TRIPLE MPG for your 20b
 

ok I am kidding.

but I am guessing ALL of us yanked out our evap return line and have a vented tank in all practicality.

take this case study: I left about a half a gallon of gasoline in an open pan to recondition some fasteners... it all evaporated in less than a day under shade.

I wonder with the added heat from fuel pumps, vented tank, constant moving, we're potentially evaporating a massive amount of fuel to the air in a warm day.

does anyone have good solution to this?

btw, that hiss we hear when we're opening fuel tank in a normal daily driver, is that vacuum caused by evap system or pressure being relieved by fuel vapor? If it's pressure, I think safety valve set at 5 psi or something may be helpful... love to hear your thoughts before I do something stupid.

My thought was wiring up vent solenoid to be open when car is running and closed when car is off, but it doesn't address fuel loss to air while driving..

This is the main idea behind returnless fuel systems (all applications, yes, boingers too). Less fuel heating from the engine bay and therefor less evaporation.

can't compare a open container versus a fuel tank with a tiny vent hose. my RX has sat for a year and barely lost any fuel to evaporation.

i just keep the charcoal canister?

yes that's a good point. do note that e85 evaps much faster. the lower the octane, the gasoline evaporates faster.. what gasoline did you have in tank when it's sitting? if you have fuel-saver, it lowers vapor pressure drastically. Besides florida has lot of humidity when it's hot, so I suspect it would also keep fuel vapor pressure down with the existing water vapor pressure in the air. not sure what all this mean in real life with car sloughing around the fuel, 2 x 044 pumps heating up the fuel to beyond boiling point, and in 120*F dry heat..., I do know I'm cavitating the pumps when it's hot out...

I can't find any good resource on this topic... some boating guys talk about it because they have large tanks and tend to care about mileage. but they don't suffer from hot fuel due to cooling by water. most car guys dont seem to care because it may means another gallon at the pump at the most..

I just think that if I can raise fuel pressure tank pressure by just a couple psi, it may prevent cavitation and lower fuel loss to vaporization.I just don't know whether it's safe with our 10-20 year old plastic molded tank..

charcoal canister just filters fuel before it reaches atm.. it won't keep any from escaping in large amount...? I suspect all our charcoal canisters are toast long time ago.

plastic molded tank? the only rotary car with a plastic tank is the RX8. unless you are talking a fuel cell, in which case i wouldn't worry about it.

but you are overthinking evaporation, i have lived in florida, nevada and california. none of those states caused large amount of fuel vaporization to occur because the bottleneck is what limits it.

if you have a beer bottle full of fuel it may take 3 days to completely evaporate. pour that beer bottle into a 8"X8" baking sheet and it may take 16 hours to evaporate. pour that beer bottle onto a 10'X10' sheet of glass and it will evaporate before even filling the sheet and within minutes. see where i am going with this? a 6mm vent hose will take years to vent vapors from a full tank of gasoline.

heat is a major factor, but not necessarily the main one.

now if you want an idea for why cavitation may occur, it could be because you are pushing more fuel than the tank can bring air in to fill the space you are emptying. but no one ever seems to address that as a possibility. soooooooooo, maybe your vent is too small!

I was thinking of doing return less myself. I still get that pressure sound when removing the cap. I think it's pressure and not vacuum because on hot days, if I disconnect the return line from FPR, fuel will continue to leak from the line until I break the seal from the fuel cap to relieve the pressure, then it stops. Even on hot days when I don't drive the car, I can break the seal on the fuel cap and hear the pressure. If both check valves are good and you have a good fuel pump housing seal, you shouldn't have any problems with vapor.

Really really want to do SEMI return system

Place the regulator before the fuel rails, the dead head the rails. Fuel is still returning to tank, but the hot fuel from the rails won't make it back to the tank. There are only two concerns here though:

1. fuel will be hotter in the rails then before, as no movement. Will this increase chances of knock? Maybe not because it might not be so significant of an increase.

2. pressure drop across the injectors, as they begin to run higher duty cycles

And wow I just thought about what Rortary Evolution just said, that definitely seems plausible as a possibility for cavitation in the tank. I know for one, I'm finishing up adding a fuel cooler now though, as the pumps do get WAY louder as temps approach 100F+

Good point man. Going to take a look at the amount of venting. It does make sense. Ya know I'm actually thinking now to add a gauge on the return line right as it enters the tank, to see if there ends up being a pressure/vaccum that develops as the temperature increases.

yea mine's a swapped rx8.

that's a good point about vent being too small, but I have a sump tank, so vent probably isn't the bottleneck for the main pressurized circuit?

as for the cavitation, less return may decrease fuel tank fuel temp... jeez I don't know about FPR before the fuel rail.. it seems dangerous unless you can OPEN it with boost which is opposite of what normal manifold-indexed FPR does... it also sounds like you're going to over work the pumps if anything.. I don't know if that would increase heat output by the pumps.