fuel outlet size

[insomniacshotrods]

Hello! I am finally getting ready to put the new gas tank i got from RPM industries in . Great tank, only problem is it has 3/8 outlet and i ordered 1/2. Ive got a 6-71 350 with 2 650 carbs and -8 fuel line. Can anyone tell me if 3/8 is too small? Should i cut it out and weld in a 1/2 ? Is it worth it? I have a holley blue pump also if that has anything to do with it. Like i said great tank, Rons the man just a misunderstanding ..any input would be appreciated . Thanks

 

[fordsbyjay]

Hello! I am finally getting ready to put the new gas tank i got from RPM industries in . Great tank, only problem is it has 3/8 outlet and i ordered 1/2. Ive got a 6-71 350 with 2 650 carbs and -8 fuel line. Can anyone tell me if 3/8 is too small? Should i cut it out and weld in a 1/2 ? Is it worth it? I have a holley blue pump also if that has anything to do with it. Like i said great tank, Rons the man just a misunderstanding ..any input would be appreciated . Thanks

When you say 3/8 do you mean 3/8 pipe thread because 3/8 pipe thread is usually about 1/2"? Most weld on bungs I have seen are usually a pipe thread. Now IIRC 1/4 NPT is usually around 3/8" and I yes I would think that is too small for your application if that is the case. I am a firm believer of having a larger feed line to the pump to make sure it gets all the fuel it needs.

 

[RPM]

I sent you a PM

 

[Mike]

The thing everyone overlooks is the entire fuel system can only flow as much as the smallest orifice in the system will allow.

3/8 NPT is the same thread the Holley blue pump has. 3/8 NPT is the same thread as the regulator shipped with the Holley blue pump.

Now, take the regulator apart and look at the size of the restriction fuel passes through. 0.21875" is all you get. Suddenly, those 3/8 NPT fittings seem H-U-G-E, don't they? :foottap:

 

[insomniacshotrods]

Mike , never thought about that factor. I was more concerened about putting undo strain on the pump with the smaller size outlet,but the inlet to the pump are also 3/8 to -8 so it wont make a difference either way but i did spend 12 bucks on a fitting for the 1/2 to -8 lol. Ron said he will send me a 1/2 bung so ill weld that in and get it all powdercoated and installed.. Thanks for all replies and thanks again Ron ,your tanks are the best .

 

[Ted Brown]

Mike , never thought about that factor. I was more concerened about putting undo strain on the pump with the smaller size outlet,but the inlet to the pump are also 3/8 to -8 so it wont make a difference either way but i did spend 12 bucks on a fitting for the 1/2 to -8 lol. Ron said he will send me a 1/2 bung so ill weld that in and get it all powdercoated and installed.. Thanks for all replies and thanks again Ron ,your tanks are the best .

I myself would keep the fuel tank as you got it from RPM, He knows what he is doing, and you welding anything to that tank will stop all his liability or making good on his work... That will supply more fuel than you will ever need... :foottap:

 

[ellis8500]

Did some calculations. At 100 MPH, you would be needing about 90 - 100 FWHp. This will require about 8 gal/hr in a liquid cooled engine running the proper air:fuel ratio. This then equates to about 17 ounces of fuel per minute...about 1 pint. 3/8" schedule 40 pipe has an ID of 0.493". This size hole will easily flow that volume of fuel.

 

[insomniacshotrods]

Mr Bill, does that include a blower motor? This is my first one, but im sure it uses more fuel than a naturally aspirated engine,not sure how much though. Im gonna try tank as is and see it might work. If not i can always pull it out and redo the darn thing. Thanks again for help guys.

 

[ellis8500]

Fuel consumption is based on engine power output and fuel/air ratio. Doesn't matter if its carbed, injected, blown or turbo-charged. Depending on fuel/air ratio, gasoline engines will burn from 0.42 - 0.50 lbs per hour per HP. So at 100 FWHP one would burn from 42 - 50 lbs of gasoline per hour [7 - 8.33 gal]. Engine HP will be dependent upon engine RPM and load. The aerodynamic and rolling drag of a t-bucket requires about 100 HP at 100 MPH [maybe more/less depending on total weight, tires, driving conditions]. But you get the point. A cruising speed of 65 MPH only requires about 30 FWHP.

Hard acceleration is a different story. In this case you will be running the engine at near full HP and a rich mixture; maybe 0.55 lbs/HR/HP. So if you have a 400 HP engine you might burn as much as 36.7 gal/hr or 78 Oz/Min during the acceleration. At this flow rate, your plumbing needs to be able to flow about 5 pints per minute. You can use these values to calculate your own needs based on your engine.

 

[blownt]

even with 5/16" fuel line I betcha you will not run dry on a street motor, blown or not. My blower motor is 12% overdriven with two 650HP double pumpers. I had 1/2" SS tubing on the inside of my frame rail and then up to the carb lines which are 3/8" BDS bent jobbies. But if you look inside a 1/2" AN fitting it is not a true 1/2". On one of my previous rebuilds I replace the line under the body to 3/8" and have never starved the motor. Don't forget you have primary and secondary fuel bowls and I doubt ON THE STREET you will be emptying the fuel bowls faster than what is being pumped in from the tank and pump. Now a racecar and motor is another matter.

 

[Mike]

It is sometimes a hard principle for people to understand, but a 350 cubic inch motor is only capable of flowing a certain amount of air. At 7,000 RPM, that number is 709 CFM.

Now you're running forced induction, but I seriously doubt you're running any serious overdrive, so I am willing to wager 735 - 740 CFM would be a more than fair number.

And mind, that number is a perfect-world number. Now you have to bring air quality into play, because the grains of water in the air will determine how much fuel those 709 CFM are capable of carrying. Why do you think racers carry all the sophisticated weather instruments and altimeters around with them?

When I was with the race team, one of my responsibilities was maintaining fuel level in the car. I used fuel for weight in the car, so I had to be aware of how fast the car would need to run for the next round and how heavy I could afford the car to be. The car made percentages of horsepower about .1 - .2 horsepower behind a 500 CID Pro Stock car, so the cylinder heads were extremely efficient. The burnout, backing up, staging and pass itself would burn between 16 and 18 oz. of fuel, depending on how long we would have to wait on the car in the other lane. To be fair, let's add more volume for a V-8 and you're still under 25 oz. We ran .500" hardline from the tank back through the driver's compartment to the bulkhead and then -8 hose (.440") to the regulator on the block plate. From there to the float bowls was -6 hose (.340"). Shift points were very high :wow:, as was finish-line RPM and the car's current MPH record is 185.77 MPH.

When people are telling you .375" lines/hoses are a God's plenty for what you're doing, they're absolutely correct.

People think they need to be jetting carbs all the time, as well. If we went from running Indy on Labor Day weekend to running Houston in mid-October, I was ~probably~ going to add one number of jet. We're talking conditions that are generally around 3,300 feet of corrected altitude to -50 feet of corrected altitude. One number, probably. From there, I might end up taking 1/2° timing out.

Don't let yourself get sucked in by trick-of-the-week numbers. Do the math and make things right the first time. 9 times out of 10, the car will run better and you'll leave money in your wallet to boot! :winkn: