Vacuum question
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
A ported vacuum fitting should have almost no vacuum on it at idle. That's the intent of the ported line. Vacuum should be almost nil at idle, but approach the same level as manifold vacuum once the throttle has been opening a bit.
But, in reality, I'm sure the vacuum level at idle for different 'ported' fittings would be a bit different. (But both should be at a very low level of vacuum.)
But, in reality, I'm sure the vacuum level at idle for different 'ported' fittings would be a bit different. (But both should be at a very low level of vacuum.)
Thread Starter
Racer
Joined: Jul 2013
Posts: 423
Likes: 6
A ported vacuum fitting should have almost no vacuum on it at idle. That's the intent of the ported line. Vacuum should be almost nil at idle, but approach the same level as manifold vacuum once the throttle has been opening a bit.
But, in reality, I'm sure the vacuum level at idle for different 'ported' fittings would be a bit different. (But both should be at a very low level of vacuum.)
But, in reality, I'm sure the vacuum level at idle for different 'ported' fittings would be a bit different. (But both should be at a very low level of vacuum.)
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
The location on the carb body doesn't really tell you anything. The sources for all of the vacuum nipples originate at the primary throttle bores in the throttle plate; internal passages in the plate and the carb body can transfer each source to about anywhere on the carb.
The only way to really know is to put a gauge on each fitting and measure vacuum at idle and then watch it as you slowly/gradually crack the throttle, or if you have a guide drawing provided by the carb manufacturer.
On most Q-Jets, the right-side port coming through the choke mechanism is manifold vacuum, as are the rear-center [large] port intended for the power brake booster and the front-center port that is intended for the PCV purge line. The two ports from the front of the carb body are both ported vacuum, I believe.
The only way to really know is to put a gauge on each fitting and measure vacuum at idle and then watch it as you slowly/gradually crack the throttle, or if you have a guide drawing provided by the carb manufacturer.
On most Q-Jets, the right-side port coming through the choke mechanism is manifold vacuum, as are the rear-center [large] port intended for the power brake booster and the front-center port that is intended for the PCV purge line. The two ports from the front of the carb body are both ported vacuum, I believe.
Thread Starter
Racer
Joined: Jul 2013
Posts: 423
Likes: 6
The location on the carb body doesn't really tell you anything. The sources for all of the vacuum nipples originate at the primary throttle bores in the throttle plate; internal passages in the plate and the carb body can transfer each source to about anywhere on the carb.
The only way to really know is to put a gauge on each fitting and measure vacuum at idle and then watch it as you slowly/gradually crack the throttle, or if you have a guide drawing provided by the carb manufacturer.
On most Q-Jets, the right-side port coming through the choke mechanism is manifold vacuum, as are the rear-center [large] port intended for the power brake booster and the front-center port that is intended for the PCV purge line. The two ports from the front of the carb body are both ported vacuum, I believe.
The only way to really know is to put a gauge on each fitting and measure vacuum at idle and then watch it as you slowly/gradually crack the throttle, or if you have a guide drawing provided by the carb manufacturer.
On most Q-Jets, the right-side port coming through the choke mechanism is manifold vacuum, as are the rear-center [large] port intended for the power brake booster and the front-center port that is intended for the PCV purge line. The two ports from the front of the carb body are both ported vacuum, I believe.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
If you NEED ported vacuum for something, use the one that has less vacuum at idle....AS LONG AS IT HAS GOOD VACUUM WHEN YOU CRACK THE THROTTLE.
AFAIC, the only "good" use of ported vacuum is for the line going to the fuel vapor recovery canister which signals the can to 'purge'.
AFAIC, the only "good" use of ported vacuum is for the line going to the fuel vapor recovery canister which signals the can to 'purge'.
Thread Starter
Racer
Joined: Jul 2013
Posts: 423
Likes: 6
If you NEED ported vacuum for something, use the one that has less vacuum at idle....AS LONG AS IT HAS GOOD VACUUM WHEN YOU CRACK THE THROTTLE.
AFAIC, the only "good" use of ported vacuum is for the line going to the fuel vapor recovery canister which signals the can to 'purge'.
AFAIC, the only "good" use of ported vacuum is for the line going to the fuel vapor recovery canister which signals the can to 'purge'.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
The 'control' port on the vapor canister should be connected to a "ported" vacuum nipple on your carb. That will cause the canister to purge only when the car is above an idle condition. The 'inlet' (or IN) line should be coming from the fuel tank and the 'output' (OUT or PURGE) fitting should be connected to the top side of the PCV valve with a T-fitting. The other side of that T-fitting runs to the large port at the front of the carb base plate. That's where the vapor fumes are directed (along with PCV valve output) for getting into the fuel charge for burning.
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Thread Starter
Racer
Joined: Jul 2013
Posts: 423
Likes: 6
The 'control' port on the vapor canister should be connected to a "ported" vacuum nipple on your carb. That will cause the canister to purge only when the car is above an idle condition. The 'inlet' (or IN) line should be coming from the fuel tank and the 'output' (OUT or PURGE) fitting should be connected to the top side of the PCV valve with a T-fitting. The other side of that T-fitting runs to the large port at the front of the carb base plate. That's where the vapor fumes are directed (along with PCV valve output) for getting into the fuel charge for burning.