Vacuum Advance??

[asecretfile]

For any of you who is carbed...do you have your distributor vacuum advance connected to the carb metering block port or full manifold vacuum or just capped off? Ive heard that on cars with a bigger cam they benefit more from full manifold vacuum rather than ported carb vacuum, true or no?

thanks

 

[nunly94gt]

On my old car I ran it to the carb. Never had a problem. I had the X cam so you should be fine.

 

[cjones]

it needs to be from a ported vacuum port. (vacuum increases as you accelerate) it's generally on the base. depending on the carb there is a large manifold vacuum port for the brake booster and/or PCV system(holley is under the fuel bowl) then a small ported vacuum source on the side (holley is on side opposite throttle linkage).

if you run it off manifold vacuum then the timing will retard at WOT.

i may run efi now but i cut my teeth on carbs

 

[67coupe]

The only difference between ported and manifold vacuum is at idle. At idle, there is no vacuum from the ported vacuum connection. Manifold vacuum will be its highest at idle. Other than idle, i.e. part throttle, cruise, WOT, ported and manifold vacuum are the same.

Vacuum advance was added for emissions and part throttle fuel economy. As you increase throttle position, vacuum decreases along with vacuum advance. This continues until WOT when manifold vacuum is near zero and so is vacuum advance. Once you reach a certain RPM, (whatever springs and weights are in your dizzy) mechanical advance is used.

On my 67, I could never get it to run right using either ported or manifold vacuum. Always had detonation at part throttle even when adjusting the vacuum advance to its lowest amount. I just plugged the ported vacuum port and used mechanical advance only.