when do you have to run a return to your tank, is this just for Fuel Injected cars ?
<hr></blockquote>Yes, it's just for fuel injection.
The deal is that the pump has to put out much more than the system will normally need in order for the injectors to do their job properly. The excess fuel has to be returned to the tank so a regulator is used to keep the pressure at the injectors where you need it and all the rest goes back.
You can also plug off the return port on the pump, that's what I'm doing since I'm using the pump off an '89 351W Crown Vic car that had a return line. I'm using a carb, so I don't need the return line. I haven't noticed any adverse effects so far.
If you're using a high volume fuel pump, it's sometimes good to run a return line to maintain constant fuel pressure. Ford did exactly this with the 1985 Mustang GT's. The last of the carbed 5.0's had a traditional fuel pump but with a an extra return line port and line.