Originally Posted By: raytseng
...I will agree with you, no harm, but I am not convinced that there truly is a need.
Are you sure that one side of your tank drains completely first, then a magical pump switches on? This seems like bad engineering.
This is why i mentioned a jet system. The fuel pump systems I'm familiar with don't drain one side first, but use a venturi aka jet system that continuously siphons and balances both sides. So it will mix via that mechanism.
Even if you dump with at -2gallons, that last fuel is going to be the fuel at the top, that goes across the saddle so it's not like it all the fuel or additive is going to settle just t to one side.
If your imagining the additive doesn't "mix" and just sinks to the bottom of one side; that's not right either. If you follow that logic, any cold start is going to be 100% additive that has sunk to the bottom.
If we take the flip side, and the passenger side is a "reserve" tank; a person were to always fills at half tank, would have super stale fuel in their reserve tank that would never be used. Don't think that happens.
I don't know how Ford does it, but they do. My BMW had the jet pump that used returned fuel to pump the off-side, but Ford has a returnless system so it's not obvious how they make it work. However, I know for sure that they do because the tank has two fuel gauge senders, one on each side of the saddle. You can put the dash into "diagnostic mode" and read each side separately. It really does drain the pax side first. Ford's factory service manual says it works this way, by the way. I didn't quite believe it myself, so I checked it.
A couple of years ago I was running a supercharger and doing a bunch of data logging. I wanted to be sure that when I changed octane levels that I wasn't running two half-tanks with different octane ratings. So, I tracked the two sides of the saddle to see if one actually emptied first, or if they mixed. Sure enough, there's "fuel right" and "fuel left". "fuel right" gets down to pretty close to zero before "fuel left" starts to drop. On that basis I decided I had to be at or below 1/4 tank before I filled up so I could count on having the same octane throughout.