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After much research, I am still unable to decide on a fuel set up for my car.
After the first dyno session, I knew there was a restriction issue (see pic)
Did make 651 whp thru this and 1/4" line increasing to a 6an line midway up the car.
I was thinking of going to a -8 feed with a -6 return and possibly even adding a 2nd walbro 400 if I can squeeze it in there. What I am having a hard time understanding is what benefit is putting a large fuel feed line on a pump that only has an outlet of less than 3/8"? I believe the id of the walbro 400 outlet measures less than 1/4"
Can someone help me wrap my little brain around this so I can decide what direction to go?
Set up is a 5.3, good rods and pistons, pt88, ljms st2 cam, 243 heads, ls6 instake, deka 80's and currently a single walbro 400 in the stock tank.
10 ft of 1/4" hose has a greater restriction than say 1" of 1/4 hose transitioning to 10ft of 3/4" hose.
What type of fuel?
How much power do you want?
It generally costs very little to bump up the next size hose when planning a fuel system. I usually figure out what I need roughly, then bump up to the next size.
IMO doing a -10 over a -8 (for example) wont reduce your power nor will it limit the power you may want to make in the future. If you're thinking of going that high up in power, mind as well do the fuel system one time and be done with it.
There is a BMW guy that made 1500 HP on -6 line back when I was doing automotive magazines. Very impressive setup. He was pushing ~90 or so PSI of fuel pressure.
Pro Mod guys use -10 line to their rails and make ~4000 HP on methanol. Fuel pressure helps.
If you are run 3/8" with standard AN bend fittings you end up with .272" in those fittings. You can support 1100 liter per hour with 15 foot run and 3 foot climb at about 5 pounds of pressure restriction. If you only run straights it can support even more as it will be all .375". There is a company that makes best which hold the diameter. I'm running all straight -6an until I split to two for each rail and running best after where only half the flow will be going through them to be safe. The other things most people get wrong is your return line should be larger than the feed line to allow the bubble to separate from the fuel in the line. Then once in the tank they float right up reducing risk of being pulled back in by the pump. A straight section of -8an requires almost a 1000 liters per hour to maintain 1 foot per second and you want to keep it between 1 and 4. We call the to large of a fuel line user sick with Pro-Moditis. Don't go overboard, but keep in mind the restrictions of bends and elevation.
Wtf.
Now -6 with any bends is too small and -8 is way too big?!
Crf450 -Dave is a amazing engine builder- good job!
I've gotta 5' rise from my tank to fuel rails....running a AEM 380 that's got a -10 inlet and a -6 out so I guess I need to use the -6 up to the rails......using the factory rail supply line....
-6an is .375" and that is capable of flowing so much with a certain amount of friction per foot depending on the inside of the line.
Standard Russell, Redhorse, and many other -6an bends are connected with what would be about -5an if they had that as a size. They restrict the size down to about .272" and if you run the same through that size straight it will have more friction than the size it claims to be. Pretty simple.
If you have a straight -8an it is .500", but those fittings are the same from the standard makers. The bends would be a -7 if they made them.
You also need to keep in mind bends of any size are a restriction.
You can flow up to 480 gallons or 2150 liters per hour through 8 foot of hose up 6 feet and the pump will be pumping about 5 psi more than you have at the rail. You can make a lot of power from that. If you don't believe the numbers just look at the size of the outlet of the intank pumps. My car has a 450LPH pump with about a 1/4 inside diameter outlet. It's has a cross in it so it's not even and open 1/4". People do way to much line.
-8an is good to 850 gallons per hour.
All though it is recommended to stay between 1 and 4 feet per second. It is way more important to stay below 4 as having it to slow is still enough fuel to not burn up the motor. I think you would have to run 1 inch line to have it so much over done you would have engine trouble. So, run larger if you like and keep in mind -6 is not really .375" unless you source the right fittings. Just don't run to small.
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The great fuel line debate. 
