what does the vac advance do?

Question

On a carb with vacuum secondaries is this what the vac addvance does?

Answer 1

The vacuum advance is part of the distributor. Here's how it works. A heavily loaded engine produces little vacuum while the highest vacuum levels occur at sustained light-throttle cruise. But for maximum efficiency in either instance, peak pressure must still occur in the desired ATDC range. A heavily loaded engine, however, will likely detonate with the high levels of spark lead a lightly loaded engine can tolerate, so a load-sensing unit that uses engine vacuum to advance spark in light-load conditions is added to the distributor. As engine load increases and vacuum levels diminish, vacuum advance reduces spark lead to lessen the risk of engine-damaging detonation.

Answer 2

The vacuum advance works only if the intake manifold vacuum is up to 10 - 12 inches of mercury. Under full throttle there is little vacuum in the intake manifold. The vacuum advance does not work. The mechanical weights and resistance springs give the motor the advance. When you roll out of the throttle the motor the intake vacuum increases pulling in vacuum advance.

Prove this to your self by pulling the distributor cap off and attach a long small hose on the vacuum canister and the other end in your mouth. As you suck on the hose the canister moves the vacuum advance to its maximum. As you slowly real ease the vacuum the spring loaded diaphragm canister pulls the breaker plate back to no vacuum advance.

To solidify these terms connect a vacuum gage with a hose to an opening below the throttle plate. With the engine running take a vacuum reading. It should be at 15-16 inches of mercury. Quickly increase the throttle open. Notice the vacuum in the intake manifold decreases. In real life the vacuum canister reduces the vacuum quickly thus no vacuum advance.

This little canister prevents spark knock and pre- ignition. Imagine a full throttle start. There is little vacuum in the intake manifold thus NO vacuum advance. Buy a piece of long hose connected to the vacuum gage. Hold it in your hand as you accelerate. As you see there's very little vacuum in the intake manifold.

Answer 3

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RE:
what does the vac advance do?
On a carb with vacuum secondaries is this what the vac addvance does?

Answer 4

Vacuum Advance Distributor

Answer 5

The Vacuum Advance, increases the timing advance on your engine. It makes the spark plugs fire earlier than normal. This allows the engine to rev higher faster and give you more power (at the cost of fuel consumption). Puts in that "ooomph" when you floor the accelerator pedal.

Answer 6

It addvance the timing when you need it like pulling a load or just mashing the pedel to the floor. Yor motor works off of timeing and when need more power it will addvance the timeing for the sitution. Then return to the set timing when not need no more till next you need it it will do it again

Answer 7

Adjust it with the vacuum advance hooked up. You may want to get an adjustable advance unit. With a big cam you will have substantially less vacuum. This will alter your advance curve. An adjustable advance unit will allow you to tailor your curve to complement the new cam.

Answer 8

If it's on the carb, it either pulls the choke off or operates the secondarys on a Holley. If on distributor, it advances the points around for better timing and performance at various speeds.