[PKRWUD]

Originally posted by red82gt
I'm gonna need the pictures, I know how it works, except the part where you say that the spark port keeps pulling harder as the air moves faster, wouldn't this mean that both advance mechanisms are doing the same thing, increasing with rpm?
The spark port pulls a vacuum that is proprtional to the load so it'll pull it's hardest under the least load once the throttle blades are opened. I also have an article in HOT ROD that says the exact thing as the FordMuscle article and I think if it was incorrect, it wouldn't have made it into print.

This is from your article:

Vacuum Advance: Most Ford distributors include a vacuum advance mechanism. This consists of a diaphragm vacuum canister, an arm from the canister to the breaker plate, and a hose connected to an engine vacuum source. The purpose of this mechanism is to provide spark advance when the engine is not spinning fast enough to create the centrifugal advance talked about earlier. In other words this is an engine-load dependent advance. This would be a typical situation when climbing a steep hill, or driving at low rpms, light throttle, conditions. In these conditions there is high engine vacuum, so the vacuum signal applied to the diaphragm in the canister, via the hose, will cause a 'pull' effect on the arm, which moves the breaker plate and results in a timing advance. During full throttle conditions there is very little engine vacuum, and thus the vacuum advance does not contribute to total advance.

Have you ever driven up a steep hill? Your pedal is to the floor, and your manifold vacuum is at zero. The last line is completely false. If manifold vacuum and spark vacuum were in fact the same, then it would be correct, BUT THEY ARE NOT. They are the opposite. if they were the same, it wouldn't matter where you got your vacuum from for the distributor.

Vacuum advance is tricky to tune because there is no direct measurement like total. In fact, the reason you must measure initial and total timing with the vacuum hose disconnected is because when the engine is in neutral there no load, thus the vacuum is high, and if the hose were connected you'd see as high as 60 degrees advance and think something is really wrong! The only way to tune vacuum advance is on the road, by feel, and AFTER the initial and total are adjusted.

This is simply false. Check the vacuum at the spark port when you are idling. There is none. The vacuum increases as the throttle plates are opened.

I'll take some pictures for you. Do you understand the venturi principal? You need to in order to understand this. I'll be back in a few.