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NCC Forum Home Page -> NCC - Projects and Updates -> 3/8 vs. 5/16 fuel line-Holding me back?
Post Info TOPIC: 3/8 vs. 5/16 fuel line-Holding me back?
SShink


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3/8 vs. 5/16 fuel line-Holding me back?
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When replacing the gas tank on the Chevelle, I realized it has a 5/16 line from the tank to the pump.  Likely because it was originally a small block car.  I'm going to replace the line with a 3/8 factory steel line while I've got it up on blocks.

Now I'm curious how much flow restriction to a big block the 5/16 line caused?  Not sure if I've been restricting the BBC of fuel under full throttle  headscratch

I have to think that since the factory designers spec'd in the larger line, the difference is significant.  It's about a 0.60 difference in diameter or a 17% change in size.  Anyone done this switch and noticed any difference in performance?

 

 



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Stan S.-Twin Cities 'South Metro'

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John D


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The area where it's trapped between the body & frame by the rear control arms & shock. The forward end of the "fun" starts in your pic #1



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John D. - St. Louis Park, MN.

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jim larson


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Not positive; but the surface area ratio of 3/8 diameter to 5/16 diameter is about .141/.098 or about 44% more. Don't really know what that means regarding how much suck it has. But they used a 5/16 fuel line on the L79 engines with 350HP. Think it would also depend on the carb, weather it is a 600 cfm or 750 cfm, etc. And also the fuel pump.

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Jim L

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Chris R


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There wont be a major pressure drop enough to cause you a shortage of pressure at the pump with that size line. Not enough to give you any real problems, otherwise you would have had fuel system problems a long time ago. I wouldnt replace it because of the reason of it being too small. I would however consider replacing it for the reason of it being 40 years old though. The problem is part of the fuel line tucks above the frame rail and a lot of cars need to have the body lifted up to get the old line out and a new line on, meaning, PITA. 



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Chris - Ramsey, MN.

Dear Optimist, Pessimist, and Realist.

While you guys were busy arguing about the glass of water. I drank it!

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SShink


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I did more research on TC, and to your point Chris, sounds like it's hard to do with the body on, so I'll probably put it off for now...



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Stan S.-Twin Cities 'South Metro'

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jim larson


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SShink wrote:

I did more research on TC, and to your point Chris, sounds like it's hard to do with the body on, so I'll probably put it off for now...

 Think you have to loosen up all body bushing, remove those on the pass side, and jack the pass side of the car up.



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Scott Parkhurst


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I still have a stock tank/line in mine too...


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Scott Parkhurst

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John D


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We could get into all kinds of fluid dynamics/percentage of fill/GPM/cross-sectional area/physics crap here if needed....

What it all comes down to is:
Have you EVER experienced a "fuel starvation/lean-out" issue when under the following conditions:
- The primary AND secondary bores of the carburetor are FULLY open.
AND
- The time of the above has approached or exceeded 15 seconds?

Think about this "real world". The carb has reservoirs/bowls. When in "day to day" driving are you going to tax the limits of the fuel delivery system? Is the 1 in a 1000th day at the track going to justify the PITA of exchanging the fuel system for the .134" increase in area?

Run whatcha Brung



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John D. - St. Louis Park, MN.

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Chris R


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Me and Dave got to learn just how fun this job really is when he had Mikes car.

This perticular example didnt happen to Mikes car but it certainly can happen.

1. Replacing fuel line means dropping frame down or possibly pulling a body mount or two.

2. body mounts after nearly 40 years wont just come right out unless you have a southern car.

3. Body mounts break and your stuck having to fix the repair.

4. No point in putting the 40 year old rubber mounts back in so might as well get new rubber, solid, or eurothane mounts to replace them with along with new bolts if they are in bad shape.

5. Cant just put new body mounts in the spots near the fuel lines. Looks like its time to remove the rest of the mounts which raises the opportunity to not just break one or 2 near the fuel line. Now its what, like 8 total. Thus creating more opportunities to break bolts. In that case, go back to step 3 again.

So fuel lines can be quite the can of worms.



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Chris - Ramsey, MN.

Dear Optimist, Pessimist, and Realist.

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SShink


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So I'm a dope!  Just used the calipers and measured what's on the car.  It's 3/8"!  I'm sure this was originally a small block car, so expected a 5/16 line.

I'm still confused about why it's so hard to change the line though.  I looked at mine, and it looks like other than feeding it through the area above the lower control arm, it should be pretty easy.  What am I missing?

 

 



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Stan S.-Twin Cities 'South Metro'

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Derek69SS


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350 cars got a 3/8" line... 5/16 was only on the 6-cyl and 307 V8 cars.

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Derek Kiefer - Mantorville, MN

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SShink


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Derek69SS wrote:

350 cars got a 3/8" line... 5/16 was only on the 6-cyl and 307 V8 cars.

That explains it.  Thanks Derek!  beers

I must have been thinking of one my many other past Chevelles that had the small line...  laughing 



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Stan S.-Twin Cities 'South Metro'

1972 Malibu Convertible 2nd time around 

2001 Mustang GT Convertible 

Forum influenced terms: 'Link Paste', 'Stanitized', & 'Revolving garage door...' 

 
Dave Seitz


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Another option is a fuel accumulator if you are worried about starvation. Yes a big fuel filter can act as an accumulator in the engine compartment. Flow and pressure are NOT the same so if you have an accumulator and a larger line out of that with a regulator that can solve some problems you may have. This is kind of a trick for certain Hyd applications that were designed badly.



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