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Thread: EFI question

  1. #1
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    EFI question

    An idle thought - is an EFI fuel tank pressurised to a higher pressure than a carb-ed engine given there is a return line?
    Or is the extra pressure only between the pump and the throttle body?
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    That probably depends on if you are talking about "production" cars or aftermarket EFI.. I know current generation production EFI uses all sorts of trick emissions gubbins and I think they aim to maintain positive or neutral pressure in the tank. To the point that if you forget to put the petrol cap back in you'll get an engine warning light for a lean-running condition due to unmetered air being sucked into the intake via the fuel filler.


    Aftermarket .. I wouldn't expect the tank to be pressurised - it should breathe and have a zero pressure differential to the atmosphere. The only extra pressure is between pump & injectors - 40-70psi is usual for indirect injection or for direct injection ~70psi to the high pressure pump and then 700-2000psi between the HPFP and injectors (more for diesel IIRC, 2600psi rings a bell).
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    the really high pressure is between pump and rail, I guess the tank must be at some level of positive pressure though.

    Modern tank/emissions design have a sealed breather system that is opened at the whim of the ECU to vent pressure via a charcoal canister- mine is a little less sophisticated than that though

    Jim

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    Yes, I was actually referring to bolt-on EFI like Fi-Tech / Holly Sniper.
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    I guess you could answer as a “no difference from a carb”. Ignoring the difference in pressure between systems the fuel that comes out of the tank will only return in the same volume if the engine is running if the engine is running then less returns than leaves - so as Aaron says - “neutral” probably

    Jim

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    Quote Originally Posted by KevinW View Post
    Yes, I was actually referring to bolt-on EFI like Fi-Tech / Holly Sniper.
    Yep then definitely as Jim says - I doubt they 'care' what the fuel tank is, so it'll continue to be vented as the original. Overall you aren't likely to be using any more fuel when EFI than carbed, despite the higher pressure, so the vent will still need to let the same amount of air in as it always did to prevent the tank from imploding. If you see what I mean..
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  7. #7
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    Quote Originally Posted by aaronjb View Post
    That probably depends on if you are talking about "production" cars or aftermarket EFI.. I know current generation production EFI uses all sorts of trick emissions gubbins and I think they aim to maintain positive or neutral pressure in the tank. To the point that if you forget to put the petrol cap back in you'll get an engine warning light for a lean-running condition due to unmetered air being sucked into the intake via the fuel filler.


    Aftermarket .. I wouldn't expect the tank to be pressurised - it should breathe and have a zero pressure differential to the atmosphere. The only extra pressure is between pump & injectors - 40-70psi is usual for indirect injection or for direct injection ~70psi to the high pressure pump and then 700-2000psi between the HPFP and injectors (more for diesel IIRC, 2600psi rings a bell).
    Good answer, don't know if productions cars pressurise or vacate the tank but as you say this is for emmisions purposes not for any benefit to the engine or by product of fuel injection or having a return flow.
    First time I came across this was on a Honda Prelude which I owned in the early '90s. When parked up the car would 'fart' every few minutes, don't know if this was due to tank being under vacuum (to prevent petrol fumes being expelled) or pressure and some excess being being expelled. Was quite embarassing at times.
    Thought it may have been specific to that car, maybe a loose seal, but had another one several years later which did exactly the same thing.

    On the question of a return line causing addtional pressure in the tank this cannot be correct. There cannot be more fuel returned to the tank than originally removed (even if pump was running with engine stopped). If the tank were sealed it would eventually form a vacuum due to the removal of the fuel regardless of carb' or EFi, EFi should do this more slowly at the MPG should be better.

    Also rembember someone telling me a tale of a car (think it was a Ford Granada, early EFi back in the ,80s). Then they removed the fuel cap to fill up they noticed a rush of air, filled up and found they could only get about 3 gallons into a more or less empty tank. The tank must have had a blocked vent and the vacuum caused had 'imploded' it. The pump must have been doing really well to feed fuel against that vac'

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    My old MR2 (~2003) didn't fart, but you did get a whoosh when you undid the filler cap - I believe that was because the evap (evaporative emissions) system would cause a small negative pressure in the tank; not enough to make it implode, obviously, but enough that it could guarantee any fuel vapour could be sucked up into the charcoal canister and then (when the solenoid opened) out of there and into the intake to be burnt at cruise.. the long & short being that it would have a constant small vacuum in the tank. At least until you stopped to refill it..

    I assume most evap systems still do the same thing, but .. for all I know they maintain positive pressure for some equally obscure evap control reasons now, in the name of saving the polar bears/penguins/children etc

    Funny that the Prelude would fart though I really wanted a Prelude back in the 90s.. never did own one though. My 1990s Japanese dalliance was with the eye-wateringly expensive to run 300ZX (I had two - a 1989 and a 1992), instead, because that seemed far more sensible at the time!
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    Quote Originally Posted by KevinW View Post
    Yes, I was actually referring to bolt-on EFI like Fi-Tech / Holly Sniper.
    Thinking of a change to EFI Kev?

    I looked at the Holley Sniper (they do a dual quad replacement) if the dual quads turn out to be to much of a PITA to set up.

    cheers
    Kiel and Duncan (dad and the lad errr young man now)

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  10. #10
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    Quote Originally Posted by kitcarsareus View Post
    Thinking of a change to EFI Kev?

    I looked at the Holley Sniper (they do a dual quad replacement) if the dual quads turn out to be to much of a PITA to set up.

    cheers
    I found out that you can fit the FITech inside a turkey pan, so it would be nicely disguised.
    But no, not any time son...gotta sort out paint first. But interesting to have a little think about. Been talking to a chap on here (Neville) who has fitted one to his RV8. Seems to do what it says on the tin, and a few more mpg on the FE would be nice.
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