| Engineering question [message #138950] |
Sat, 13 August 2011 20:46  |
armandminnie
Messages: 864
Registered: May 2009
Location: Marana, AZ
Karma: 2
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Assuming no fuel vapor in the line, does it matter where in the fuel line I measure fuel pressure - at the pump or at the carburetor? I think that gasoline cannot be compressed so it should be the same at both ends. I have had some experienced mechanics tell me it matters but I don't think so.
Armand Minnie
Marana, AZ
'76 Eleganza II TZE166V103202
visit my gmc blog
click here to visit gmcws.org
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| Re: Engineering question [message #138956 is a reply to message #138950] |
Sat, 13 August 2011 21:11  |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
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| armandminnie wrote on Sat, 13 August 2011 21:46 |
Assuming no fuel vapor in the line, does it matter where in the fuel line I measure fuel pressure - at the pump or at the carburetor? I think that gasoline cannot be compressed so it should be the same at both ends. I have had some experienced mechanics tell me it matters but I don't think so.
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Armand,
Simple answer = No*
*Complete and technically correct answer follows:
At the flow rate that the system should see, there should not be any measurable head loss (pressure drop) in the fuel line between the pump discharge and the carburetor inlet.
There will be a very small head difference between the pump discharge and the carburetor inlet based on the vertical rise of the fuel line. That will vary with temperature as the density of the fuel does also. That decrease should be less than 1 psi, but 1 psi is pretty big to a conventional carburetor's needle and seat.
** If I bothered to measure the vertical rise, I could calculate that difference at a given temperature.
So, if you put in a pressure gauge to be a tell tale for fuel delivery problems, go ahead. The indication will reliable where ever you tap into that line. It will read slightly increasing pressure with increasing temperature as the mechanical pump creates a quasi-constant pressure but the weight of the fuel in the line will decrease. The variation from temperature will be in the range of 1/8 of a psi.
Matt
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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| Re: Engineering question [message #138959 is a reply to message #138956] |
Sat, 13 August 2011 21:27 |
armandminnie
Messages: 864
Registered: May 2009
Location: Marana, AZ
Karma: 2
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Actually, I installed the fuel pressure gauge because I had a fuel pump failure - I have only an electric pump and it failed with too high a pressure so I installed a fuel pressure regulator and a gauge to make sure I knew what the pressure was. This time the fuel pump was putting out less than 3 pounds so I replaced it. Hoping that fixes the problems I have been having on this trip - I'll give it a try tomorrow.
Armand Minnie
Marana, AZ
'76 Eleganza II TZE166V103202
visit my gmc blog
click here to visit gmcws.org
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| Re: [GMCnet] Engineering question [message #138980 is a reply to message #138967] |
Sat, 13 August 2011 22:45 |
armandminnie
Messages: 864
Registered: May 2009
Location: Marana, AZ
Karma: 2
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I have a Carter P4070 fuel pump and the first one that broke was putting out enough pressure to cause some flooding. This was diagnosed and fixed by Donny at Applied GMC. Was great for a long time until today. I have talked to two mechanics - JimK and to a local guy who just happened along as I was finishing the swap. He knew all about Carter fuel pumps and said the 455 could never run with less than 3 pounds at the pump. I'll find out if it is fixed tomorrow.
Armand Minnie
Marana, AZ
'76 Eleganza II TZE166V103202
visit my gmc blog
click here to visit gmcws.org
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| Re: Engineering question [message #139089 is a reply to message #138950] |
Sun, 14 August 2011 13:40 |
winter
Messages: 247
Registered: September 2007
Location: MPLS MN
Karma: 0
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I would want to measure as close to the carb as possible. You can assume there is no pressure drop or restriction in the line but I'd rather know than assume. A longer distance to the meter can also reduce any pressure pulses that a pump may put out.
Jerrod Winter
1977 Palm Beach
Green Jelly Bean
Twin Cities, Minnesota
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| Re: Engineering question [message #139106 is a reply to message #138950] |
Sun, 14 August 2011 14:54 |
winter
Messages: 247
Registered: September 2007
Location: MPLS MN
Karma: 0
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The OP has an electric pump instead of the mechanical pump. The distance to the pump and the kind of fuel line used is varible depending on how it is set up.
Jerrod Winter
1977 Palm Beach
Green Jelly Bean
Twin Cities, Minnesota
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