Return line too small?

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UOP

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Got a minor problem. Spent the day upgrading my fuel system by installing a new fuel pump and fuel pressure regulator. For those who don't know this is in a sbm swapped RX7. The problem is that the FPR will not regulate the fuel down past 7.5 psi, instead of the 6 psi I want. I'm thinking that my ~5/16" return line is too small for the fuel pump I'm using (255 lph). So next weekend I'm going to upgrade the return line, but I'm not sure if whether or not upping the size to 3/8" will do the trick. Don't want to spend the time and money if it won't work so I figured I'd ask. Anybody have any experience with kind of thing? I need to get this worked out.

Thanks
 
Regulator should reduce pressure regardless of a return or not. Try another regulator
 
If that pump is 255GPH, you should put a larger return line on it than the feed line. You want no resistance to flow in the return line. If the line is stepped down, you may not be able to get the pressure where you want. The related line back pressure will cause pressure creep in your regulator, thus, never stay consistent.

I run a BG return style regulator and I recall the instruction say that a 1/2 line is required for the return and a 5/8 return for any pump in excess of 140GPH.

Just something to consider.
 
Haha, a 255 GPH pump would be a complete waste on my car. My pump is right at around 68 GPH (255 LPH). That's why I'm thinking I need to up the return, and am thinking 3/8 would be enough. After starting this thread I went outside and bypassed the regulator to see if there was any resistance in the system and it showed 4 psi. I've got a roll of 3/8 rubber fuel line (probably long enough)so I'm going to temporarily run it back to the tank to see if that fixes the problem. If it does I'll run hard lines.
 
from what i understand yes the smaller line can cause that. when i helped djvcuda install hes efi he ran new 3/8 supply line and had originally planned on using the stock 5/16 line as the return to basically save time. i know he talked to a few guys with efi experience and they said to run the 3/8 line as a return also because it may bottle neck with only a 5/16 return...
 
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