Walbro 400 install and pump pressure/flow testing
 

I finally got around to installing a Walbro 400 that David Buschur gave me (Thanks!). Its being held in with a hose clamp rather than the fuel pump cap. It takes very little clamping force to hold it in very tightly. Since it doesn't have the cap, its also lacking the bowl for the fuel siphon return. Based on other people's experiences with fuel pressure overrun at idle, I drilled out the siphon hole to 3/32" (2.4 mm). Best as I can tell, 3/32" is barely larger than stock, and from watching my fuel pressure at idle after installing the pump, my guess is that drilling it probably was not necessary to maintain fuel pressure at idle (and during cruise when the fuel pump is in low voltage mode).

Anyhow, after a bit of fiddling around with the install, I fired up my Evo. Net fuel pressure at idle was 44 psi which is what it should be. (Quick clarification: Net fuel pressure is fuel rail pressure minus boost and should always be 44 psi for a stock fuel pressure regulator.) The crossover load for full voltage mode was tuned for my Walbro 255, and as I suspected, net fuel pressure jumped to 60+ psi (should always be 44 psi) when the ECU kicked the higher flowing Walbro 400 over to full voltage, and pressure didn't drift back down to 45 psi until the fuel demand had increased by a fair amount. With my Walbro 255, it would bump up to about 46 psi net fuel pressure for just a split second and then settle back down to 44 psi. I ran out of time to fiddle around today, so tomorrow I'll be raising the crossover voltage to try to get rid of the pressure bump...

Well, I couldn't increase the crossover load nearly as much as I anticipated. Fuel pressure fell off drastically in low voltage mode for anything more than about 4 psi of boost. Final values for the crossover load are only about 5-15 load higher than what I used for my Walbro 255. Fuel pressure bumps up only slightly at the transition to full voltage.

Another suprising result is that even with this pump, net fuel pressure is still falling off somewhat at WOT high rpm with my current power of ~485 who on E85. By 7500 rpm and 27 psi of boost, my Walbro 255 falls off to about 36 psi net pressure, while the Walbro 400 is falling off to 40 psi net pressure (should be holding 44 psi all the time). This is with the fuel system in top condition - upgraded fuel pump wiring, new fuel pump carrier (i.e., new internal filter), new Walbro 400 fuel sock, new OEM grommet on the fuel pump outlet, and using a Wilson monster diameter fuel rail. Nothing much left to upgrade. With fuel pressure so similar to my Walbro 255, I'm wondering if there is a significant flow-induced pressure drop in the factory fuel line as fuel travels from the pump to the rail, and as a result, the pump seeing something like 20 psi higher fuel pressure than at the rail. I'm starting to think that replacing the stock hardline with -6AN isn't such a bad idea.

With that said, the extra 4 psi of fuel pressure from switching to the Walbro 400 did richen up the AFR by half a point. I suspect I'll pick up a few more psi of fuel pressure when I lean the AFR back out by 0.5 pt (or slightly more).

Leaning out to 12.0:1 allowed the fuel pump to hold 43 psi out the top, so its basically right at the envelope of sustaining 44 psi. I'm still puzzled why a Walbro 400 can't easily hold the pressure, so I ordered everything I need to make fuel pressure measurement at the fuel pump outlet. This weekend, I'll be doing voltage and pressure measurements at the pump.

Why didn't you chose to go with the aero motive 340 instead?

CBRD lent me one to try. Flow rate and pressure holding capacity were horrible compared to a Walbro 255 or DW300. Not sure why it was like that. Perhaps it was a defective unit.

https://www.evolutionm.net/forums/ve...mp-wiring.html

please keep posting results!

cb

I just purchased a aero motive 340 to replace my 255 thinking its a significant improvement. Looks like that was a dumb move

Holy crap finally! I've had mine in since January with good results.

This is a good pump to get the most out of a BBK on e85! 255 and the others fall a bit short.

Are you still on the stock fuel rail? I had great luck swapping in an aftermarket unit on my X, made all the difference! Veeerrrry curious to see how re-running the line does for you....I *think* I will be able to get by w/o doing that on my car...still on stock FPR.

Other thing you can try is lowering base fuel pressure...I did it by elongating the stock FPR body (inadvertently, see my build thread)...but an AFPR would be worth trying if you're needing to get a bit more out of the pump.

Did your Aeromotive unit have the polarity reverse from stock or Like stock?

Only reason i ask, i built a double pumper off two 340's. The first set i got was reverse polarity, they actually didnt work too well. Sent them back and they sent me a set that had the stock polarity....and run great, for the last 6 months.

cant wait to see the results.

I was under the assumption that the new walbro was only minimally higher flow then the tried and true 255 at stock voltage, but run them 16-20 volts and they really start to shine, kind of like a big turbo at low boost..

I've had a Wilson fuel rail for the last year or more. It has 11/16" ID, one of the biggest on the market. Depending on the fuel pressure results, I may get an AFPR. I have a pretty strong feeling that a -6 AN fuel feed line will make a huge difference and may actually force me to get an AFPR to prevent overrun.

I didn't pay attention to whether it had reverse polarity. Interesting to know.

FYI

From Aeromotive FAQ page:

11.) Q: I heard the 340 Stealth Pump may have reverse-polarity compared to the pump I have in my car now. What does this mean, what should I do about it, and if the fuel pump is accidently wired wrong, could it be damaged?

A: The original version of the 340 Stealth Pump, which began shipping in February 2011, had a positive/negative (+/-) orientation on the pump that was later found to be opposite factory orientation in some of the more popular applications. A change was made to the +/- position of the pins on the pump and the included pig-tail, in the fall of 2011, correcting this issue. At the same time additional updates were performed to the inlet end caps of the 11141 and 11142 pumps in order to better fit and secure the inlet filter/sock. In order to help distinguish the new version, and for enhanced cosmetic appeal, at that time the inlet end cap color was changed from white to red, so all current Stealth pumps feature inlet and outlet end caps molded in red.

In all cases, the original and the current version 340 Stealth pumps have had the correct markings for +/- (correct wiring polarity) molded into the pump’s outlet end cap, just below the pins in the plug. Simply confirming that the wire connected to the terminal marked with the + sign is the 12V hot lead from the car’s harness is all that is necessary to ensure proper electrical polarity of the motor. This is true for any 340 Stealth Pump, regardless of version.

So what if the pump was accidentally wired backwards? Since the 340 Stealth Pump employs a DC 12 Volt motor, reverse wiring is not immediately damaging to the motor, however the pump will run in the wrong (backward) direction, resulting in no positive flow or fuel pressure. Continued running of the fuel pump in this manner will eventually damage the pumping mechanism and motor shaft bushings due to lack of lubrication and cooling flow. If your fuel pump runs when power upt, but does not make flow and/or pressure, be sure to check the polarity of the wiring before running the pump repeatedly, or for extended periods of time.

http://aeromotiveinc.com/tech-help/f...th-fuel-pumps/

Thanks. The one I tried had a red cap (and it flowed fuel), so that eliminates it as being an older model with reversed polarity at the terminals.

So is the 400 a better pump than the 340 or we havent gotten to that conclusion yet