Thread: LPFP Tech info
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      12-25-2012, 02:53 PM   #1

Drives: X1, 335i
Join Date: Sep 2006
Location: SF Bay, CA

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Smile LPFP Tech info

Hi guys,
I hope everyone is enjoying their Christmas and is looking forward to the 2013!

There also seems to be a little confusion with regards to fuel system design. Specifically, the Low Pressure Fuel Pump (LPFP) system and how to upgrade it. I don't have too much time so I'll make this quick. There are 3 ways to upgrade the fuel pump for those who want to support higher HP levels or run high concentrations of ethanol. We've tested all 3 over the past 2 years. Here's what works well and what doesn't (as others have, and will soon, find out).

1) Running the factory fuel pump at a higher voltage. By running the stock fuel pump at 15-16v, you'll see 10-15% more flow. So a healthy pump that flows 350lbs/hr at 14v will now flow somewhere around 390lbs/hr. Similarly, a less-fresh factory pump that flows 300lbs/hr will flow around 335lbs/hr. Not a huge gain and probably not a good tradeoff given the added stress to the pump (which means lower lifespan).

2) Replacing the factory LPFP with a Walbro E85 pump. This approach is better than the first. A Walbro flows 420 liters per hour (LPH) at 43psi. But like most fuel pumps, flow falls off rather abruptly as pressure goes up. At the N54's 72psi of pressure, the Walbro flows approx 350LHP. Which translates to approx 570lbs/hr which sounds great until you realize that got rid of the stock pump and that this pump doesn't offer a dedicated secondary vent, you have to put in a T in the outlet to power the Venturi jet (in order to keep fuel bucket nice and full at all times). This will drop effective flow (flowing going upstream to the regulator) by 15% (or more if the line isn't sufficiently restricted). Which means that the system flow can easily drop to 485lbs/hr. So compared to a fresh stock pump which flows 350lbs/hr, you're picking up approx 38% more flow.

3) Running a modified Walbro in series with the factory pump. This is essentially what we are doing with our Vishnu/FFTEC in-line fuel pump upgrade. The disadvantage of this approach is obviously cost since you have to machine an adapter that converts what is designed to be an in-tank pump into an in-line pump. This also requires additional lines and fittings. The upside is that you don't have to modify the factory LPFP assembly (cutting it open). The other upside is that running two pump is series drastically outperforms (and outlives) either of the previous two options. Here's why...
When you put two pumps in series, the total system flows approx 20% than the higher flowing of the two pumps assuming no inlet restriction (more on that later). Which means that the Walbro, which flows 570lbs/hr will now flow approx 685lbs/hr assuming the pump upstream keeps doing its job. This is because it will operate a lower pressure ratio than it would if it had to work by itself. This is because it is getting pressurized at the inlet (to ~35psi) which means that it is only stepping up pressure by another 37psi instead of having to work from 0psi as it would have if it were working as a lonely in-tank pump.
The factory pump is also working less hard since it is operating at approx half the system pressure it would otherwise operate at. And with the drop in outlet pressure, the factory pump sees a big increase in flow (up to 480lbs/hr from the 350lbs/hr it provides at its usual 72psi). This is well matched to support the needs of the Walbro upstream up to ~800hp on gasoline or ~620hp on straight E85. Above that, it will start to run out of steam and start to drop outlet pressure, forcing the Walbro to operate a higher pressure ratio which will limit fuel flow to the output we saw in the first part of approach #2 (before a portion of the outlet was bled for the venturi jet). All said and done, running both pumps in series improves flow to approx. 570lb/hr, or a improvement of 62% over stock.

And yes, we've make some big numbers on this set-up And it is alway a good idea to "oversize" your fuel system because pumps do constantly degrade over time.

I've also attached a pic below of the prototype fuel system running in our shop car. It's an evolution of (add-on to) our in-line fuel pump upgrade. Designed for 800+whp on E85. With it, the car has 3 pumps, 3 regulators, a surge tank and Teflon coated SS line conversion! You don't want to know the parts/labor cost on that

Hope that sheds some light on things....
Happy Holidays!
Attached Images

Last edited by shiv@vishnu; 12-25-2012 at 08:47 PM. Reason: Edited.... used the flow numbers from the wrong pump by accident. Fixed. Bigger differences now.