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

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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.