Fuel Return Line Sizing
 

I'm in the process of upsizing the fuel lines in my RX3. Lot's of folks here are recommending 1/2" supply lines with 3/8" return lines. Is there an issue with using 1/2" for both supply and return? My goal for the system is to support up to 500hp on E-85.

https://cimg8.ibsrv.net/gimg/www.rx7...02e67480b6.jpg

Not sure why people think that a return line should be smaller. It's a low pressure line and if anything requires a larger line to ensure smooth fuel pressure at varying loads.

I would do AN-8 or larger or if your doing imperial I would do both in 1/2 inch

Thanks for the feedback. I just wanted to check if there was some magic I was missing that made 3/8" a better choice. I have enough 1/2" stainless tubing left over for the return line, so using 1/2" saves me money too.

larger is fine, but if you have staged multiple pumps or a PWM setup it wouldn’t likely be necessary,

but one consideration for not going larger than necessary might be cost vs. pipe/hose type because if you didn’t already have it and didn’t need it to be that large, then the bypass flow requirement even under low pressure in a staged or PWM setup isn’t likely to require such an oversized return line.

couple of links for calculating it out, first one is an overview and the second an online calculator:

https://l-m-engines.myshopify.com/pa...ine-calculator

https://lmengines.com/pages/fuel-lin...5-c7f7d5d28a3a


along with an overview by Howard Coleman here on the forum for determining the ballpark flow requirement:

https://www.rx7club.com/single-turbo.../#post11671585


I ended up choosing cunifer (copper/nickel) hard piping; typically used more often for brake lines, because it’s a lot easier to work with, even formable by hand, and quite corrosion resistant as well. So sort of like your situation, at the time it was more cost effective to buy a 25 foot coil in -8 (1/2”) than two shorter lengths in -8 and -6 (3/8”).

if you start looking at hoses though, which some people find easier to run even for the full front-rear length, especially the lightweight racing type which is what I initially wanted, the difference in cost/foot can add up quickly. Then I calculated out the actual weight difference and it was so negligible that the cost difference and likely longer term durability of cunifer won out.

https://www.cunifer.com/

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I disagree. Staged pumps and/or PWM controlled pumps still require large mount of return flow. Since they do not speed up and slow down instantly when coming on and off the throttle etc you end up with large fuel pressure swings either way .

When you have plenty of return flow no matter what the pumps are doing you get very little deviation in pressure when engine conditions are changing quickly (Gear shift, varying partial throttle etc)

As always it may not effect the final result very much but if you're going through the effort of running lines, might as well do it right.

it’s sound rationale; agreed.
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