Hi David, I think there are a couple ways to go about answering your questions. Searching on the forum will certainly get you a number of hits, but I'll try and break things down quickly into four categories of fuel system implementations (there are probably several other ways to further break things down, but this ought to get you started anyway):
1. Carburetor: No need for a return line. Fuel goes in one direction from the tank to the carb.
2. Standard Mechanical Fuel Injection (e.g. Bendix RSA-5): No need for a return line. These really aren't much fancier than carburetors, except for the fact that they distribute fuel more evenly to each cylinder by keeping the air and fuel separate until it reaches each cylinder. More prone to hot fuel problems than carburetors due to the tiny little fuel lines that run all over hot parts of the engine, see #3 for how some people solve this.
3. Fuel Purge system: This isn't a fuel system in itself but is a sort of "add-on" to an otherwise return-less system. A single return line can be plumbed from near the engine back to [typically just one] fuel tank. In this way, hot fuel can be manually returned to one tank by opening a purge valve and running the boost pump in order to reduce problems associated with vapor lock or other hot-start issues.
3a. Less optimally, if you don't plumb a return fitting to the tank itself, you can also just run a purge line back to the fuel line further back, creating a sort of closed loop. These apparently do help, but perhaps not as much as the system that returns straight back to the tank. The tradeoff is that it is the simplest type of return.
4. Electronic Fuel Injection (e.g. EFII): Need a return line. These systems operate like almost any modern automobile. Pressurized fuel is constantly flowing in a loop around the engine, past a pressure regulator, and then back to the tank. These systems also require a duplex fuel selector, so that switching between R and L tanks switches both the send and return lines simultaneously. Some systems require "full flow", meaning that the return line has to be the same diameter (and flow at the same rate) as the send line. While this type of system costs the most and is by far the most complex, it is also by far the most "future proof" since it supports all the above types of systems as well.
Some (in fact, many) might advise you to keep it simple and ditch the return line. I can't disagree with that ideology in the least bit, but in the end it's going to come down to personal preference for what you really plan to do with your airplane and engine. For our project, we decided to go with a full-flow duplex system (#4) even though we are at least starting out with a mechanical injection system as described in #2. It was a lot more work, and arguably introduces a few more potential failure points since there are more fuel lines and connections etc; but I feel that as long as you take your time and follow approved practices for building the system, you can keep that additional risk to a minimum.
Hope this helps!
