Vacuum Test
Thread Starter
Heel & Toe
Joined: Dec 2007
Posts: 17
Likes: 0
From: MA
Vacuum Test
Where is the correct place to get a vacuum reading on a '71 LT-1 with a holley carb. I hooked the gauge up before the vacuum advance where the vacuum line splits with one line going to the A.I.R. and the other going to the vacuum advance. I also hooked the gauge up to the carb where the vacuum line attaches and get a different reading. The former gives a good reading and the latter a lower reading. Should there be a difference, and where is the best place to do this test?
Thanks.
Thanks.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
If you are just measuring the vacuum level of the idling engine, you can hook into any fitting on the intake manifold or a carb fitting that is connected directly to the manifold. [You should not plumb into the PCV 'dump' line going into the base of your carb, if you have one; but that fitting should be blocked off, also..]
To determine to max vacuum your engine is capable of producing, you also need to disconnect (and block vacuum) lines to all vacuum operated systems so that any leakages within those systems will not affect "baseline" readings. You can also adjust timing to maximize your vacuum level at idle, but you won't want to do that if you have already set up your distributor curve for "performance".
Once you have established a vacuum 'baseline', you can re-connect those vacuum lines...one at a time...while monitoring to see if that baseline level changes (drops) significantly. If so, there is a leak somewhere in the system that was just re-connected, and that should be diagnosed and resolved. Once all vacuum systems have been reconnected, and the engine vacuum level has not dropped more than about 2" Hg from the "baseline" reading, you should be good to go!
To determine to max vacuum your engine is capable of producing, you also need to disconnect (and block vacuum) lines to all vacuum operated systems so that any leakages within those systems will not affect "baseline" readings. You can also adjust timing to maximize your vacuum level at idle, but you won't want to do that if you have already set up your distributor curve for "performance".
Once you have established a vacuum 'baseline', you can re-connect those vacuum lines...one at a time...while monitoring to see if that baseline level changes (drops) significantly. If so, there is a leak somewhere in the system that was just re-connected, and that should be diagnosed and resolved. Once all vacuum systems have been reconnected, and the engine vacuum level has not dropped more than about 2" Hg from the "baseline" reading, you should be good to go!
Last edited by 7T1vette; Jul 8, 2018 at 11:36 AM.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
Block off everything. Unhook PCV, power brakes and especially headlight-wiper-duct vacuum. Now you know what your engine vacuum is. Then hook everything up one at a time looking for changes...
