I've just finished rebuilding a conventional Y-block distributor to replace the Load-O-Matic that came on my 1956 F100. My question is: Where is the best place to pick up the vacuum signal for the advance mechanism?

Thanks for your help.

 

This question is almost as dangerous to ask as the "what's the best oil" question.

That said, I prefer to take vacuum signal for the distributor centrifugal advance directly from the manifold. Of course, this will make your engine idle faster, which means you will need to turn down your idle speed.

It is important that the throttle plates not be too far below the idle transfer slots in the carb. If they are, you will have a slight hesitation just off idle. The tip of the throttle blade needs to be right at the transfer slot.

But if this issue is not a problem, then I feel it best to get the most idle speed. I feel that the engine idles a bit cooler and in some extreme instances, this might help to prevent run-on when the engine is shut down. (Altho this is not normally a problem with Y Blocks.)

 

Thanks for the tips. I make sure that the throttle plate is in the appropriate position before making any other adjustments. I don't understand, though, why taking the vacuum signal from the manifold would increase the engine idle speed. I realize that if the intake manifold draws air through any port other than the carburetor, the idle speed would increase, but a sound vacuum advance diaphragm - I just ordered a new one - should not admit air into the intake manifold.

 

Because this will apply vacuum to the vacuum advance diaphragm, and result in more advance at idle.

Consider, when you change points and set the timing with the vacuum advance disconnected: have you ever noticed that you can speed up the engine idle, just by advancing the distributor, sometimes to 25* or 30*?

That is what attaching the vacuum lead to the manifold will do, because most vacuum advance assemblies will give full advance at around 16 to 20 inches of vacuum. If you set the timing to spec with your timing light, you will see the advance jump immediately upon connecting it to manifold vacuum, and you will sense an increase in the idle speed at the same time.

On the other hand, when using ported vacuum, the vacuum signal is tapped from the throttle bore just above the throttle blade, so no manifold vacuum is shown to this port until the throttle is tipped open to an off-idle position. When the throttle blade passes this port, then it is exposed to manifold vacuum and acts accordingly.

The vacuum advance will drop to 0* if you mash the pedal to the floor from a standing start, as it should. Idle and lighter throttle applications around town are the only thing that will be affected by the manifold vacuum being connected to the advance assembly.

Remember that the vacuum advance's purpose is to be a sense of engine load. The more load on an engine, the less advance is needed. Think of it this way: When the vacuum is high, the throttle blades prevent very much air-fuel mixture into the cylinder. Even when the mixture is then compressed, the gasoline molecules are still relatively far apart. They are not colliding with the air molecules very often and therefore it takes a longer time for the igniting flame to pass from molecule to molecule. Because there are not very many molecules in the cylinder, the effective compression ratio is low -- and we know that one can usually use more advance when the compression is low, all other things being equal.

On the other hand, when the throttle plates are wide open, lots of air-fuel molecules are getting into the cylinder. The most get into the cylinder at the peak torque rpm, because this is the speed where the engines volumetric efficiency is highest. In other words, more air-fuel molecules fill the cylinder at the torque peak/volumetric efficiency point than at any other. The piston is moving fast enough to create the highest pressure differential between it and the atmosphere while the intake mixture is still able to have the time to get into the cylinder before the intake valve closes. In other words, the piston is going fast enough to make lots of vacuum, but still slow enough that the mixture has time to fill the cylinder. Any faster, and the mixture doesn't have time to fill the cylinder. Horsepower still climbs, because there are more power impulses per second, even tho they are less powerful, but torque drops, because each impulse is less powerful.

Anyway, when the cylinder is as full as it can get at this point, then the effective compression ratio is at its highest point (the maximum number of air-fuel molecules are in the combustion chamber) and with the molecules all crowded close together, the igniting flame travels very quickly thru the chamber. So, little advance is needed.

The vacuum advance senses engine load by saying, "When the throttle is closed, little air-fuel is in the engine and the load is light, so I'll give the spark lots of advance." On the other hand, the engine says, "When the throttle is wide open, the engine is getting a full charge of fuel, so the load on the engine is high."

These conditions can be sensed by manifold vacuum.

Note that the centrifugal only part of the curve is usually staged by the "Strong spring" and the "weak spring" in the Ford distributor: The light spring allows the advance to come in quickly, up to the torque peak or a little sooner. Then, the strong spring's tension is also added to the advance curve, so that the advance is much slower for the last few additional degrees. The increased rpm and the slight lessening of the effective compression ratio as the rpm passes the point of highest volumetric efficiency allows for the slight increase in advance.

Another factor plays a part here, too: when the mixture starts to burn and expand in the chamber, the pressure in the chamber starts to rise. This can cause mixture in another part of the chamber to ignite spontaneously, especially if there is a hot spot like an exhaust valve or sharp chamber edge. The result of the spark-induced flame front and the spontaneous flame front is ping or knock: pre-ignition.

It takes a certain amount of time for this to occur, and less time is available when the rpm rises, so the advance curve can possibly take this into account, also, and start the spark sooner.

In any event, the spark seeks to get the maximum cylinder pressure on the crank at or just before the rod is at 90* to the crank pin's radius line, so that the maximum amount of expanding pressure will push on the crankshaft.

The long rods (relative to the stroke) in the Y Block, along with the efficient combustion chambers that most Fords have been traditionally provided with, allow the engine to operate optimally with modest amounts of advance (as compared to brand x, for example, or engines of the famous "hemi" design).