Ted B

iTrader: (6)

When I elected to go with a LARGE turbo and considered the prospect of running E85, I knew that one 255 lph pump would be insufficient, and so like several others, I opted for Full Blown's tidy dual pump system. When we installed the system, I had Drifto (www.mpfab.com) upgrade my fuel lines to AN-8 feed and AN-6 return, and we swapped in an aftermarket fuel rail so I could use a liquid-filled FP gauge and adjustable FPR.

When we fired things up, we realized that the dual pump setup caused an overrun at idle, forcing the FP to 60+ psi. We couldn't drop the pressure to an acceptable level (a band-aid fix anyway), so we temporarily found a remedy in disconnecting the power to one pump until we worked a solution. I ran the car for awhile like this, but I noticed that at WOT and high rpm, the AFRs would lean to an unacceptable point, which could not be tuned out. I realized that what was happening is that the line pressure was forcing fuel backward through the dead pump, which caused a reduction in line pressure at the most critical point in the rpm range. If you've disconnected power to one pump, BE ADVISED that you should NOT run the car at WOT, or you risk the prospect of going uncontrollably lean.

Like others, we considered the use of a Hobbs switch to activate the second pump at an appropriate point, but I didn't want to add to the complexity of things.

The fuel return line comes to a 'T' inside the tank, whereby the return goes straight through the 'T', and the 90 degree fitting is plumbed to the other side of the tank. The velocity of the return fuel through the restricted orifice inside the 'T' is what sucks fuel from the other side of the tank into the main section. The pressure buildup in the rail is caused by the orifice in the 'T' being too small to accomodate the volume of return fuel as a result of the dual pumps.

To resolve this, we reasoned that we should be able to enlarge the orifice inside the 'T', just to the point where the idle pressure returned to normal, and this should not be large enough to kill the Bernoulli effect. After I eyeballed the orifice inside the 'T' for a few minutes, I selected a 9/64 drill bit, which is only slightly larger than the orifice. We reinstalled the 'T', and found that the idle pressure dropped to just a couple of PSI above normal, which we were easily to bring into spec. So far, this seems to have worked, and both pumps are now running 100% of the time, without causing any excess pressure issues.

If you have or are contemplating a dual pump setup, I'm sure you'll find this much easier and cleaner than rigging a Hobbs switch. Use a 9/64 bit, and go no larger. Just keep in mind that you better not try to return to a single pump after doing this, or you may find that 1/2 of your tank is unusable.

I hope this writeup proves to be helpful.

crcain

Nice info Ted. I never liked the idea of the second pump switching on at a certain manifold pressure. It worried me what might happen if it failed to come on. Tell me something, now that you are essentially tuned for two pumps running at once and the FPR is dialed in to suit that, what would happen to you if one pump failed? I suspect the car would immediately bog and not run. So to me, your idea seems likely to be more safe?

Also, I keep hearing about how the fuel flowing through the return line helps move fuel from one side of the saddle to the other. Can someone draw a diagram of this for me as I'm real curious to understand how this works exactly. I'm specifically interested in it because of the method I use for getting rid of pump fuel to use race fuel is by simply detaching the return line from rail of the FPR and sending that into a jerry can. And I just use a wire to ground the fuel pump switch on my SM4 harness.

Ted B

iTrader: (6)

If one pump fails, the car will run fine, but WOT FP will be reduced. You hope this doesn't happen, or you'll get what feels like boost cut at high rpm.

The white plastic fitting that is key to all of this is located in the tank, on the pump assembly. See diagram.

AWD Motorsports

iTrader: (211)

I am pretty sure on the last Full Blown Setup we did we just had the AEM trigger the 2nd pump which wasnt complex at all.. We made approx 650whp and didnt find any issues at all.. On stock ecu I can see it being a problem.. Thanks for the info Ted!!

Ivan@AMS

Walbro fuel pumps have one way valves in the outlet which do not allow fuel to go backwards through the pump. Either your check valve is damaged or there is something else wrong with your system.

AWD Motorsports

iTrader: (211)

Interesting point..

Waz996

iTrader: (1)

Ted, I am undergoing exactly the same problem! And your explanation cant be any more descriptive. I'm doing this surely this week..

Excuse my ignorance, but we happen to be in a metric system country, how much is a 9/64 bit in millimiters (i got 3.5mm?)

Thanks,,

Wael

Ted B

iTrader: (6)

That is interesting. I considered the possibility of a faulty pump, but I switched power from one pump to the other, and the results were the same. With the map values set at full rich (7.4), IDC climbed to 100+ by 7000rpm, and AFRs dipped into the 13s and then 14s. Not a good situation. There was little question that the problem was reduced rail pressure.

When I powered up both pumps (after enlarging the return orifice), AFRs suddenly dipped to ~10.0 with no other changes.

Perhaps the difference is due to the volume of the AN -8 feed line reducing the ability of the single pump to maintain adequate pressure. I'm not sure of what else could have possibly caused this phenomenon.

Ted B

iTrader: (6)

9/64" = 3.57mm, so the 3.5mm bit will probably do it.

You will know if you drilled it to the proper size if the difference in rail pressure at idle between one pump running and two pumps running is within 2-3 psi (0.2 bar) with the FPR vacuum line disconnected. If it is, set it to 43-44 psi (2.9-3.0 bar) and you should see it drop to ~39 psi (2.6-2.7 bar) at idle. If the pressure difference is more than about 0.2 bar, use the next larger size bit.

Ivan@AMS

Curious how you have the pumps wired up. Possible voltage drop issue if both pumps are being powered by the factory wiring. IMO, factory wiring is not adequate for two pumps. I run dual relays and use the factory power and ground to trigger the relays. I am also curious if your pumps are the HP version or the regular 255L pumps.

Ted B

iTrader: (6)

We upgraded the wiring well before we went to the dual pump setup. Even single pump users should realize that upgrading the power AND ground wiring and connections is worth 0.5V at the pump, which represents a nice boost in overall capacity.

Both pumps are HP units.

I know it's a weird thing, but we'll go back and recheck everything to be sure. I don't want to see this issue crop up when we run E85, which places significantly greater demands on the system.

EvoDan2004

iTrader: (94)

few questions ted.

what is a HP walbro 255 to a reg walbro 255. how do i know which one i have??

how hard was it to upgrade the wiring?? from where to where does it need to be upgraded.

also is it possible to have the part your making bigger made??

Ted B

iTrader: (6)

You can identify your pump from the part number. FWIW, all turbo cars use HP pumps, so if you bought yours from a familiar vendor, the chances are small that they'd have sold you the incorrect one.

You'd have to ask Drifto about the wiring, but the new, beefier wiring harness at the pumps would look familiar to you if you saw it. Think lighting harness for a trailer.

There is no reason to have the plastic 'T' fitting made, as modding the factory one takes all of about 10 seconds once you remove it.

PharmEcis

iTrader: (3)

We haven't necessarily experienced overrun with the Full Blown setup as we were able to control base FP just fine w/ a Fuel Lab unit and stock lines. I can state that the check valve in the Walbro's does not operate exactly as advertised in all instances as we had a pump go bad in one install and it WAS feeding fuel pressure back through the pump. As soon as we got into boost the AFR shot lean and fuel pressure dropped crazily. This would happen whether or not the faulty pump had juice or not. After replacing the faulty pump, everything went back to normal.

Drilling out returns has been something 3G Eclipse users have had to do for years in order to avoid FPR overrun when installing a Walbro. Haven't had to do it yet on an EVO but am in no way interjecting that it might not be necessary for certain setups.

Differences in the Walbro 255 vs the 255 HP is the flow rate characteristics at higher fuel pressures. The way to tell is the part number. GS317 is the 255 and GS342 is the 255 HP.

http://www.roadraceengineering.com/f...pflowrates.htm is an excellent resource for fuel pump flow rates at different line pressures.

Ev0cRaZy

iTrader: (24)

I run a Dual Walboro Full Blown setup with stock wiring with no issues at all. Install was smooth and it works fine. I also run a Hobbs switch.