Vacuum question for Holley
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Mountain Pass
Joined: Feb 2015
Posts: 167
Likes: 5
From: Virginia
Vacuum question for Holley
Something I can't figure out with all the argument over ported or full manifold vacuum...
Based on reading, it seems you get some extra benefit of running full manifold vacuum (cooling, better mpg?), but Holley's instructions on their carburetor recommend a ported vacuum for the distributor.
I'm probably over thinking it but can think it though. Thanks all
Based on reading, it seems you get some extra benefit of running full manifold vacuum (cooling, better mpg?), but Holley's instructions on their carburetor recommend a ported vacuum for the distributor.
I'm probably over thinking it but can think it though. Thanks all
Laughing Gas
Joined: Dec 2020
Posts: 869
Likes: 186
From: Near my truck
Vacuum advance at a cruising rpm provides far better gas mileage than having no vacuum advance.
Ported and manifold vacuum are exactly the same with regard to ignition timing vacuum advance once the throttle plates are open.
The main difference between the two vacuums can be found at idle.
Assuming the engine is not fitted with OE emission things, one can run a Holley, or an Edelbrock for that matter, with as much as 40* ignition timing with no problem. (20* mechanical + 20* manifold vacuum say.)
As suggested, try both setups and see the difference for yourself. (It doesn't take long.)
Ported and manifold vacuum are exactly the same with regard to ignition timing vacuum advance once the throttle plates are open.
The main difference between the two vacuums can be found at idle.
Assuming the engine is not fitted with OE emission things, one can run a Holley, or an Edelbrock for that matter, with as much as 40* ignition timing with no problem. (20* mechanical + 20* manifold vacuum say.)
As suggested, try both setups and see the difference for yourself. (It doesn't take long.)
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