Minecraft wiki entry for Mob_grinder, associated text: Timing is important for this trap to work efficiently. For single mob smashes, a loop of at least 30 repeaters set at maximum are needed, giving 12 seconds per loop. With this setup, turn on the loop for a little more than 10 seconds to ensure they die, and have the loop off for the rest, giving 10 second kills and 2 second load times. For a full row of 8 pistons, the load time is approximately 6 seconds, meaning the loop needs to have 40 repeaters giving a 16-second loop. Load time for a stream of 8 blocks long and 2 blocks wide, with pistons on both sides, is about 8 seconds. This amounts to at least 48 repeaters.