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Ported vacumn is generated by flow velocity thru the venturi of the carb.




That is correct. And intake manifold vacum will be right around o at wide open throttle where ported vacum will be high. Thats why ported is used so much as it will give more advance with more throttle. . Ron



Again, this is wrong information. At wide open throttle regardless if it is ported or manifold vacuum there will be no vacuum or very little vacuum. The ported vacuum is there for just idle or very close to idle conditions. Once you are on the road there is very little difference between the two. I still think that using ported vacuum is better for more situation as long as the distributor advance slots have been shortened to allow much more mechanical advance then is the norm on the stock dist.






The ONLY difference between "ported" and manifold is the ported passage is just above the throttle blades. When the throttle is closed it closes the port = no vacuum = no vacuum advance at idle. As soon as you crack the throttle past the port it then sees manifold vacuum. This was an emissions era invention.

The manifold vacuum port passage is below the blades so it sees vacuum or the lack of it all the time.

A simple test to confirm this is get 2 vacuum gauges and hook one to each port. You will find that they will be basically the same everywhere but idle.

Kevin




I agree I used the wrong words as I was refering to venturi vacum but I said ported.
Ported is actually manifold vacum but it is placed above the throttle plate so you have no vacum at the port at idle.
Venturi vacum is from airflow thru the carb venturi which increases with more airflow. It is how the vacum carbs work from venturi vacum. Mopar also ueed a venturi vacum signal to the EGR amplifier in the 70's as it was the signal it got and it told the amplifier when to put intake vacum to the EGR valve.

Another thing to remember is intake and ported vacum will be strongest on closed throttle decell and then at idle. And when at part throttle the intake and ported vacum signal is still strong as thats why the vacum advance can help part throttle power and milage. But when the throttle reaches a certain point close to and at wide open throttle the vacum drops to about 0 because with the throttle open there is no way to create a low pressure area under the throttle.

Many cars in the 70's with all the emission devices used the dual vacum advance which at times also put manifold vacum on the other side of the vacum diaphram to retard timing at idle. That way they could open the throttle more to maintain idle speed letting in more air thus leaner mixture at idle. They also used Decell valves that would advance the timing under decelleration as another emission device.

Bottom line is most vacum advances use ported vacum which is intake vacum taken above the throttle plate and would give alot of advance at part throttle but when going to wide open throttle the vacum advance would drop off and the mechanical advance was what took over.
I was wrong in stating most vacum advances use venturi vacum as some did but not alot. Ron