Vacuum Advance

[Anonymous]

I've been running without vacuum advance because when I hooked it up to the 2300 it caused a bog when applying gas at part throttle. We just set it to 16 degrees ,as much advance as possible without pinging. Problem was the starter wanted to kickback occasionally. So today on a whim I hooked it up to the manifold vacuum. Dialed initial back to 12, reads 32 now at idle. I was sure the bog would come back and my brother was sure it would ping, but it did neither. Took it to town and back, 40 miles, the engine is stronger when cruising and it may be an optical illusion but my gas needle didn't seem to move as much as normal. It'll take a couple tanks before I know for sure but I think my milage is going to be much better. I'm really surprised at the off idle transistion not being a problem, just doesn't make sense to me.

 

[Anonymous]

From what I understand the vacuum advance is designed to work at part throttle. It is often detached or not used on race cars that are operated at WOT. Cruising to town and back you would likely be operating at part throttle and therefore the vacuum advance would be helping out some.

-Dan in Atlanta

 

[Anonymous]

But it does make sense. Maximum power and fuel economy are going to occur when the expanding gases develop maximum pressure at the optimum point in the power stroke. Ideally, you would want it developing peak cylinder pressure as the piston is accelerating down. Too early, and the gases are working to slow the pistons travel up in the stroke. Right at the top of the stroke, and too much of the force is transmitted to the bearings, rods, & block. Develop peak pressure too late in the stroke, and part of the stroke is wasted. Max pressure does not occur the instant the spark is applied, the fuel takes a little time to burn and generate gas pressure. With no vacuum advance component, only a small percentage of the operating range is addressed by the initial timing and the centrifugal advance, and timing is not optimized over the whole operating range.

Engines do not expel the exhaust well at lower rpm’s. More ignition advance is needed in these diluted mixtures because the flame front progresses at slower speeds. At low load part throttle cruising, the engine is running at higher rpm's, and can be advanced more than what the initial timing and centrifugal timing will advance it. These operating regions are where vacuum advance helps improve power and fuel economy. Your engine operates a greater percentage of the time in these regions than it does at WOT.

You can get more total spark advance running a vacuum advance than if you did not run one. But when engine load increases, the vacuum drops and the vacuum advance component drops out of the equation, eliminating part throttle ping that might occur if timing were fixed (that is if the distributor is set up properly).