Vacuum advance functionality questions
Vacuum advance functionality questions
(OP)
I'm sure this must have come up before, but I did a search and couldn't find it, so here it goes. My question is on distributor vacuum advance, old school, I know. I know because of lean mixtures at light loads it's desireable to advance spark timing for optimal pressure peak timing, with advance decreasing with increasing load. Also, most cars don't advance timing at idle. My question is how the correct vacuum signal is obtained. When I look at the vacuum port location in a throttle body or carb, it's just a hole upstream of the throttle plate in the closed position, on the atmospheric side. This location confuses me, the pressure signal looks like it's static and would have increasing vacuum with load. I don't see anything that's measuring stagnation pressure. I've seen advance curves for distributors and they increase advance with vacuum. So if I look at the port location and the distributor advance curves, It should be getting advance timing at higher loads instead of lower, which I know it doesn't. What am I missing?





RE: Vacuum advance functionality questions
Mike Halloran
Pembroke Pines, FL, USA
RE: Vacuum advance functionality questions
RE: Vacuum advance functionality questions
I think you are confusing dynamic pressure with static pressure.
The dynamic pressure increases with increased flow as you stated, but the port is measuring static pressure which is independent of flow. As Mike stated, the vacuum decreases as the throttle blade opens, until wide open throttle where the static pressure is approximately ambient pressure.
Reidh
RE: Vacuum advance functionality questions
RE: Vacuum advance functionality questions
RE: Vacuum advance functionality questions
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RE: Vacuum advance functionality questions
RE: Vacuum advance functionality questions
RE: Vacuum advance functionality questions
Try to find a carburetor and a factory shop manual pre-1969, and study both.
Mike Halloran
Pembroke Pines, FL, USA
RE: Vacuum advance functionality questions
One caution if you look for older shop manuals, as Mike stated. There are some older cars (e.g. some VW cars/buses in the mid 60's) that had true vacuum advance, i.e. applied more vacuum to the vacuum solenoid on the distributor with increasing speed, instead of a mechanical advance mechanism. These worked by having a vacuum port in the center of the venturi, and a really sensitive vacuum solenoid on the disti.
Woe to the parts swapper that put one of these on a car that had a carb with a standard, off-idle vacuum port. The disti would go full advance at off idle (ping city) and have almost no advance at full throttle.
RE: Vacuum advance functionality questions
RE: Vacuum advance functionality questions
RE: Vacuum advance functionality questions
RE: Vacuum advance functionality questions
RE: Vacuum advance functionality questions
I have a copy stashed away. I can't remember the exact title, but it was by HP books or SA design or something like that. It has all circuits and ports explained, at least to pre pollution models.
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RE: Vacuum advance functionality questions
"Advance decressing with more load"
You just hook it to the manifold vaccum, that is any place below the throttle plate. If you can't figure it out at the carburetor. Just use a fitting that screws into a port in the intake manifold. At idle advance will be most, and when pulling a load it will be least.
RE: Vacuum advance functionality questions
I think part throttle could be either rich or leaner stoichomtric, and generally more advanced ignition timing would still be required for best torque due to the cooler, slower burning less dense mixture.