Normally Open Valves
Normally open valves are available with, and without, vacuum vents. Valves without vents release air for 1.1-2.8 seconds when a sudden high vacuum of about 20 in. Hg is applied to the signal port. This prevents backfiring during deceleration.
Normally open valves with a vacuum vent release air at intervals during deceleration, and when a vacuum pressure difference is maintained between the signal port and the vent port. The signal port must have 3 in. Hg more vacuum than the vent port. The pressure difference, along with the timed release of air during these stages prevent the catalyst from overheating.