So
@Finface if the pump motor is off, the valve (which is before the drain/supply hose) is closed, gravity allows most of if not all of the fuel to drain out, right? I don’t remember anytime when I picked up a gas nozzle and more than a few drops came out before inserting into my car, do you? Where is this mysterious dilution taking place?
@sad you have a drawing or flow diagram of a gas pump?
Hi DGatzby,
Far be it from me to engage too deeply on a topic like this with an engineer who designs hydroelectric systems for a living! I ain't no fool!
I did intend my sharpshooting pump gas octane "tip" to be somewhat tongue-in-cheek. We car enthusiasts tend to be pretty anal-retentive (discredited Freudian term...new less potty-referencing PC term Obsessive Compulsive) about caring for our cars. Why not obsess about the octane content of every last precious drop of gasoline *grin*? I really did read, when I first got my car, that there is residual fuel left in the hose. Found a couple of links today to back up it could be "a thing". Usually it is just as easy to plan my fill-ups with potentially diluted octane fuel in mind as not. If doing it satisfies a mild OCD urge, even if the rationale is about as irrational as counting steps from your car in a distant parking lot to your office door, and going back to do it again if it isn't an even number, or hand-washing dozens of times every day (Wait! We're supposed to be doing that!), no harm. It would just be wasted brain cell energy to worry about it. I freely admit I'm just indulging my OCD.
For the OP asking about gasoline I did make two notes to myself when I got my car as follows;
"Don’t run the car out of gas - the fuel lubricates the electric fuel pump. Fill up at 1/4 tank with 93 octane - 91 octane minimum."
"Don’t top off gas - can damage the fuel system."
I hereby nominate myself for a new competitive monthly and yearly category that Bull might want to set up - Most Obsessive Compulsive Hellcat Owner - call it the MOCHO award. Exhibit A - Finface writes excessive notes to himself which may, or may not, be true. Exhibit B - he writes long-ass forum posts sometimes about goofy shit.
It would be an award I would accept with humble gratitude, given the level of OCD expressed here daily about waxes, wheel gloss products and catch cans (I have one!).
Exhibit B in MOCHO competition for Finface - before I select a fuel grade I've (sometimes, not always dammit!) inserted the fuel nozzle, lifted the hose, and squeezed the handle and had "some" gas flow in. Free gas! Yippee! No, not really - that gas was going into my tank anyway. More to the "free gas" point, after I shut off the fuel at the pump I've also squeezed the handle and had "some" gas flow into my tank. I paid for that gas still in the hose, right? Right? Come on, 'fess up! Who else has done this?
Nasty Flight Attendant "cheap airline captain joke" time!
"How was copper wire invented?"
"Two airline captains fighting over a penny!"
Flight attendants can be so cruel.
I did a quick google search just now "is there gas left in the hose after you replace it that the next car will get?", and found a 2008 WSJ article saying there is "about half a gallon" from the previous buyer in a single hose system. And a motorcycle forum post where a poster claimed to have contacted Shell and gotten this answer;
"On the use of a single hose to dispense multiple grades of fuel...the fuel grade change point is directly above the hose connection resulting in a residual volume of 0.56 liters for small diameter hose, or 0.78 liters if the pump happens to be equipped with a non-standard large diameter hose. While this amount will have only a very small effect on a typical car fill up, the effect is larger on 2 cycle equipment or a motorcycle tank with limited capacity."
About half to 3/4's of a quart, or possibly up to half a gallon? As we've observed there isn't anything like these amounts gravity flowing from the hose when we squeeze the handle with the pump shut off. Shell (supposedly) in any case says, "only a very small effect on a typical car fill up". Effect on a 700 HP Hellcat purchasing 91 octane fuel? Way above my pay grade. Here in Kentucky premium is 93, so...
...hey, time to go wash my hands!
Best,
Finface