Can some one explain this to me...

[RedZ85]

People are talking about putting in bigger fuel lines...I can see the need if you have a monster under the hood. For us running a mostly mild Chevy 350, whats the point? i have a Mallory Comp 110 fuel pump that will be going to a return style regulator (also mallory) all using the STOCK 5/16" fuel line and 3/16" return line. The whole point of a return line is to constantly keep fuel flowing through the pump. The engine is using some fuel...why does your return line need to be the same size as the fuel line like people are saying? The stock setup from the factory didnt have same sizes. Also, the amount of fuel that pours out or the fuel line if its not connected to the carb once the pump is on is way more than my engine is going to use at WOT. Whats the advantage of going to a larger 3/8" line? Finally, if i were to go to a larger 3/8" line, whats the point being that the syphin tube inside the tank is only 5/16"? so unless that tube is removed and replaced with a 3/8" or larger syphin theres no change. Am i correct in thinking that the fuel flow from a tube 5/16" in the beginning to 3/8" at the end will be EQUAL to the flow of a tube thats 5/16" all the way? Input please!

[grumpyvette]

"Am i correct in thinking that the fuel flow from a tube 5/16" in the beginning to 3/8" at the end will be EQUAL to the flow of a tube thats 5/16" all the way?"

 

yes and NO,a very short section of 5/16" line feeding a larger 3/8" line has less restriction than a much longer 5/16" line

yes the 5/16" feed line in the tank that goes to a larger 3/8" line will limit the flow potential of the 3/8" line, theres no question if thats your only source of fuel flow to the pump, and your also correct that a 5/16" fuel line has the ability to feed a MILD 350 engines fuel flow needs.

now the reason the return line to the tank needs to be about the same size as the feed line is that the idea of the return line is to provide a nearly zero restriction pressure route back to the tank, to allow the pump to work more efficiently without fighting a back pressure & restriction.

now obviously the ideal setup will have a fuel line size and pump thats easily able to exceed the engines fuel flow requirements by a few percentage points under the hardest accelleration, and high heat conditions and taking into account the restrictions the filters,fittings,regulators,ETC. provide, so it should be obvious that if your engine can in theory use lets say 50-60 gph that youll want a pump AND FUEL SYSTEM flow capacity that is at about the 80 gph or higher range.

now ,

HOW to CHEAT slightly, yeah its a rube goldberg patch to add flow without the correct tank/line mods

keep in mind that the engines useing (X) amount of fuel, the pump and system can provide that (X) voluum only if the 5/16" feed can flow that (X) voluum but your trying to have a slightly higher voluum circulating thru the system to cool the pump and allow the bye-pass style fuel pressure regulator to work correctly,lets say for the example that a 3/8" line has a 18% larger flow voluum (which is about correct) and your engine requires 90% of the capacity the 5/16" line theoretically flows, well it should be obvious that the 5/16" feed line will barely keep up, and the return line will see very low flow rates at max rpms under accelleration (G) loads.

BUT, if we were to instal a 3/8" (T) fitting feeding the return line into the feed line after the fuel filter but before the pump in port we will in effect have that 18% greater flow added to the 10% the 5/16" line can provide, to allow the extra circulating fuel voluum past the pump and regulator.

now this is a comon mod and used frequently by guys who don,t care to do the job CORRECTLY, by installing a 3/8" or 1/2" line and pick up from tank pickup point to tank return point.

your still limited by the 5/16" feed line but the engine "SEES" a higher flow and in a marginal system it helps. but ONLY IF THE 5/16" line can FLOW enought at max flow levels to feed the engines needs

IDEAL.HELL NO! but its an option if you for some reason don,t choose to do it correctly

 

ideal

 

 

 

Rube Goldberg Patch

[Alf]

just this past weekend i dropped my tank and removed the factory in tank electric pump in my z31 becuase it couldn't keep up with my engine. i made a new sump out of 3/8 line but there is about a six inch section of 5/16 line thats part of the sump. i'm still running with the stock line, 5/16 i think. i was wandering the same thing Redz85 was. om my system i am now using a mechanical pump with a return, is this a good setup? should i have just gone with a mechanical dead head setup? or will the pump just not return if it doesn't need to?

 

 

here is my modified sump

 

[grumpyvette]

a RETURN style fuel pressure regulator system with dual lines (feed & return) will be superior in almost every application, your NOT stuck with the factory parts or sizes, there are readily available fittings that can replace the factory pick-up lines/parts.

and theres nothing preventing you from running DUAL pick-up tubes/filter socks, feeding into a (Y) a quick trip to a salvage yard and an hours time should allow you to collect the necessary parts cheaply, then a hole drilled and some minor tig welding will allow you to have a custom setup with dual feed tubes, OR theres always the option of installing a aftermarket sump or a fuel cell that has those larger line sizes already designed in!

 

http://www.magnafuel.com/products/accessories/fittings/index.htm

 

http://www.jegs.com/webapp/wcs/stores/servlet/CategoryDisplay?lang=-1&catalogId=10002&storeId=10001&categoryId=16651

 

 

http://www.jegs.com/webapp/wcs/stores/servlet/CategoryDisplay?lang=-1&catalogId=10002&storeId=10001&categoryId=25757

 

http://www.jegs.com/webapp/wcs/stores/servlet/CategoryDisplay?catalogId=10002&storeId=10001&categoryId=19797&parentCategoryId=10297&langId=-1

 

 

many times its faster and cheaper in the long run to just buy and install a fuel cell designed for the higher flow rates rather than modify the stock parts

[silicone boy]

Grump just amazes me with his posts. Very thorough. You should have your own show.

[Z-8]

I second that motion. Or at least one of those call-in talk radio shows.

[jap tin]

With my first Z I ran it in the 10's with the stock fuel line and a Mallory 110 pump. Put on larger fuel line , no faster. Just some real world results, no book learning.

[Michael]

Shouldn’t this be in the “fuel delivery†forum?

 

Now that I’ve annoyed the original poster, here’s a related question… With electric fuel pumps typically one uses a pressure regulator and a return line, for example as in Grumpy’s diagrams in this thread, or in http://forums.hybridz.org/showthread.php?t=108825 . And most people end up with an electric pump, either because they’re adapting a stock fuel tank with an internal electric pump, or are using JTR-type engine mounts which interfere with the stock Chevy mechanical fuel pump location.

 

In my application there is no interference between the engine mounts and the stock fuel pump. (454 BBC mounted to the frame rails). I plan on using a Carter “street†mechanical fuel pump, rated at something like 110 gph at 6 psi, going into a 4160-style 750 cfm dual-feed, single power valve, vacuum-secondary Holley.

 

This should be plenty, because, well, let’s do the calculations… a 0.030-over 454 at 6000 rpm at 90% volumetric efficiency (optimistic) uses about 43,000 ft^3 of air per hour, which at standard sea level is about 100 slugs of air, or around 3200 lbm of air (per hour, not per minute). Assuming 12.5:1 mixture and fuel density of 6.3 lbm/gal (should be close), this gives just over 40 gph fuel flow requirement, if fuel were being burned at a steady rate. In an actual fuel flow path, the various pressure drops might be such that to maintain continuity, the requisite pressure rise across the pump at equilibrium is greater than 6 psi, in which case the actual max fuel flow rate will be less than 110 gph (there’s a nearly inverse relationship between volumetric flow rate and pressure rise). But I’m still fairly confident that the 110 gph at 6 psi pump should be sufficient. I’m using a 3/8†fuel feed line.

 

Now here’s my question: (1) with this combo, do I need a fuel pressure regulator at all? (2) I have a fuel cell with a built-in sump, but don’t plan on using a return line. Do I need a return line, after all? This thread: http://forums.hybridz.org/showthread.php?t=106679 suggests that a return line is always a good idea, for heat-soak reasons.

[grumpyvette]

"I’m still fairly confident that the 110 gph at 6 psi pump should be sufficient. I’m using a 3/8” fuel feed line."

 

 

everything you stated is close to correct in a real world example ,at least in theory.

BUT ID point out that in almost every case youll find the restrictions to flow in the fuel lines due to both the filter and the smaller internal passages in the various fittings are a far greater restriction than you would at first suspect they might be.

that 110 gph at 6 psi flow rate measured at the carb just standing still , drops off fast , when measured under hard accelleration, high heat in the pump and from the engine compartment and once you factor in a filter that is not totally clear of trash

now as an example

I measured my old corvettes fuel system flow into a 5 gallon bucket, against zero restriction , (at least in theory)by allowing it to pump untill it filled the bucket and carefully timing the result.

 

I was using this pump (the advertizing suggests it exceeds 350 gph free flow)

and TWO of these regulators

useing a 12 gallon fuel cell for a tank, with the proper 1/2" an fittings and 1/2" INTERNAL SIZE FUEL LINES, and AN FITTINGS, the set up took 2 minutes and 13 seconds to fill the bucket, thats a flow rate STANDING STILL OF ABOUT 140GPH,due to restrictions, so don,t assume that the pump rateing is your true flow, and yeah before you ask it held a rock steady 5 psi at the twin dominator carbs on the 496 bbc, the main restriction was the filters,(I had two) its really hard to find decent low restriction fuel filters. I had to use two marine fuel filters that were the size of most oil filters with a removeable filter element that was the size of a can of coke

 

why did I need to go to that extreme, well every other combo I tried tended to show signs of fuel pressure dropping off , and the vette was only running very low 10 second times at 135mph, keep in mind all that feeds up too the (4) float pindle valves that control the fuel flow into the carbs , that are less than 1/4" in dia.