Ok guys, need a little help here. So many confusing things out there about what type of line, what type of fittings, do/dont use any amounts of rubber, etc.. My main question is fittings and adapters. I bought a 3/8" steel fuel line coil from the auto parts store. I have a re-produced factory tank, which is setup to accept to the 5/16" line using a 1/2-20 connector. I know that because I have a 5/16 brake line at home which has a 1/2-20 fitting on it, and it screws in perfectly. The threaded plug on the gas tank is a 1/2-20 flared fitting. I need to know what the best way is to run my new 3/8 hard line and connect it to the gas tank. For years I used a short 1 foot piece of rubber fuel line to go from hard line to tank fitting, but should I try to hard line this all the way to the tank. If so please tell me what to use to make the connections and fittings work. I dont have double flaring tool, or know how to do that yet, but I have a buddy that can do it just fine if thats the best way, just cant figure out what the right fittings would be to use. I know many of yall have used larger lines with factory tanks, do tell.
The stock tank in my 55 was like yours. The 5/16 tube goes in to a bulkhead sweat soldered into the tank. The restriction is the bulkhead ID. It would not p*** a 3/16 drill bit. I plugged it. On the top of the tank is a flat spot big enough to cut a hole in, then make a 1/8 plate to cover the hole. I brazed a fitting in the plate, then ran a 3/8 tube down in the tank curving to the rear, and wrapped a screen around the flattened end. I use eight sheet metal screws and a cork gasket to put the plate on. I used a 90 degree fitting on top, and ran a 3/8 line down to a fuel pump mounted under the back seat. The pump can be used to prime after a two week shut down, and as a boost pump when racing. I use a switch and relay to run the pump. The motor has never run short of fuel with the 3/8 line. It ran out of fuel in third gear at the HAMB drags one year.
I've run an electric fuel pump located on the frame very close to the tank for years utilizing a section of rubber fuel line between the tank and pump. I think I originally did that in the mid-80's. Kept the same setup when I installed a new tank earlier this year and changed to a new piece of rubber fuel line. I don't see any issue using a section of rubber fuel line to connect to your steel line.
I stopped by napa and found a conversion fitting that screws into the tank fitting, and has a larger female end allowing a 3/8 line fitting to screw into it. I may go that route, it will allow me to run hard line tank to pump. However, if I run into issues I am not scared to resort to the short rubber line and a hose barb end fitting. I just read somewhere that this ethanol gas is going to eat it faster. True? I am like Don though, I am worried that the ID of the tank fitting is still going to create a bottle neck, but I may try it like it is before I go wild trying to fix that. If it begs me for more gas I will try something different.
The time to make that fitting bigger is now before there is ever any gas in the tank. You are running a 347, my 320 used more fuel than the bulkhead fitting could flow. Cut it out and make a bigger one.
If you do run rubber hoses, it's best to use the "fuel injection" hose that NAPA sells. It is E10 proof. Make the hoses as short as possible. Best to run a "flex" line between the frame/body to the fuel pump as the engine moves and will break a hard line. There are many flaring tols out there... I use a hydraulic pump style that delivers perfect 45 degree double flares everytime. Very easy to use tool..... a bit expensive but you and all your friends will be happy you have it!!!!
I ran my fuel line earlier this year and I am using the same tank you are. I was also wondering the same thing. I bought 3/8" line which runs along the frame rail, but the line coming out of the tank is 5/16 due to the thread on the tank. So to me that seems like it would be a bottleneck. Also, my 302 has the stock fuel pump, which also has the fitting for a 5/16 output line. So the line from my pump to the carb is also 5/16, which seems like it would defeat the purpose of having 3/8 line at all. So I think I just need to rip It all out and start over. Should I just run 5/16" line all the way? Or should I try to change the fittings on the tank and the pump somehow to fit 3/8" lines? It's a pretty mild motor, and I'm very confused. It's surprising to me that a 1/16" difference in line size would make such a big difference. Hopefully I'm not hijacking the thread too much. Regarding your other question, I believe it's normal to have a soft line between the line on the tank and the line on the frame. Also I think you should have a soft line between the engine and the frame to allow for some movement there.
Think of your fuel line like a garden hose,a 1/2" or 5/8" will actually make more pressure than a 3/4",for the same reason a high pressure spray nozzle has a smaller outlet and that is to increase pressure.Unless you have a full blown race car that requires a higher volume of fuel 1/16" is not going to make a difference so the 5/16" to 3/8" adapter is not a problem,run the 3/8" all the way to the fuel pump and Carb from the tank.
On the suction side of your fuel pump, the "PRESSURE" is atmsopheric - the vacuum created by the pump is just about the only force available to make the fuel move forward. If it could pull a perfect vacuum - that force would be 14.7 pounds per square inch (at sea level) and less as you climb. That suction is way less effective than the pressure coming out the other side of the pump (which is actually 14.7 pounds plus your gage reading) - so the suction side is a lot more sensitive to any obstructions. As Jeff mentions, the flow restriction of a 3/4 water hose - is noticeably less than a 5/8, even when hooked to the same valve. In the same manner, the 3/8 tubing is about 2/3 of the flow restriction of the 5/16. So you could expect - at the least - that the flow in that portion of the line would increase by a third at the same suction. Because it will be flowing slower by a third - the actual restriction is even less than that (probably doubles the advantage of the bigger pipe). On the pressure side, smaller lines work because (unless you are feeding some dual carb elephant) - that line is typically feeding a tiny "needle and seat" that is way smaller than the feed tubing. A typical Holley .097 needle orfice has an opening of .0024 square inches - the 5/6 line has an opening of.077 square inches (33:1 ratio!) - and pressure on both is the same. Its easy to see the "overkill" available on a daily driver sort of motor with an ordinary feed system.
Greenbird, I think that is a great explanation, but I think I need to go back to school to understand some of it lol. I see the point though. Also I checked last night and my mechanical fuel pump is not 3/8 inlet, its smaller. So in order for a 100% 3/8 setup I would have to redo the tank, buy a new fuel pump with 3/8 in/out and run 3/8 line. And there is the needle and seat size you mentioned also. I am running a custom built Holley HP 4150 with .120. Pro Systems carb from here You just read on the internet, 5/16" line is good to **HP and above that run 3/8, etc.. Also another reason I wanted to upgrade is because my engine has a stumble from idle to wide open throttle. It hesitates for about 2 seconds and then plants you in your seat, and never does it again. You can drive normally without this hesitation but from a rolling start to nailing it, it will hesitate as if its starved, then just make you smile. So I was ***uming this is fuel starvation. Now on the other hand, the gas tank I was using was OLD...a gas tank from another car that was cut and modified to work back in the 90's (by someone else). The fuel line on there now is not factory and I measured the inside diameter of it and its smaller than the ID of a 5/16 line. So hopefully if I run the 3/8 line and leave the tank and pump fittings at 5/16 it will feed my hungry stroker motor. I just hope I dont have to fix it later, because its much easier (and safer) to do it now, should it need to be done.
I'm running the .120's in my Demon too - they were the smallest cheapest steel needle and seat ***emblies I could get my hands on - .0113 square inches area. Thats a lot more than the common .097's for sure. It takes about 1/2 pound of fuel per horsepower per hour (usually seen on a dyno sheet). So a 300 HP engine would need 150 pounds per hour divided by 60 minutes equals 2.5 pounds per minute. Gas is roughly 6 pounds per gallon so we're talking about .42 gallons per minute (at 300 HP). You see the point - its pretty easy to pump out a gallon of gas from a 5/16 line in a solid minute of steady running......pulling that amount out of the tank by suction - is a whole different ballgame. The "floored it" stumble you speak of - isn't so easy to solve as the bulk delivery.
Todd, could the stumble be because the carb is cold? This time of year, most engines run better with a heated intake manifold. I use heat on the Y block all the time when I'm not racing. It runs fine with no stumbles. You also could check the accelerator pump stroke.
So you're saying that maybe it would be better for me to just have 5/16" line all the way through? From what I understand, it would deliver enough volume, and better pressure than a line which is part 5/16" and part 3/8"
I have 5/16" line from the tank to the pump and then 3/8" from the pump to the carburetor. What I gather from Greenbird's post is that this setup will be okay because the larger size line is on the pressure side of the pump. The fuel pump should be able to move more volume but at the same pressure as the suction side. The flow rate on the pump side should then be about the same as that on the suction side.
Todd- I think your stumble is carb calibration related, not fuel delivery related. Fuel delivery issues typically manifest themselves after extended time under load: your engine doesn't require the max possible flow just cruising around, only when running through the gears, spending a lot of time at WOT..
Yea I never could figure out the stumble. I havent driven the car in over 2 years (you know paintjob BS). However at first I had a BG Speed Demon 650 carb with a victor jr intake and it stumbled. Then I swapped to a custom built carb for my engine (HP 750 dbl pumper) and an air gap intake. Same stumble. I have tried 3 different jet size combos and no change. I have adjusted the 4 corners every way possible, no change. I chocked it up as a fuel issue because I had a small line, hot motor, 30 year old gas tank, etc.. Although I did have the gas tank "boiled out" before running the new motor. It is strange though Matt as you mention...I only had the stumble when I punched it. I honestly cant remember now since its been so long if it did that at WOT through every gear or just the first punch. It would make more sense if it is happening at every initial WOT punch, but now I just cant remember. Oh, Don, when I was driving it (not recently) it was in the summer. And even when I drove it around 15 miles and it was up to temp, it would always respond the same. So, to be clear, the situation I have right now is: Are yall saying I should roll with this setup and not be concerned about fuel delivery? Tank fitting 5/16" Fuel pump inlet 5/16" ? hose barb style fitting Fuel pump outlet to carb 1/2-20 inverted flare fitting 3/8" steel line in between This is the pump http://www.summitracing.com/parts/crt-m6588
If you have a stumble on tip in, its an accelerator pump issue. My car ran out of fuel after a short burn out a fast line up and half way through third gear it said NO MORE! Your carb may be set up for an over the counter engine, and yours is not. If its a Holley, buy an ***ortment of accelerator pump cams.
Don, someone on the main board (a while ago when it was happening) mentioned pump cams also. I never changed any and tried it, once I get the new fuel system installed I will try that if the stumble is still there.
I think that rundown is good enough for the horses you have to feed... If its a Holley - then I hope you've been through the business of setting up this transfer slot on the primary throttle. If it needs more fuel to keep running - adjust the secondaries leave the front ****erflies like the picture suggests. On some carbs - the stumble will come and go with an adjustment to the front bowl level. Try raising that a bit. Find out which "shooter" size is in the primary - its supposed to cover the "hole" when you floor it. The accelerator pump is a fixed size - the shooter orfice determines how fast it empties into the air-stream.
As mentioned above- pump cam changes would change the total volume of fuel available to the acc. Circuit, I think your stumble, since it is at initial tip-in, is shooter related... My comments about fuel delivery and WOT were regarding diagnosing your stumble. My opinion on using the stock tank is that the OEM tank fitting will be a bottleneck if you are racing or spending a lot of time on the gas- you'll empty the carb's float bowls bc the pump and lines can't keep up....
I found the carb specs from when I purchased it. They use you engine specs and custom build/calibrate a carb for it. Here is my engine specs and custom build carb specs. Could I have said specs any more? good grief Engine and Cam CARB
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