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fluid pressure question

M

mjsnodgrass

Member
if I were using a 1/2' hose at 7 lbs pressure, feeding a 3/8" hose at the carburetor, would the pressure at the end of the 3/8 hose be 7 lbs? I seem to remember something about this from school, but it's been a long time. thanks for the help
 

buzzin is exactly right. This is something that firefighters and plumbers have to learn and understand. At the flow rate that you will be using the gasoline, unless it is some 10,000 CID monster, you would need extremely sensitive pressure testing equipment in order to be able to measure any drop.
 
Pressure is only accomplished with resistance, so the pump may provide 7 PSI, but with an open end the whole line is most likely at 0 PSI. The pressure will vary depending on the restriction at the end, so it will be between 0 and 7 PSI depending on how open the restriction is.
 
So many variables...

The most influential factor is flow rate. At zero flow rate, and zero head (head is the gravity factor as in the difference in height from the tank to the carb), then yes, the pressure would be the same. But that is a useless number, because nothing is being done, just pressure sitting against a closed needle valve.

Once flow begins, then the end pressure will begin to drop. The more flow, the more drop. Caused by friction in the flow path, the more bends, turns, fittings (especially reducers) the more resistance, the more drop in pressure.

Reduced pressure=reduced flow.

If this is a mobile application (race car, aircraft, high speed marine) also factor g forces on the fuel.
 
I agree, we need a bit more info. If there is no consumption the pressure in a closed system is uniform (except for gravity if it really matters). In a flow, there is restriction at the change in diameter. .375 area is .44 sq. in. a .5 hose is .79 sq. in. So---- if the flow is gallons per minute the restriction will be very important and dramatic. If it is gallons per hour not an issue at all. Jim
 

No need to overthink this. 1/2 inch hose feeding 3/8" AT, the carburetor tells us that almost the whole line is 1/2" except for a small amount of 3/8" at the carburetor. We also know that 1/2 is a VERY , VERY, good sized gas line unless the engine is HUGE. Therefore we know that flow will be nowhere near great enough to generate significant friction loss in the 1/2 inch line. Therefore loss of pressure due to friction will be insignificant.
 

No need to overthink this. 1/2 inch hose feeding 3/8" AT, the carburetor tells us that almost the whole line is 1/2" except for a small amount of 3/8" at the carburetor. We also know that 1/2 is a VERY , VERY, good sized gas line unless the engine is HUGE. Therefore we know that flow will be nowhere near great enough to generate significant friction loss in the 1/2 inch line. Therefore loss of pressure due to friction will be insignificant.
 
wow. you guys are good. the subject came up as we are suspecting racecar was going lean on the top end. we speculated if we replaces the 3/8" line from the tank to engine compartment with 1/2" line to ensure we had enough flow to overcome the forces of acceleration. then the subject of fuel pressure on the carb floats came up. the 1/2" hose would be about 7 ft. long, but with quite a few bends. the 3/8" hose feeding the carbs is about 3 ft. total. we just trying to figure if the would be a difference in pressure. thanks for the conversation.
 
Somehow, this reminds me of the car builder of yesteryear who showed up to race with 2 inch fuel hose in his car. He said the hose held about 2 gallons of extra fuel. . . .
 
(quoted from post at 15:31:20 02/14/18) wow. you guys are good. the subject came up as we are suspecting racecar was going lean on the top end. we speculated if we replaces the 3/8" line from the tank to engine compartment with 1/2" line to ensure we had enough flow to overcome the forces of acceleration. then the subject of fuel pressure on the carb floats came up. the 1/2" hose would be about 7 ft. long, but with quite a few bends. the 3/8" hose feeding the carbs is about 3 ft. total. we just trying to figure if the would be a difference in pressure. thanks for the conversation.
Click to expand...

Fuel pump a mechanical on the engine pulling fuel from the tank? Or an electric pump at the tank pushing the fuel ?
 
it's electrical. pushed the fuel. we did the plumbing last month. the subject of fuel pressure came up as a afterthought.
 
12" line with 7# of pressure into a 1" line will still only yield 7# pressure, with no flow. How much flow is moving thru the pipe, and how big the pump is, will determine your pressure at the end of the 1" line.
 
(quoted from post at 13:40:26 02/15/18) 12" line with 7# of pressure into a 1" line will still only yield 7# pressure, with no flow. How much flow is moving thru the pipe, and how big the pump is, will determine your pressure at the end of the 1" line.
Click to expand...

And since we know that it is a car, we still know that the 3 feet of 3/8 will not measurably reduce pressure while flowing.
 
Gasoline has a viscosity of .6 CPS, so @ 2 GPM in 3/8 tube 3' long, you would get about 1/2 PSI of drop @ full flow.

This may be a little off, as I used water, which has a viscosity of 1, giving it .8 PSI drop.
 
By the way, the flow is turbulent at the rate, so you will have good heat exchange to the outside.
 
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