There may not be one.
If you want to keep your automation in one dimension running while you're hanging around in another, and you're in SSP, forget it. Back in those days, singleplayer Minecraft would only ever load the dimension that the player was currently in, and whenever you go to another dimension, the dimension you were previously in unloads.
If you're on a server, or if you want to keep a distant base running while you're elsewhere in the same dimemsion (but why?), you might have some options. Buildcraft pipes, for instance, will load any chunks that they try to shove items into. So, if you set up a loop of transport pipes around the corner of a chunk and pump a bunch of items in, they'll circulate around and continuously load those chunks, as long as they don't all unload at once. Do note that this will have a tendency to repeatedly load and unload those chunks all the time, which is absolutely terrible for your TPS.
If you've got Teleport Pipes (or whatever that mod's called), is has a block called a Teleport Anchor (or something like that) that is an actual chunkloader. It'll load the chunk it's in as well as the four adjacent chunks in a plus shape. I do not remember if this mod is actually in the FTB retro pack, but if there's a pipe you can craft with two diamond gears (and is not a Logistics Pipe), it's in there.