I'm installing one of Marks 505 is my 69 and am wondering if I will need 1/2" fuel line from the fuel pump to the carb? I have 1/2 line from the tank to the pump.
Do I need 1/2 fuel line from pump to carb?
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csmnlm
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G
Guest
·No, your needle and seat opening is only around .100 or so. That being the case, 1/2" going up to the carb is a total waste, even if you did manage to get the fittings and carb linkage to clear each other. I'm using -6 hose myself on my 540, as are most guys I know. The Aeroquip hose I'm using has .34 ID, which is still WAY big for those little needle and seat openings.
if there is only .2 feeding the carb why is anything bigger than factory 5/16 needed?
G
Guest
·OK, take Aeroquip Startlight braided hose. Their -10 ID is .56 (.250 sq in) Their -8 ID is .44 (.152 sq in), but a high-flow regulator hole is typically around 5/16 (but it also has the diaphram shaft going through that, so the hole is really even effectively smaller than this) or .3125 (.077 sq in), and a carb's needle and seat opening is typically around .120 (.011 sq in) give or take, depending on what you have, and two of those give you .022 sq in.
Looking at the sizes, you'll see the the regulator only has .077 sq in, so if your hose going up to it is -6, -8, -10, or a sewer pipe, it still only has .077 sq in to flow fuel through at the regulator. So, what exactly is larger hose going up to it, going to do for you, since that .3125 dia hole is the smallest restriction up to that point? And after that, you only have a total of .022 sq in available at the needle valves when they are WIDE OPEN, which isn't even usually the case. So, you can see that it doesn't really take as much hose diameter as most people think.
The worst case would be that the regulator was wide open and the needle valves were wide open. So, in that condition, the smallest restriction would be that total of .022 going through the two needle valves. That -10 hose above, has over 3 times as much sq in area as the regulator hole, and it has over 11 times as much sq in area as the needle valves. That is HUGE overkill by any measure.
Then consider that a 3/8 or .375 (.110 sq in) hole in the bottom of a gas tank with 6 inches deep fuel in it, will gravity feed at the rate of 108 gal per hour. And a buddy's 890 hp 540 BBC, consumed fuel at a rate of 57 gallons per hour on the dyno. So, its easy to see that its not that hard to feed high performance motors the fuel they need, even when you consider G forces at launch for drag cars. As long as you can maintain 7 psi coming out of the regulator (of course for a carb setup), you're good to go. And it doesn't take much hose diameter to do that as you can see above by comparing the different sq in sizes.
Most people way over-do it. Its not wrong to do that, its just not needed. Any number of guys have reported running 10.0's at the drag strip with only 3/8 factory hard line, and a 14 psi, 130 gal per hour free flow rated mechanical fuel pump. As long as you have any reasonable hose size, along with reasonable pressure up to the regulator, its hard to go wrong. The numbers are what they are.
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1,277 Posts
Wow, that's very contrary to a lot of things I have read but does make a lot of sense. I'm surprised that nobody has said argued this yet. I don't have a clue about what is best and I'm going to watch this for the differing opinions.
Yes you did. You gained about 6 to 8 lbs.:yes:
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281 Posts
UDHarold has always been adamant that a 1/2 inch line is necessary with his cams. The way I see it, if I still have 6psi on the fuel guage at the finish line with my 3/8 line I'm OK. My engine simply can't use any more!
One thing I think is a good idea is to use a fuel log into the fuel bowls out of the regulator for extra reserve volume.
One thing I think is a good idea is to use a fuel log into the fuel bowls out of the regulator for extra reserve volume.
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32 Posts
One of the purposes of bigger lines ; pipe sizes etc in any fluid flow system is to limit the pressure drop or velocity in that line to improve characteristics in that system. ie the size opening in th regulator nor the needle seat opening in the carb should hav squat to do with line sizes. The line should be large enough between the regulator and the carb that the pressure drop in the line is negligible so the regulator takes all the control.
James
James
Peace of mind..lol
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1,594 Posts
X2... I will take the security of -8 stainless tubing and braided hose and threaded AN fittings over rubber hose and hose clamps any day thank you.
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5,366 Posts
I have -8 going from the mech pump to a filter, then to the holley # 704 regulator , then to the carb.
Would I be better off changing the lines from the reg to the carb to -6 to ?
The pump os a BG superspeedway 8.5/9 psi with the regulator turned down to 6.5 psi.
Would I be better off changing the lines from the reg to the carb to -6 to ?
The pump os a BG superspeedway 8.5/9 psi with the regulator turned down to 6.5 psi.
So, what is the answer, what is the safety part, why does UD Harold insist on half inch,what are the benefits? Bob W. said there was no gain so why is it better?
I,m asking because I am looking to do the same thing. Or is it the pump capacity that matters more.
I,m asking because I am looking to do the same thing. Or is it the pump capacity that matters more.
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182 Posts
I have always had 1/2 from regulator each side of the regulator right to the carb inlets. Done that for so long I forgot why. I just assumed better overall flow because it would not "bottleneck".
That was simply to have more fuel on board. If you're only allowed a specific size gas tank by rule your only option to increase fuel capacity is GIGANTIC fuel lines! Smokey found the "gray area" in the rules book
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