Fuel Line Size Pump to Carb - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**The Rocknrod** · 07-03-2019, 11:31 AM

What is the consensus on the size of fuel line I should use from the fuel pump (mechanical) to the Carb. Using a 625 cfm Street Demon. Right now I’m all set up for a 3/8 tube, but I was thinking of maybe using a 5/16” tube instead. Blueprint Engines 350.
The fuel pressure would be the same but the volume might be more? Maybe that is over-thinking, just raising a float.
I don’t know that’s why I’m asking what the general thoughts are on the subject. What do you think?

**kwmech** · 07-03-2019, 11:52 AM

Why do you think a smaller line will give you more volume? GM went to 3/8 for a reason on the V8. 5/16 was for the 6 cyl. Pressure will be the same regardless.

Had this happen to me at the plumbing store a couple of years back. I was looking for some fittings for a gravity water system I was installing. 4'' pvc down the hill from my storage tanks. I needed to end up at a hose bib with 1'' to the valve with a few Ts underground. Guy commented that by doing that I should have 4 times the pressure. I just looked at him and said you're fired. I got the weirdest look from him.

**The Rocknrod** · 07-03-2019, 11:54 AM

More volume on the larger line. Maybe I didnt write it correctly on my first post.

rpmerf · 07-03-2019, 12:31 PM

More volume on the larger line, so why would you want to use a smaller line?

3/8 = 6/16 > 5/16

**The Rocknrod** · 07-03-2019, 12:34 PM

I'm askin the question why would I need a 3/8 vs 5/16?

**WorkinLonghorn** · 07-03-2019, 12:41 PM

Either size is fine IMO. Your 625 cfm carb can only use about one once of fuel per second. That is ~2 tablespoons per second.

07-03-2019, 11:31 AM	#1
The Rocknrod Moderator Join Date: Aug 2011 Location: China Spring, TX Posts: 7,280	Fuel Line Size Pump to Carb What is the consensus on the size of fuel line I should use from the fuel pump (mechanical) to the Carb. Using a 625 cfm Street Demon. Right now I’m all set up for a 3/8 tube, but I was thinking of maybe using a 5/16” tube instead. Blueprint Engines 350. The fuel pressure would be the same but the volume might be more? Maybe that is over-thinking, just raising a float. I don’t know that’s why I’m asking what the general thoughts are on the subject. What do you think? __________________ Master Chief, US Navy, Retired 1964 C10 Step-side Short Bed - In Progress 2009 2500HD Duramax LTZ 1948 GMC Panel 1956 Big Window Stepside 1953 5 Window Chevy Stepside 1932 Coupe New Seat Foam for Bench Seats?

07-03-2019, 11:52 AM	#2
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,389	Re: Fuel Line Size Pump to Carb Why do you think a smaller line will give you more volume? GM went to 3/8 for a reason on the V8. 5/16 was for the 6 cyl. Pressure will be the same regardless. Had this happen to me at the plumbing store a couple of years back. I was looking for some fittings for a gravity water system I was installing. 4'' pvc down the hill from my storage tanks. I needed to end up at a hose bib with 1'' to the valve with a few Ts underground. Guy commented that by doing that I should have 4 times the pressure. I just looked at him and said you're fired. I got the weirdest look from him.

07-03-2019, 11:54 AM	#3
The Rocknrod Moderator Join Date: Aug 2011 Location: China Spring, TX Posts: 7,280	Re: Fuel Line Size Pump to Carb More volume on the larger line. Maybe I didnt write it correctly on my first post. __________________ Master Chief, US Navy, Retired 1964 C10 Step-side Short Bed - In Progress 2009 2500HD Duramax LTZ 1948 GMC Panel 1956 Big Window Stepside 1953 5 Window Chevy Stepside 1932 Coupe New Seat Foam for Bench Seats?

07-03-2019, 12:31 PM	#4
rpmerf Registered User Join Date: Feb 2011 Location: Parkville, MD Posts: 1,024	Re: Fuel Line Size Pump to Carb More volume on the larger line, so why would you want to use a smaller line? 3/8 = 6/16 > 5/16 __________________ 1970 C20 Custom Camper - 350, TH350 1997 GMC Suburban 1994 Acura Integra GSR 1987 Dodge Daytona Shelby Z

07-03-2019, 12:34 PM	#5
The Rocknrod Moderator Join Date: Aug 2011 Location: China Spring, TX Posts: 7,280	Re: Fuel Line Size Pump to Carb I'm askin the question why would I need a 3/8 vs 5/16? __________________ Master Chief, US Navy, Retired 1964 C10 Step-side Short Bed - In Progress 2009 2500HD Duramax LTZ 1948 GMC Panel 1956 Big Window Stepside 1953 5 Window Chevy Stepside 1932 Coupe New Seat Foam for Bench Seats?

07-03-2019, 12:41 PM	#6
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,855	Re: Fuel Line Size Pump to Carb Either size is fine IMO. Your 625 cfm carb can only use about one once of fuel per second. That is ~2 tablespoons per second. __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, 2nd owner, Work-Truck supreme. Best $300 I ever spent.



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