a stock or very mild cam - the vacuum levels of both ported and manifold vacuum follow each other fairly close.

Mechanical and vacuum advance systems overlap each other depending on engine load and rpm so as to give you the proper amount of advance for a particular situation.


When you time the engine the vacuum advance line should be disconnected and plugged at the distributor end . . . golf tees work great for this.
If you set total all-in timing with the vacuum advance line connected to manifold vacuum you'll be giving away a lot of horsepower.
Same thing is true with ported vacuum although to a lesser extent since it takes a bit of rpm to get ported vacuum up to full vacuum levels.

when I checked the total with the vac plugged it would go up to around 35 or so. I liked the way it idled better with the full vac but the guy at summit said I would burn pistons if I used 40 at idle. Obviously as I get on the gas in this case it would drop off quickly and my total would be a little less than that.
Well, you are right about some people saying I can't do it. I guess I'll just play with variations until it works well.

Originally Posted by tcodi

when I checked the total with the vac plugged it would go up to around 35 or so. I liked the way it idled better with the full vac but the guy at summit said I would burn pistons if I used 40 at idle. Obviously as I get on the gas in this case it would drop off quickly and my total would be a little less than that.
Well, you are right about some people saying I can't do it. I guess I'll just play with variations until it works well.

Burning pistons at idle is a new one on me.

More than likely something that never happened.

Nothing against Summit, I like them and trade with them, but like anyplace else some employee's know what they're doing and others are passing along what they think . . . right or wrong.


Got a Motor's Manual?

Do a little digging in there and you'll find both mechanical and vacuum advance curves. (Keep in mind some of the figures listed are in distributor degrees so you'll have to double them for total crankshaft timing.)
A little extrapolation to fit your needs and you'll be in business.

You can get an ajustable vaccum can. The one you you have must be 25 deg. adv. If you also at 20 deg of mech. That is a total of 57. What is your total at 3000 rmp? What is it at 3000 rpm with the vaccum unhooked?
Port vaccum increases as you accellerate, but can also cause the car to run hot at idle. Not enough vaccum advance.

yeah but the vac and mechanical wouldn't stack up, when mech's full on the vac is off

At cruise speeds you will have a combination of both, mech. and vac. + initial. Port vac. will increase until WOT. Manifold vac drops under load.

Originally Posted by Mikej

At cruise speeds you will have a combination of both, mech. and vac. + initial. Port vac. will increase until WOT. Manifold vac drops under load.

Ported vacuum follows to an extent, manifold vacuum except at zero.

Rather than post the whole write-up here, I'll make a separate post.
Look for: "More about vacuum sources and timing"

yeah but the vac and mechanical wouldn't stack up, when mech's full on the vac is off

True under full load, but if you're spinning the engine up in the driveway and holding the throttle steady with a high manifold vacuum, vacuum advance adds to the mechanical advance.