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Thread: Limit of Returnless Fuel Systems

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    Si Speed 317's Avatar
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    Limit of Returnless Fuel Systems

    Mixed with my personal experience and information; courtesy of Geoff from Full-Race
    A lot of people have been asking me when it's necessary to add an external Fuel Pressure Regulator (FPR) and a fuel return line, so I've taken it upon myself to edit Geoff's original These estimations assume an equal length manifold turbo kit and ID 1000cc injectors tuned properly.

    According to Jeff Evans (from Evans Tuning):
    - a Walbro 255 high pressure in-tank pump can flow enough fuel for ~440whp on stock intank FPR, and ~500whp on a crushed FPR "Comptech Style".

    - a Bosch Motorsports in-tank fuel pump, can flow enough fuel for ~480whp on stock intank FPR and ~550whp on crushed 'Comptech Style' FPR
    (seems a bit steep to me)

    -if you install the "Boost-A-Pump" manufactured by Kenne Bell, this will jump the fuel pump's voltage supply to 20V (perfectly fine for the fuel pump, in fact good for it!) and allow the walbro to flow close to 600hp and the Bosch beyond this

    Now, since this post (back in 2009), many more options have been introduced.
    I am personally currently running an AEM320lph fuel pump (great buy for $99 shipped) and that outperforms the Walbro255 and performs very closely to the DW300lph, which flows 320lph. It's very quiet, even quieter than the Walbro255.

    Don't get overly excited though, there are downsides of a returnless fuel system as well. Let's go over how your fuel system works so we can fully understand this concept.

    Fuel is taken from your tank through the fuel 'sock' (filter) by your fuel pump. The fuel is then pumped from your fuel pump to your FPR (Fuel Pressure Regulator), and then to the point of no return: your fuel rail. Your fuel sits here and gets used by your injectors, and is commonly fed more fuel than it needs. That's great, right? Not exactly. In a return setup, that excess is routed back to your fuel tank and completes the cycle, allowing some lucky molecules of fuel to live longer before being blasted away. A returnless system is more prone to detonation because of the dead-end that is created. This is fine on an NA setup, but remember, we're comparing apples to apples here. Also, your Injector Duty Cycle (IDC) is much higher with a returnless setup. Boosted setups normally come with a stock return setup just for this reason. Also, you have more room for fuel, so your IDC will decrease with the same size injectors by adding a return setup.

    To paint a picture for you to drive this concept home, think of a store with 4 cashiers. A returnless setup is like having that store with, let's say 100 (for example) customers all waiting on lines until they're checked out. A return setup is like the same store and the same 100 customers, but many of those customers see the line and say they'll come back in a bit. Make sense?

    I hope this helps shed some light to show that it's not absolutely necessary to add a return line and external FPR, of course for anyone looking to run 30+ psi or have a race car the FPR is still a wise investment, but not exactly needed.

    I'll be updating this thread as more information is brought in and proven. I want to keep this constantly updated, so the information we're all looking for is up-to-date and we're ready to go.
    Last edited by Si Speed 317; 09-06-2013 at 05:18 PM.

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