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  • pressure for blown hat EFI

    Next question for you guys: I am injecting E85 above the blower (no port injection). I have been reading that port injection should plan around 45PSI and throttle body is somewhere around 12-15 PSI. So, what pressure should I run for my application?

    thanks
    ed

  • #2

    Re: pressure for blown hat EFI

    I'm sure one of the resident experts will chime in, but I'm going to say 45 and the regulator needs to be referenced to the same location as the injectors. Between the throttle plates and the supercharger.

    Kevin

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    • #3

      Re: pressure for blown hat EFI

      Depends on the injector type. Are your running a TBI or are you running a pair or set of multi-port bosch style injectors just above the blower?

      And I'm unclear, are you injecting the fuel BEFORE the blower or are you injecting the fuel into a hat that is pressurized? If you are above the throttle blades and before the blower the regulator doesn't need to be referenced, if you are injecting into a pressurized hat or below throttle blades the regulator has to be referenced to where the injectors are located, ie the hat if it is pressurized.
      Escaped on a technicality.

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      • #4

        Re: pressure for blown hat EFI

        Thanks. I have an old-school style bug-catcher "hat" sitting on top of a 4-71 blower. The EFI injectors are located in the stock MFI locations (above the blower but behind the throttle blades) injecting into the hat. Should the regulator be boost referenced?

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        • #5

          Re: pressure for blown hat EFI

          Thanks. I have an old-school style bug-catcher "hat" sitting on top of a 4-71 blower. The EFI injectors are located in the stock MFI locations (above the blower but behind the throttle blades) injecting into the hat. Should the regulator be boost referenced?
          THis might be a BK Bridges question, but: the goal of referencing the fuel pressure regulator is to keep the pressure differential at the injector nozzle consistent. So that would mean your regulator needs refernced to the pressure between the throttle blades and the blower. That will see a lot more vacuum at idle than under the blower (where your MAP sensor needs to be plumbed).

          That being said, I know for a fact that you can just set pressure at 60psi, not hook any reference to the regulator, and be just fine. The pressure differential at the nozzle will change, but a TON of people run boost without a referenced regulator and do just fine.

          Your combo is a lot like that 40 Ford I tuned a few weeks ago. Spraying above the blower sounds like it wouldn't work great - but it really does, so long as you have a decent seal on your throttle blades and good control of airflow through them.
          www.realtuners.com - catch the RealTuners Radio Podcast on Youtube, Facebook, iTunes, and anywhere else podcasts are distributed!

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          • #6

            Re: pressure for blown hat EFI

            As I know zip about roots blowers I'll ask the dumb questions. Given the back in the day blown mechanical injection setups sprayed the fuel above the blower inlet behind the throttle blades on the bugcatcher I presume that the rotor tip seals like being wet and the inlet air temps benefit from the latent heat of vaporization of the fuel as an offset to the pressure seen in the manifold and the beating effect of the rotors themselves?

            It would also explain why a certain roots blown 351 C Pinto a buddy of mine had running on alcohol back in the day would create frost on the blower case when driven on cool nights. That car was featured as "Lunatic Fringe" in CC way back when the Red Rider song of the same name was new.
            Drag Week 2006 & 2012 - Winner Street Race Big Block Naturally Aspirated - R/U 2007 Broke DW '05 and Drag Weekend '15 Coincidence?

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            • #7

              Re: pressure for blown hat EFI

              Re what pressure... Its the pressure that the injectors are designed to run at that sets the requirement for fuel pressure. Older TBI injectors run at lower pressures that modern injectors for the most part. You need to determine what injectors you've got under your hat and run the specified fuel pressure. As DG et al said, the fuel pressure regulator should be referenced to the hat to compensate for the vacuum developed by the blower/motor, increasing the injector flow via the magic of pressure differential. That will simplify tuning and keep your VEs / pw in the "regular" ranges. Without the reference you will need to pull fuel at high vacuum (idle) which isn't the most efficient way to run an EFI computer and may cause un-tune-outable transient behavior. If the injectors are really big, it may cause poor injection at high vacuum due to the super short pw required to get the right air fuel ratio. Not referencing the regulator can work, but when in closed loop it also works the "data stream" more, potentially slowing things down a bit...or two. At least this is what Ive been told (and the laws of physics seem to back it up)...
              www.FBthrottlebodies.com
              Bruce K Bridges

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              • #8

                Re: pressure for blown hat EFI

                Makes sense. It should be fairly easy to just run a reference line to the hat. I am runing 8x80LB injectors for my blown v6, 600 HP, on E85.

                On a related note: I wonder if the characteristic "surging" of a blown motor at idle is due to the vac. surges from the blower creating a rich/lean cycling behavior.

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                • #9

                  Re: pressure for blown hat EFI

                  Those (8x80lb) injectors are pretty big for 600hp, almost XXl. Not a big problem, but still they've got a lot of margin. 43.5lbs of fuel pressure will be just fine.
                  BKB
                  www.FBthrottlebodies.com
                  Bruce K Bridges

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                  • #10

                    Re: pressure for blown hat EFI

                    Yea, When I did my original calcs I wanted to leave myself plenty of room for more boost. With E85 and my 9.5:1 static CR, I can probably dig deep into my blower hubs and easily run 20 lbs of boost. But, this is a pro-street application, not a full out race car. In addition, I dont have to initially wire up all 8 injectors. For start ups (and since I am new to EFI anyway), I may just start with moderate boost and 6 injectors instead of all 8. The MS3 is going to be utilized for fuel only, batch injection.

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