Until now, Valve has been a bit vague with details on the Survival Pack expansion that’s headed for release next week. The pack is headed for release on both the Xbox 360 and PC. Valve goes into detail on this new “Survival” mode on the Left 4 Dead blog.
Their goal for the Survival expansion was to create a mode distinct from the Campaign or Versus. They heightened the pace of the spawning of Zombies in this mode, starting from slow and steadily rising into short spawn times. They said the amount of zombies killed in this mode often outnumbers an entire campaign. The sessions are supposed to last around 10 minutes or less.
They go into great detail about the spawn times and pace, in comparison to the campaign mode. If you manage to survive long enough, Tanks will begin to respawn every15 seconds and Hordes like every 5 seconds. They included a chart to show how the spawn times progress on the blog. It sounds like A LOT of fun, especially when you’re playing with your friends or something.
Surviving The L4D Survival Pack [Left 4 Dead Blog]