Which Vaccuum?
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Senior User
Joined: Aug 2006
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Which Vaccuum?
OK, so I'm old and confused. If you pull the vaccuum line off the carb that goes to the distributor and you have vaccuum coming from the carb port, is that called manifold vaccuum or port vaccuum? I bought a smaller carburetor and am trying to figure out which ports to block off. Trading a 750 DP for an old Holley 600.
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Joined: Jan 2006
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Here's some info from a article you can find here:
http://www.bob2000.com/carb.htm
<CENTER>
Vacuum ports and what they do.</CENTER>Not all Holleys have the same ports, but you can tell by location and size what each one does. Any port at the very bottom of the carb (in the aluminum baseplate) will pull manifold vacuum, which is strongest at high vacuum situations such as idle and slowing down, and weakest at low vacuum situations such as wide-open-throttle (WOT). Common manifold vacuum connections at the bottom of the carb are PCV (usually a 3/8" diameter tube) and EGR/distributor retard/smog device hookups (usually smaller tubes under the primary fuel bowl). The other type of vacuum connection on a Holley is ported vacuum, which is weak at idle, strong at cruise, and weak at WOT. This one is always (if supplied at all-some competition carbs don't have this port) on the passenger (US) side of the carb, above the idle mixture screw, and is generally used for distributor vacuum advance.
http://www.bob2000.com/carb.htm
<CENTER>
Vacuum ports and what they do.</CENTER>Not all Holleys have the same ports, but you can tell by location and size what each one does. Any port at the very bottom of the carb (in the aluminum baseplate) will pull manifold vacuum, which is strongest at high vacuum situations such as idle and slowing down, and weakest at low vacuum situations such as wide-open-throttle (WOT). Common manifold vacuum connections at the bottom of the carb are PCV (usually a 3/8" diameter tube) and EGR/distributor retard/smog device hookups (usually smaller tubes under the primary fuel bowl). The other type of vacuum connection on a Holley is ported vacuum, which is weak at idle, strong at cruise, and weak at WOT. This one is always (if supplied at all-some competition carbs don't have this port) on the passenger (US) side of the carb, above the idle mixture screw, and is generally used for distributor vacuum advance.
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To answer your question: That's ported vacuum.
It only shows up upon revving the engine up.
Manifold vacuum is present at idle from a port on the manifold.
One usually in the middle & One (Vacuum tree type) at the back for the Brake Booster & Auto Trans
It only shows up upon revving the engine up.
Manifold vacuum is present at idle from a port on the manifold.
One usually in the middle & One (Vacuum tree type) at the back for the Brake Booster & Auto Trans
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Try direct vacuum first, because this will give you extra vacuum advance at idle, keeping your spark plugs cleaner over time; however, some motors tend to idle too rough or too fast hooked up direct, in which case, you should run ported vacuum.
www.PerformanceDistributors.com
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