The return is so you don’t dead head the pump. A fuel pump (at least the pumps we are talking about in this thread) is just like an oil pump in that when the bypass opens, fuel is dumped right back into the inlet side of the pump. It’s not a good idea for an oil pump and it’s a worse idea for a fuel pump.
Think about the fuel system as a system and not a bunch of individual pieces and you have to include everything from the float bowls and floats back to the tank.
Once the pump has filled the system and the floats have moved up and shut off the needle and seat, there is pressure building in the feed line from the bottom of the regulator (talking about a dead head regulator, not a bypass regulator) back to the pump. Whatever the bypass is set at (lets assume 15 psi for now) the pressure builds IN THE LINE back to the bypass and then it opens and sends fuel back to the inlet side of the pump. How long do you think that took?
Now the pump is bypassing. As fuel is used from the float bowls (the bigger the load the more fuel used and the more fuel you need) the float drops, the needle and seat opens up and fuel flows into the bowl. That starts to drop the pressure in the line before the regulator. And that pressure drop has to be lower than 15 psi, and it has to go all the way back to the bypass. When that happens, the bypass closes and fuel pressure builds up in the line again until the line pressure hits 15 psi and it starts all over again.
How much line from the pump to the regulator? 8 feet? 9 feet? 10 feet? Whatever it is, it is. So the time it takes for all that to happen may be a small number but it is significant in terms of how fast you can get fuel past the needle and seat, and how fast you can build pressure back in the feed line to the regulator.
Now consider a bypass system with a bypass regulator. I never put the bypass at the pump, or it’s essentially the same as a dead head system as far as the response time of the fuel system.
With a bypass regulator, near the carb where it should be, keeps the line pressure much closer to bypass pressure. It also reacts much quicker since the pressure drop to get fuel flowing only has as much line as there is from the carb(s) to the regulator and not the 8 feet (or more) of line with the dead head system. Response time is significantly reduced with a bypass system, the pump stays cooler, fuel is not bypassed back into the inlet side of the pump but rather back to the tank. This also heals keep the fuel in the feed line cooler because it’s not sitting in that line, waiting for a pressure drop to get fuel moving.
I can’t think of a reason why anyone making over 400 hp would ever use a mechanical pump, and why you wouldn’t buy the biggest pump and run a return system is beyond me. Aside from the initial investment there is no reason not to do it. It’s just a better system.
