Normally Open Valves
Normally open valves are available with and without vacuum vents. Valves without vents provide a timed dump of air for 1.1-2.8 seconds when a sudden high vacuum of about 20 in. Hg is applied to signal port. This prevents backfire during deceleration.
Normally open valves with a vacuum vent provide a timed dump during deceleration and also dump when a vacuum pressure difference is maintained between signal port and vent port. Signal port must have 3 in. Hg more vacuum than vent port to hold dump. This is used to prevent catalyst from overheating.