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Discussion Starter #1 (Edited)
I'm looking at mechanical fuel pumps for the street-driven 347 I'm building, and I'm a bit confused at the options available. I keep reading that something like 3/8" lines would be a good idea for a motor in the 350-450 HP range.

So anyway, this might be really dumb, and it's possible I've confused something entirely, but here's the deal: I was on Summit comparing the selection of stock SBF fuel pumps to the aftermarket options, and noticing that it seemed as though the stock pumps had mostly 1/4" NPT fittings on them. That seems like it would restrict the standard 5/16" line even more, but for a stock application I guess it's suitable. Anyway, then I noticed that Carter's "high performance" pumps are also using 1/4" NPT fittings. That piqued my interest, and I started looking at the pumps they used on bigger motors, and noticed that even 428's and such are also using 1/4" NPT fittings from the factory.

I'll probably just end up getting something like an Edelbrock pump and 3/8" lines and call it a day, but I'm just curious, because after looking at everything available it doesn't seem like there's much difference, on paper, between these performance pumps and the standard ones from the factory. Even their free flow rates are all roughly equivalent. Is 3/8" overkill?
 

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No, in fact it is undersized for todays' HP applications. You want the CARTER HI-VOLUME pump and plumb at least to the tank and pickup (and naturally to the carb).

Most FORD HI-PO (and Police) came with 3/8" line. Think of modifying the pickup (5/16") also.
 

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Discussion Starter #3 (Edited)
Hmm, all the Carters I saw had 1/4" NPT fittings only... and then one with -8AN/-10AN. that's why I was going to go for an Edelbrock or Holley.

I guess my question pertains more to the fitting size itself, then. If they're equipping cars with 1/4" NPT fittings and 3/8" lines from the factory.... isn't that kind of pointless, since the flow is still going to be no greater than the biggest restriction?
 

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Discussion Starter #5 (Edited)
Oh, yeah, I know Holley has one in 3/8... just not Carter. That's all I was saying. But thanks.

I'm only asking out of curiosity here, I'm not actually intending to use a stock fuel pump... But it seems like all the stock replacement pumps I can find, for any early Ford engine, use 1/4" NPT fittings. Since I know for a fact that some Fords were equipped from the factory with 3/8" fuel line, is it standard practice to step those fittings down that much? Like I said before, it seems like that would just negate the effect of the bigger line size? Unless the proximity to the fuel pump is simply overcoming the restriction, and it doesn't matter at that point, or something.
 

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Oh, yeah, I know Holley has one in 3/8... just not Carter. That's all I was saying. But thanks.

I'm only asking out of curiosity here, I'm not actually intending to use a stock fuel pump... But it seems like all the stock replacement pumps I can find, for any early Ford engine, use 1/4" NPT fittings. Since I know for a fact that some Fords were equipped from the factory with 3/8" fuel line, is it standard practice to step those fittings down that much? Like I said before, it seems like that would just negate the effect of the bigger line size? Unless the proximity to the fuel pump is simply overcoming the restriction, and it doesn't matter at that point, or something.
Sorry, I mis-read your post.

The CARTER used by FORD in their HP applications had 3/8" inlet/outlet. The CARTER online catalog is not very concise. You would need a jobber catalog to figure it out. I would think the only HP application on a SBF would be the 289HP and BOSS 302. The MPC will not likely give that info, only a Service PN.

Fuel Pump

All small-block Fords of the period were fitted with Carter fuel pumps. From 1964 to 1965, it was a screw-together Carter pump with the fuel filter. From 1966 to 1967, Ford used a sealed Carter pump without the filter canister. The Hi-Po engine differs in volume: It delivers more fuel than its 2V/4V counterparts.

SOURCE- How-To ID A 289 High Performance Engine - Mustang Monthly Magazine
 

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Do you need a 3/8" line for that many horsepower? No. Do you need a pump capable of feeding it? yes. I had the Edelbrock pump on my 302 making 375HP, and it feeds it no problem. There are many articles about the capabilities of 5/16" fuel line and how it's been used for most mechanical pumps and engines under 500HP. Many will debate this (especially the people trying to sell you fuel lines!) so do your own research.

The line is the single largest source of pressure drop because it is so long. The 1/4" restriction is very short and the losses are low. The larger the line, the lower the pressure drop at a given flow rate, and the longer the line, the higher the pressure drop at any flow rate. The pump has to suck fuel up the line and once your positive suction head drops sufficiently, you start boiling the fluid in the line and you 'max out' your line capacity. This is even more an issue with impeller (electric) type pumps, but it's true of mechanical ones too.

So first, how much flow do you need for 450HP? Assuming a BSFC of .45 lb/hp/hr, we'll need a minimum of 138lph to supply that. Racers Math 1

Next lets look at line pressure drops @ 138 lph
10 feet of 3/8" line + pressure drop from 3/8" to 1/4": .16 + .03 psi = 0.2 psi
10 feet of 5/16" line + pressure drop from 5/16 to 1/4": .38 + .02 psi = 0.4 psi

So you can see that the line pressure drop is much more significant than the step down to 1/4" at the pump, and that the pressure drop is half in a 3/8" line.

Pressure Drop Online-Calculator

The next thing you have to consider is that when you're accelerating, gasoline is trying to rush back down your line into the tank. Lets say .5G acceleration (120mph trap in 11.3 seconds.) In this case P = rho*g*h = 750 kg/m^3*9.81m/s^2*.5*3m = 1.6psi. This is independent of line size.

So the total pressure drop under .5G acceleration for each line:
3/8" line: 0.2 + 1.6 = 1.8 psi
5/16 line: 0.4 + 1.6 = 2.0 psi

Not a big difference. At all.

The vapor pressure of an average summer blend gasoline at 100F is 10.5psi (In Texas. Your state will vary.) Vapor pressure is the pressure at which a fluid starts to boil. So if you are in Texas where the air pressure is 14.7 psi, once you drop the pressure 4.2 psi you start to boil the fluid and you start leaning out. http://www.epa.gov/ttnchie1/ap42/ch07/final/c07s01.pdf Guide on State Summer RVP Standards | Fuels and Fuel Additives | US EPA

You're not close to 4 psi with either the 5/16" or the 3/8" line, so if it saves you trouble and your 5/16" is in good shape, keep it. If it's not, why not upgrade while you're going through the trouble? However, this is only true if everything in the system is 5/16". If your pickup is pinched, your line is pinched, or anything on the tank end is under that, the pressure losses rise quickly, so make sure it is in good shape.
 

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I'm running the Carter high performance street pump, has 3/8" fittings and I even drilled out the insides to open them up some. It's a clone of the old Boss 302 pumps. I had one of the Edelbrock pumps years ago. It shot oil out of the breather holes in the top all over my engine. Edelbrock told me of a $35 gasket I could buy to correct the problem. I went back to using Carters.
 

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... Anyway, then I noticed that Carter's "high performance" pumps are also using 1/4" NPT fittings. ...
A couple points you may not be aware of - 1/4" NPT is a large fitting with a small name. The body of that size fitting is acutally over 1/2", and fittings are available for hoses from tiny to 1/2" (AN -8) hose. So, 1/4" NPT is not a performance limiter you need to be concerned about, and does not relate to the line size.

Second, no matter what line size you use, the inlet needles and seats at the carb will determine the total flow you can actually achieve. Unless your carb is modified with custom or racing needles and seats, large lines won't make any difference anyway. Again, this shoots-down the large line theory.

Finally, if you increase your demand in the future with a much more powerful engine, adding an electric boost pump and larger pickup will continue to feed whatever you need through the same lines.

David

PS: Nice synopsis Wes. :tup:
 

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Discussion Starter #11
Thank you for that incredibly detailed answer, thekingofazle, that's exactly the kind of explanation I was hoping for! And thanks for explaining it in layman's terms, too. I gotta bookmark this one for the future.

Running new line isn't particularly an issue for me, and in fact I was completely anticipating it, I just don't always like the "more is good, so too much is just right" approach, so I do try to be careful about spending my money where it matters, rather than just buying every performance part imaginable.
 

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...sheesh...

I guess I should forward this thread to FORD ENGINEERING. Seems they never had a clue and wasted all of that money... :rolleyes:
 

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...sheesh...

I guess I should forward this thread to FORD ENGINEERING. Seems they never had a clue and wasted all of that money... :rolleyes:
Science and Engineering have come a long way in 40 or 50 years, you have to admit. We have some really bright and insightful people at this forum, that's why I keep coming back. :tup:

John
 

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...sheesh...

I guess I should forward this thread to FORD ENGINEERING. Seems they never had a clue and wasted all of that money... :rolleyes:
I think they did just fine, as they did not build to bare minimum to save a penny on a part as they began to do a decade later, then reversed their position again 2 decades later when they found it to be going too far. Wes's calcs show the 5/16 and 3/8 are sufficient for anything you could use on the street, and just happens to be what Ford used those years. Personally, I run 3/8 on carb cars for anything on the street, and any track cars that have less than ridiculous power. The 1/2 and 5/8 full-length lines being pushed by performance sellers is simply unnecessary and a waste of good cash. If you need more flow, push it with a pump at the tank.
:tup:
David
 

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Discussion Starter #15
Well, one of the most unexpected things I found out from this thread is that total fuel flow capacity is NOT restricted to the smallest diameter anywhere in the system. See, I was under the impression that it behaved similarly to exhaust systems in that way, and I guess I was sorely mistaken.

Explains why Ford (from what I've gathered) sometimes ran 3/8" line up to the pump, and then 5/16 from there on out.

And for what it's worth, I'm probably going to run 3/8" anyway, but it's good to know that I have a larger selection of pumps available to me now.
 

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Well, I disagree.

Let's just leave it at that. It is your car and you got the answer you were looking for.

Wish you good luck with it... :tup:
 

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A couple points you may not be aware of - 1/4" NPT is a large fitting with a small name. . :tup:
LOL good description :)!
OP, I don't think anyone knows what the inch dimension NP threads means. However I do believe the ID of a 1/4" NPT is larger than 3/8". Not too intuitive, I know....
 

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Just for clarity sake-

1/4'' NPT x 3/8'' Hose (Barb)



Now you can fully understand the sizing differences.
 

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Discussion Starter #19
Well, right there's something else I didn't know.

It is your car and you got the answer you were looking for.
Wasn't looking for any particular answer... I'll take what I can get! Mostly I was having trouble finding definitive information on this sort of thing.
 

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Well, one of the most unexpected things I found out from this thread is that total fuel flow capacity is NOT restricted to the smallest diameter anywhere in the system.

That would be news to me, too!
 
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