ported vacuum switch
Oldsdruid
Joined: Dec 2007
Posts: 10,618
From: Southside Vajenya
Need to know which engine you have. Vacuum connections are different between 2-barrel, 4-barrel, and stick cars. Assuming you have a 1973 455 by username.
At any rate, a 1973 engine should have a 3-port vacuum switch.
455-4bbl with 3-port switch:
Top port (marked D on OEM switch) goes to distributor vacuum advance.
Middle port (C on OEM switch) connects to ported vacuum on carburetor. It also has a thermal check spark delay valve inline between carb and switch.
Bottom port (MT on OEM switch) connects to intake manifold vacuum tee at front of engine. There is a tee in this line that also directs vacuum to the transmission vacuum modulator.
Most 4-port switches I've seen have a V port. This line connects to the air horn at top rear of carburetor. Under certain conditions the switch will vent vacuum back to the carb. It was routed there instead of to atmosphere to dampen noise. There are two nipples there, one for the choke fresh air supply and the other for the DTVS vent if equipped. If you don't need it, cap it to prevent a vacuum leak if the thing goes silly and tries to vent off vacuum.
The thermal check spark delay valve switches between full and restricted distributor vacuum advance depending on underhood temp. Below 50 degrees ambient it allows full vacuum signal; above 50 degrees it switches to restricted signal to delay vacuum advance. It's strictly an emissions thing and the car will run fine (probably better) without it.
At any rate, a 1973 engine should have a 3-port vacuum switch.
455-4bbl with 3-port switch:
Top port (marked D on OEM switch) goes to distributor vacuum advance.
Middle port (C on OEM switch) connects to ported vacuum on carburetor. It also has a thermal check spark delay valve inline between carb and switch.
Bottom port (MT on OEM switch) connects to intake manifold vacuum tee at front of engine. There is a tee in this line that also directs vacuum to the transmission vacuum modulator.
Most 4-port switches I've seen have a V port. This line connects to the air horn at top rear of carburetor. Under certain conditions the switch will vent vacuum back to the carb. It was routed there instead of to atmosphere to dampen noise. There are two nipples there, one for the choke fresh air supply and the other for the DTVS vent if equipped. If you don't need it, cap it to prevent a vacuum leak if the thing goes silly and tries to vent off vacuum.
The thermal check spark delay valve switches between full and restricted distributor vacuum advance depending on underhood temp. Below 50 degrees ambient it allows full vacuum signal; above 50 degrees it switches to restricted signal to delay vacuum advance. It's strictly an emissions thing and the car will run fine (probably better) without it.
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