[C2] Ported vacuum and manifold vacuum
06-22-2018, 01:48 PM
#21
Le Mans Master
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Update. I was using the full manifold vacuum fitting at back of intake manifold. My mechanic is a fan of ported vacuum. He took some measurements off the "timed spark vacuum" fitting on the Holley Street Avenger carb and says it is not a true ported vacuum fitting. He moved the vacuum line to the "full manifold vacuum" fitting at the base of the carb rather than the fitting on the manifold. He says the carb fitting responds better. If I understand what he is saying he wants a manifold vacuum source that starts to drop vacuum as you get into the throttle thus reducing vacuum adance. Is there a difference between to the vacuum sources?
Glad to see you are sorting out the different ports. Well an easy way to find if a carb's vacuum port is above or below the throttle plates is to hold the throttle blades open and insert a fine wire into the port. If the path is direct you should be able to see the wire enter the carb venturi. Now if the port has an indirect path I have successfully used a spray can of WD40 to squirt in liquid and watch where it exits.
I have found at least 3 types of vacuum ports.
Direct under the throttle blades - usually the ports on the carb base are easy to verify as this (but not always).
Ported on the side of the carb that enter the venturi just above the throttleblades. These are not exposed to vacuum until the throttle blades open high enough but will source the manifold vacuum once uncovered.
Venturi vacuum ports that are exposed (somehow) to the vacuum produced by the increased airflow through the venturi. Yes the vacuum (signal) actually increases at this port as rpm increases. It is a physical nature of an increased flow velocity in any pipe the static pressure decreases. You can read up on Burnoulli's equation if you need to research this. But many owners are surprised when they connect to a port like this. And no you don't want to be connecting your distributor to a port like this. I can guess it's used more for pull-off diaphragms like those found on qjets.
Now as far as a difference between the 2 vacuum ports you are considering/using a vacuum gauge should be able to give you the information to determine this rather than have me guessing over the internet.
Hope this can help. Let us know what you find please.
06-22-2018, 04:56 PM
#22
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06-22-2018, 05:16 PM
#23
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Excellent article, John. Should be required reading for everyone on the C1-C2 forum. I liked the Model T reference, as I drive a stock '15, and I left the spark advanced while hand cranking ONCE. And only ONCE. It came around and hit me but didn't break anything. The key is to not wrap your thumb around the handle, RETARD the spark, and pull it only from 9 to 12 o'clock. Going around the horn is flirting with disaster! Again, great piece on timing and a great contribution. Thank you.
06-22-2018, 07:03 PM
#24
Ported vacuum is above the carburetor throttle plates and full-time vacuum below the plates.
To wit, ported vacuum isn't operational at idle (throttle plates closed) as the orifice isn't exposed to vacuum.
Hence you miss cooler, smoother running at idle due to the increased advance provided by the vacuum operated advance....once the car is moving (throttle plates open) the two types of vacuum begin acting similarly...
As mentioned ported vacuum was an emissions contrivance to keep timing retarded at idle; there is no advantage to it currently. However, you will still get some owners that insist their car runs better with ported vacuum, but have a hard time explaining why in detail.
To wit, ported vacuum isn't operational at idle (throttle plates closed) as the orifice isn't exposed to vacuum.
Hence you miss cooler, smoother running at idle due to the increased advance provided by the vacuum operated advance....once the car is moving (throttle plates open) the two types of vacuum begin acting similarly...
As mentioned ported vacuum was an emissions contrivance to keep timing retarded at idle; there is no advantage to it currently. However, you will still get some owners that insist their car runs better with ported vacuum, but have a hard time explaining why in detail.