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Thread: Erik Gross method revisited

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    Erik Gross method revisited

    We were asked to start migrating content here... This is my first contribution. Originally from a couple years ago, but I'm still using it and very happy with the results.
    --------------------------------------------------------------------------

    I made a post about this on 3Si that sparked some interesting discussion, but it was never intended to be a case study/searchable resource... More of a "woot, look what I found" thread..

    Cliff notes of other post:

    1: I install Supra pump in my car.
    2: I think I'm really smart wiring in the stock resistor to keep from overrunning my stock FPR..
    3: I'm not smart enough to search, and discover the pitfalls of this plan until AFTER I experience them myself..
    4: I'm bent because it "should" have worked.. and go on a mission to find out why it didn't..

    Now there are several methods of wiring a fuel pump that are attributed to Erik's work... For the purposes of this discussion, it is solely the practice of wiring in the stock fuel pump resistor to slow down a hotwired pump at slow engine speeds.. My entire fuel system is wired as standalone from the stock wiring. I used two relays signaled from the MPFI and the ECU.. Even the resistor is fed from the hotwire. Here's a diagram of my system:



    From reading the old threads, Erik gave up on the idea because there was an obvious fuel pressure variation between the low and high speeds that screwed with the fuel trims and would cause drivability issues like lean spots right before the transfer... These are the same symptoms I experienced.

    Many of you will say "why bother.. Put in and aftermarket FPR" And you would be justified in that position... But I'm far too stubborn to let a solution lay undiscovered. And I had a pretty solid theory about what was going on. (and wasn't about to take a couple hundred bucks away from the white car build if I didn't have to)

    So here is the Data I gathered: (bench tests w/stock regulator)

    The stock pump hotwired at approx 13.6Volts, draws 10 amps making 45 psi (w/no vacuum)
    The Supra pump under the same conditions draws 14 amps and makes 50 psi(overrunning the FPR)

    Deadheaded,(up against the internal bypass) the stock pump draws 12 amps making 90 psi.
    Under the same conditions the Supra pump draws 20 amps making 100 psi.

    Conclusion: the supra pump is a huge power hog...

    On to my car... Why are we getting the pressure fluctuations?

    KOEO, battery charger maintaining 13.6v, jumper relay to hotwire mode: The installed Supra pump screaming along at 7500rpm, drawing 14-15 amps, and overrunning the FPR...



    Jumper the resistor on line: The supra pump gets the wind knocked out of it.. Slows down to 3500rpm, drawing 7.5 amps... At this speed it will make less than 50 psi deadheaded... Starting the car, I discovered that on a snap throttle the pressure would drop, rather than go up like it should. The Supra pump simply could not supply enough fuel to run the car while turning that slow..



    Conclusion: Too much resistor for that pump...

    Now what to do about it..? I already had a plan... I wired a second stock resistor in parallel to the original, cutting the resistance in half.




    Results:

    5000 rpms, 10amps, normal fuel pressure(no overrun), and able to make 80 psi deadheaded... WOOT! I win!



    I've been driving it around for a week looking for any signs of fuel control or pressure issues. I have to call this one a fix..

    Thanks for reading..

    Larry

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    Very interesting work.

    -Ricer Evo 8: 8.75 @ 166 Best MPH of 167. Quickest 4G63 Powered Evo in The US!!!
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    And the list goes on...

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    Well this is a timely Post! I have a Supra Pump dropped in with all Stock wiring (no hot wire). I'll keep eye out for this symptom.
    Thx
    Dan
    97VR4

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    Crap... I just noticed the typo butchering Erik's name in the title. Need to find a Mod...

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    Great work! Can you build me one?

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    this needs to go in a knowledge database section

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    so the moral here is, AFPR?

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    The moral is, no need for the AFPR.

    Help them, for they know not that which they do not know!
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    Good post by Larry, I recently re-wired my Twin Walbros in parrallel with 2 resistors and the car dove terribly with the resistors getting red hot ( odd as I had no problem with a single Walbro in 2 speed mode).

    Gone 2 pumps full speed at the moment but the noise is annoying!.
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    Quote Originally Posted by Greg E View Post
    Great work! Can you build me one?
    I'd be much more willing to help you do it yourself, but if you really wanted me to build it, I suppose it could be arranged...

    Quote Originally Posted by IPD View Post
    so the moral here is, AFPR?
    That's what I usually recommend, but there are downsides... If you drive your car on long trips and lots of highway use, slowing the pump down will significantly prolong it's life. There is also the all too common complaint of the aftermarket regulators leaking, causing long crank times.

    Quote Originally Posted by KeithMac View Post
    Good post by Larry, I recently re-wired my Twin Walbros in parallel with 2 resistors and the car dove terribly with the resistors getting red hot ( odd as I had no problem with a single Walbro in 2 speed mode).

    Gone 2 pumps full speed at the moment but the noise is annoying!.
    You have any way to test the current draw on those things?

    The nature of an electric motor is that it will increase it's current draw as it's slowed down. If you bring it to a full stop, it's a dead short. This is why you will see starters and battery cables turned to bacon from a depleted battery, it goes against all logic until you examine the nature of a segmented armature electric motor.

    In my above data, those resistors were only allowing 7.5 amps each through them with the pump slowed down to under half normal speed. It sounds like two resistors is still too much for dual pumps.

    The Walbro has significantly less current draw under normal operation than a Denso, but I would theorize that it's much less tolerant to being slowed down. The Denso has 12 armature segments, while the Walbro has only 8. You get much less leeway for speed changes before you approach dead short status.

    In your situation, I'd be tempted to experiment with an injector resistor pack... You'd have 6 separate segments you could run in parallel one at a time until you found the sweet spot.

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