Searched for quite awhile and could find no answer...getting very frustrated please chime in.

Ive been having fuel problems on the dyno, I constantly go lean on the top end. Is the 3/8 line not enough for the power im pushing. The dyno owner mentioned upgrading to a -10 AN feed and -8 return. Is this necessary. I dont want to throw parts at this thing.

Dyno numbers 505rwhp/640rwtq so far...

Setup has stock fuel tank with...
42 lb/hr injectors
255 Hi-Volume Walbro (just installed)
ATI Inline Booster Pump
ATI FMU

I just dropped the walbro in and wanted to ask before I tried the dyno again.

I know the ideal thing to do is drop the fmu and tune with 2 bar map and so forth but I want to figure this out with what ive got for now.

 

What was your fuel pressure like on the top end when it was going lean?

 

At the dyno it was around 60 psi. I see 80+ when I first get in it.

 

what fuel pump did you have in there before?

 

Stock pump was in there when it went lean. 255 walbro got dropped in three days ago.

 

thats your problem then... even with a booster pump, theres no way the stock pump can sustain that much power..


they usually cap out far before that... your booster pump was probly pulling fuel thru the stocker...


you should be real happy with the walbro... everyone i know uses them without problems.

 

Thats what I wanted to hear. The lines are enough then I guess. My friends are convinced that I dont have enough line we shall see.

 

3/8" is more then enough... With bigger lines you'll also have to run a bigger pump to maintain the same pressure/flow...

 

yup.


and if it makes you feel better, there are FI LS1 guys running 200 more hp then you with thoes same size lines. it'll feed enough to max out your 42lb injectors @ a reasonable pressure.

 

I'm not getting that. The bigger lines just have a lower pressure drop before the fuel rails and fuel pressure regulator. The injectors aren't using any more fuel with larger lines (once you set the fuel pressure at the rail to the same as before the larger lines to account for slightly less pressure drop), so there is exactly the same mass flow rate through the lines, so the pump can remain the same.

I agree that the stock lines are plenty for this application, with the good walbro in-tank pump only. FWIW, my fuel pressure started dropping at high rpms with the older Walbro GSS307, 6x lb/hr injectors, and 5000+ rpms, on a 1:1 SX fuel pressure regulator, but I just made up for it with a wider pulsewidth in the tuning. Not ideal, but it works.

 

More weight in fuel to move.

My brother’s car was actually running out of fuel with a 190L/hr intank and swapping from a ½” feed to a 3/8” (in both cases, aluminum hard line) actually fixed it

 

he just said it, but i was going to say the same thing..

when you launch, that fuel is pretty heavy, like a 12ft long cyl of fuel pushing back at the pump...

what'll happen is you'll get a lean bog on launch, and pull your hair out trying to tune it out because its fine on the dyno and on the street... lol

 

The launching idea makes sense, as you need to accelerate the fuel fwd. Flipside would be a bigger return line creating more of a vacuum on launch, pulling in more fuel when accelerating. Interesting idea.

For steady state, it seems like size shouldn't matter, using the above example. The area of the tube is larger, so you have a slower velocity at the same flow rate (no different than arguing that a 4" catback creates more exhaust). The fuel pressure regulator is still the same restriction, so the same amount (mass flow rate again, gallons per hour, liters/min, etc...) is going back to the tank, and should be leaving the pump as well.

Thoughts?

 

Hehe.......he said size doesn't matter. Get it Butthead? Stop it Beavis!

 

so you have 2 directions for the fuel to flow away from the injectors.

You know, I know that there is more to it then that, since I know that the car was running out of fuel with the 1/2” line on the dyno and doesn’t with 3/8”

 

Heh. Thanks! Forgot about that...

For the non-g forces 1/2 vs 3/8, maybe it has to do w/ the pressure drop of various fittings and adapters vs a smooth 3/8" hard line? I'd be curious what the velocity is for both cases, given a 255 liters per hour flow rate. Grab the Reynolds number, and calculate the rough pressure drop due to diameter. My instinct says the fittings and connections w/ hard corners creates more of a loss than the smaller ID hard line, but I don't have time today to calc it.

 

In this case it both the ½ “ and the 3/8” was plumbed the same way and with the same stuff on either end, no fittings in the middle. On the whole, it just seemed to be more difficult to maintain the same pressure with the ½”