Should I run larger fuel lines?

Here is a question for my first post. Glad to be here, thanks in advance for any assistance. :)

I'm replacing the fuel lines on my FL with a turned up 6NZ. It currently has -8 supply and -6 return from the tank selector to the filters. How much power are these good for, should go to -10/-8 or leave the current size?

I am planning on reversing the lines to the head and upgrading there, also adding a fuel cooler on the return.
 

Truck Shop

Well-known member
I would just change to 10 on supply, leave the 8 return. But that 8 should feed it. How big of fuel tanks do you have? Cat only requires
a cooler when total capacity is under 100 gallon.

Truck Shop
 
I would just change to 10 on supply, leave the 8 return. But that 8 should feed it. How big of fuel tanks do you have? Cat only requires
a cooler when total capacity is under 100 gallon.

Truck Shop
That's the way I'm starting to think, the return is mainly restricted by the filter head and -6 fitting anyway. The supply line runs to my newer style filter/water separator with the priming pump, so it's easy to use a larger fitting in. The tanks are 130gal, I already have the cooler, so it's just a matter of plumbing it. I would use a 3-way ball valve to divert in the winter.
 

Truck Shop

Well-known member
If you already have the cooler and fittings then it's just a matter of hose, go for it. Running larger fuel line definitely won't hurt.
 

Longhood

Well-known member
if you decide to go with larger lines, try to keep the distance from the water separator to the lift pump as short as possible and avoid droops as much as possible.
 
Hose1.Jpg

I had -6 return and -10 supply made up at the machine shop today. I noticed this afternoon when I'm getting ready to install that the fittings are WAY more restrictive. I don't see how I can use these. The original -6 is about 1/4" ID on the fitting, the new one is more like 1/8" and the -10 is about the same size as the original -8. Waste of $200
 

Truck Shop

Well-known member
You need to go back and have them make up the hoses with the right fittings. They used the wrong JIC fittings. That's not the first time I've seen that,
damn good deal that you looked them over before installing them:confused:
 
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I told him it was for fuel lines, and had the old lines there. I only paid attn to the thread size while getting it done. I was surprised the -10 fit the same flare as my original -8, so it looks like that's no improvement either since the ID of the fitting is the same as my original -8.
 
Hose2.Jpg

That was the only fittings they had for the -6 fiber braid line, JIC I think. The best solution we could come up with was going to -8 with a larger fitting. Other than going out of town to get some made and spending another couple hundred plus, that's about as good as I'm going to do I guess. Also with the -10 the ID in the fitting is about the same as the stock -8. So I'm not benefiting any on flow through the fittings, but maybe gain a little less resistance through the hoses being larger.
 
I also noticed while looking at the fittings that the flares looked different. I'm glad I paid close attn because they are. My factory lines appear to be 45 degree, where the new lines are 37 JIC.
 

Truck Shop

Well-known member
Fuel hose fittings are SAE J512 45* flare. Hydraulic normally uses the JIC 37* flare plus the I.D. is different. Then you have international connections
which is a whole different game.
 
Well, that explains a lot. I got the wrong hoses from the get-go. It's really sad that the machine shop didn't notice they were 45's to begin with. Even if all they have are JIC fittings, he could've told me that and I would have went elsewhere.

A little too late now though, I've gotten them installed, after changing all the fittings to the steel JIC's. I know they'll work. I may go back later and replace them with the SAE if the ID of the fittings are larger though. This has been a big waste of time and money, other than stopping the leak. It's too bad we don't have a good shop that can do fuel line here, would've saved me a lot of headache.

I do appreciate your help Truck Shop.
 

Truck Shop

Well-known member
Don't feel like the lone ranger, I run into the same problem where i'm at. We had a real good hydraulic and hose shop but that guy retired.
 

BoxCarKidd

Active member
Jic-SAE

Don't feel like the lone ranger, I run into the same problem where i'm at. We had a real good hydraulic and hose shop but that guy retired.
They are duel seat and interchangable except never #6 and #12 is questionable. But just because it does not leak does not help the flow.
 

oldnotslow

Active member
I am curious how much benefit or need there is for one size larger fuel lines? Was there some indication fuel volume might not be sufficient under load? My B model at one point had larger injectors and turbo, so I went one size larger on fuel lines. Didn't really notice a difference for the expense. I did notice a difference after installing a positive pressure electric fuel pump (Fuel Preporator). Smoother idle, slightly less grey smoke. My B model is now back to stock after a re-man and with a smaller wastegated turbo I am very happy with the throttle response even though there is a little less boost on top. I don't run the freeways much though.
 
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