I have my own personal opinions on fuel line sizing and whats appropriate
but figured this would be a good disscussion and maybe some great input
so I will start with my experiences and what I think, and please jsut dont bash me, but give input and why you think what you think
but in the old days of mechanical pumps they needed a larger line to ease the pushing of fuel thru the lines, thus a larger line created less drag on the fuel, thus increasing the pressue at the carb
but now days in the world of fuel injection, we have electronic pumps, these pumps are capable of 100psi, most of us only require around 50-60 psi, the line size now is not as near as important due to the pressure the electronic pumps can produce
basically to me know , up to 7-800 hp you can easily run the oem lines, as long as you have a guage to help determine the pressure you are running, a drop in pressure can mean a couple things, one increase pump pressure, or enlarge the lines at that point to get more volume,
I do agree that going to big hp large lines would be a safe bet to do, but also on my own personal twin setup I ran a large electronic pump and only a small 3/8's line and pressure held perfectly at 58 psi at 1030 rwhp
I also found this on the net and copy and pasted this, but some of the more technical guys can give more input, but my personal opininion is to run a guage, make sure your injectors are getting the correct psi, and work from there depending on your build and your hp, but line size was alot more critical in the days of carbs and mechanical pumps than the setups we use today
This post will address the required fuel line size from your fuel tank to the injector rails as well as the injector rails themselves.
Let's start with a random off the shelf injector - say a 60 lb/hr. In order for the flow rating of any injector to be meaningful we must also have a pressure rating. Most injectors' flows are rated at around 42 to 45 psig (pounds per square inch gage) and typically 80% duty cycle.
So we take this injector and hook it up to a test rail. The test rail has a regulator on it which we set to say 42 psig. We also attach an accurate fuel pressure gage right at the rear of the injector to verify the pressure setting. Now we run the injector at 80% duty cycle with its business end pointed into a calibrated cylinder so we can measure its' flow over time. Some labs have very accurate in-line flow meters that make the measuring task much easier/quicker. With everything in place and functioning we measure a flow rate of 60 lbs/hr.
Now I ask you did it matter whether I fed the fuel rail with a 1/8" id fuel line or a 3/8" id fuel line or a 1/2" id fuel line or a 1" id fuel rail as long as I was able to maintain the 42 psig? No - it does not matter what the supply line size is as long as I can maintain the desired pressure at the injector head !!!
I took this round about way of explaining flow because some people have it stuck in their heads that they must increase their line size when making big HP numbers like a thousand or more. A #6 AN line can easily support 2000 HP as long as you have enough pump head !!!
That is to say as the flow thru any given line size increases so do the friction losses. These losses are published in engineering manuals such as the Cameron Hydraulic Data Handbook published by Ingersoll-Rand. Any engineer who works with designing pump installations has one of these manuals.
Let's take a typical Corvette. I'll say there is 20 feet of steel 3/8" id fuel line from tank to rail (there is actually less but we'll be conservative because bends in the line account for some pressure losses also - each bend adds 'x' amount of equivalent feet).
Now you go into the table and look up the pressure drop for say 110 gph (gallons per hour) for 20 feet of 3/8" id steel line (12 HP/ gallon of gasoline is a conservative number): it's going to be around 2 to 3 psig.
This is the same way an electrical engineer determines what size wire to run for a given load. If the load is say 40 amps he could use a #14 AWG or he could use a #8AWG. Both size wires can handle the load, i.e., 50 amps (or if gasoline amps = volume in gallons). However the smaller wire has a much higher voltage drop (in gasoline thats pressure). If our voltage is fixed at 120 VAC the engineer will go with the bigger wire to minimize the voltage drop. Your Corvette pump head (i.e., voltage) is not fixed until after you pick your pumps etc.
So what this means is that if you need to maintain 42 psig at the fuel injector you must have enough pump head to not only make the 42 psig but an extra 2 or 3 psig for the line losses. In other words if your pump(s) can deliver 110 gph at say 60 psig, you have plenty of pump with some to spare.
Where all these line size "wifes' tails" started is back in the days of carbureautors and low pressure electric pumps or low pressure mechanical pumps. These pumps had plenty of flow but at low pressures. These pumps could not afford a 3 psig drop from front to back. So they increased line size - without looking it up perhaps a #8 AN line has only a 1 psig drop at 110 gph for the given 20 feet.
The whole point of this discussion is that if you have an accurate fuel pressure gage placed at the end of your fuel rail and you are able to maintain the head i.e., desired fuel pressure PSIG, your fuel line size is fine!!! If the fuel pressure starts to fall as you go up in HP you need more pump - if for some reason you cannot get more pump then line size can be considered - but if you look at the numbers for #6 vs. #8 there is very little difference.