EFI swap - fuel line size - Vintage Mustang Forums
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post #1 of 15 (permalink) Old 09-09-2019, 03:02 PM Thread Starter
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EFI swap - fuel line size

Will the factory 1/4' fuel line deliver sufficient flow when converting to an EFI system on a 302ci motor?
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post #2 of 15 (permalink) Old 09-09-2019, 03:22 PM
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I believe the original fuel line is 5/16. Unless you're making big horsepower numbers it should be sufficient, mine has been. If you're shooting for high power and higher flowing injectors, you'd want to go up to 3/8.


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post #3 of 15 (permalink) Old 09-09-2019, 03:42 PM
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Remember EFI runs higher pressure. Higher pressure = more flow in the same size line. Subaru guys make 550 hp with the stock 5/16" fuel lines.
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post #4 of 15 (permalink) Old 09-09-2019, 05:22 PM
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Don't know what system you're upgrading to, but my Sniper EFI says 3/8 line. I went with the Sniper EFI tank for the whole package, so since I had to run a return line anyway, I changed out the 50 year old feed line too. If you need to use any flex line in the system (I did so I could run an in-line fuel filter and the AN fittings are so much easier to work with), make sure it has carbon infused PTFE lining. It will resist the degradation caused by ethanol and the static electricity from the higher flow rates for EFI. FYI: If you're working on getting rid of the fuel smell, you might consider adding a vapor recovery system and a non-venting cap if yours was a vented one. I talked to a guy from II Much about their venting system - he was a wealth of information. Got their system; haven't gotten to that point on the build yet, but I have what I need for what I think will be a destinkafied Mustang.

302, 351 World heads, Edelbrock 302 Performer intake, Holley Sniper EFI, Hedman full length headers, custom side exhaust, Shelby style hood
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post #5 of 15 (permalink) Old 09-09-2019, 05:29 PM
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I ran 3/8" Earl's Vapor Guard hose in both directions with no fuel smell.
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'66 GT Fastback, 302, Edelbrock Top End, Sniper EFI/Aeromotive, MSD, JBA Headers & 2.5" Exhaust, T5Z, Currie 3.55 Trac-Loc
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post #6 of 15 (permalink) Old 09-09-2019, 06:08 PM
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I run two stock 5/16" fuel lines side by side for feed and return line on my 347" with EZ-efi and have never had any problems with that.
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post #7 of 15 (permalink) Old 09-09-2019, 06:14 PM
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Quote:
Originally Posted by Oregon67Cpe View Post
Don't know what system you're upgrading to, but my Sniper EFI says 3/8 line.
But ain't that for securing fuel for the max 800 hp the Sniper can handle?
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post #8 of 15 (permalink) Old 09-09-2019, 06:47 PM
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Holley doesn't differentiate between systems. They specify 3/8" for all Sniper installations. Before my install, I contacted them about using my factory hard fuel line and was told not to use it.


'66 GT Fastback, 302, Edelbrock Top End, Sniper EFI/Aeromotive, MSD, JBA Headers & 2.5" Exhaust, T5Z, Currie 3.55 Trac-Loc
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post #9 of 15 (permalink) Old 09-10-2019, 04:12 PM Thread Starter
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Thanks for the line size correction and other input. I'm not going for any major HP, just making the carb to EFI switch for peace of mind. Appears a returnless system using the 5/16 line should be fine, but plan to double check with manufacturer.
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post #10 of 15 (permalink) Old 09-10-2019, 04:25 PM
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Quote:
Originally Posted by Westmus View Post
I run two stock 5/16" fuel lines side by side for feed and return line on my 347" with EZ-efi and have never had any problems with that.
5/16" line is just fine for feed...however, if you are running a return line, it should be larger than your feed line...this is easier on the fuel pressure regulator to maintain the correct fuel pressure...in the case of under 500HP though, it wont make a noticeable difference...but you can actually see the difference if you have an in-line fuel pressure gauge when you change to a larger return line...the regulator is a LOT more consistent for fuel pressure...less bouncing around means the regulator will last longer
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post #11 of 15 (permalink) Old 09-10-2019, 07:23 PM
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Holley and FiTech both suggest 3/8in feed and return lines for their setups. There were multiple reasons mentioned. Regulator longevity, erratic pressures, higher temperatures in the return line and venting of the tank.

I have my stock 5/16ths as a return on my Sniper and will be swapping it out for 3/8ths shortly to match my 3/8th feed. Its worked fine for the 10k miles I've put on the car, but after much reading/discussion I'll be switching over to whats been recommended.
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post #12 of 15 (permalink) Old 09-10-2019, 08:42 PM
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So does Edelbrock. Even their original ProFlo setup specified 3/8 line on both return and pressure.
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post #13 of 15 (permalink) Old 09-11-2019, 06:56 AM
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Quote:
Originally Posted by wicked93gs View Post
5/16" line is just fine for feed...however...
I don't clame it to be absolutly best possible solution, just that 5/16" feed/return have work flawless on my car for probably more than 10 years. The car did run an OLD Holly Pro-Jection system before I changed to the FAST efi. My message was just that 5/16" lines can easily handle 500 hp on a high pressure EFI system. The outlet of Walbro 255gph pumps that a lot of people use are also only 5/16". I believe those pumps are marketed as able to fuel 650 hp.

My inputs on forums are probably often a little "different" because I always look for solutions that will do the job, without overbuilding the car.
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post #14 of 15 (permalink) Old 09-11-2019, 07:17 AM
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I am running 5/16 for both feed and return. As mine is a foxbody style EFI setup, and the foxbody uses 5/16, I figured the engineering calculations were already done.
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post #15 of 15 (permalink) Old 09-11-2019, 10:58 AM
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Quote:
Originally Posted by Westmus View Post
I don't clame it to be absolutly best possible solution, just that 5/16" feed/return have work flawless on my car for probably more than 10 years. The car did run an OLD Holly Pro-Jection system before I changed to the FAST efi. My message was just that 5/16" lines can easily handle 500 hp on a high pressure EFI system. The outlet of Walbro 255gph pumps that a lot of people use are also only 5/16". I believe those pumps are marketed as able to fuel 650 hp.

My inputs on forums are probably often a little "different" because I always look for solutions that will do the job, without overbuilding the car.
I agree, 5/16 is fine for anything up to 450HP(at that point I would start stepping up the size just for peace of mind) but 450HP is more than the vast majority run anyway...at least on a 302. My suggestion about a larger return line(if running a return line) was solely to help the regulator maintain a more consistent pressure, nothing to do with the actual flow capability of the line. The thing about fuel pressure is that if you vary the fuel pressure you actually vary the injector flow too...Injectors rated at 19 lbs/hr at 39psi will flow something like 24 lbs/hr if you up the pressure to 50psi(there are calculators for that type of thing and that number is just off the top of my head...it might be 55psi that gives you 24lbs, cant remember exactly offhand) so its critical to maintain as consistent of a fuel pressure as you can. Pressure isnt going to bounce around over a very wide range with a 5/16 feed paired with a 5/16 return...but it would bounce around a lot more if that return were 1/4...and it bounces like crazy if you run a 1:1 fuel pressure regulator into a rising rate fuel pressure regulator(RRFPRs are the devil...a band-aid tuning solution for those unwilling to buy real computer control for forced induction cars...they are a NIGHTMARE to tune correctly). Anyway, when anyone does an engine swap of any type, I ALWAYS reccomend that they buy an aftermarket 1:1 FPR with one of those little in-line fuel pressure gauges(a $30 ebay unit works just fine...I have used half a dozen and never had any issue out of any of them). With that in-line gauge you always know what your fuel pressure is doing...and the ability to adjust it on the fly is awesome. It allows you to get the most out of your fuel system.
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