Fuel pressure
- Thread starter Polara383500
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IMO....
BE VERY CAREFUL when installing a fuel pressure gauge in your carb's fuel supply line, especially in an under hood location. Because the stock pump (Carter or equilivant) is a positive displacement pump that introduces severe discharge pulsations that will damage (rupture) the bourdon tube, the pressure sensing element in many gauges, causing an INSTANTANEOUS leak. In the innitial pixs, this would place the gauge very close proximity to the hot exhaust manifold....also using rubber fuel hoses in lieu of steel lines, is another potential leak source. The fix for a fuel pressure gauge ptoblem is to use a diaphragm isolated gauge or one equipped with internal pulsation dampener. Another alternative is to use an indirect reading gauge, one that displays pressure remotely. HOW is the gauge shown in the pixes, any benefit if it's under the hood? How can the fuel pressure be determined, under max demand, if it, the gauge, cannot be observed?
Again, perhaps if a fuel starvation problem is suspected, it's a volume not a pressure issue. Just my opinion of course.
BOB RENTON
BSB67
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Its not a positive displacement pump. Pressure to the carb is supplied by a spring actuated diaphram. The spring dictates the max pressure. The nature of the pumps design will cause plusing.
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The reason for the pulsating output is because it is a positive displacement pump....vs a centrifugal pump which is relatively smooth output. The internal spring determines the ultimate pressure but flexing the diaphragm by the actuating arm actuated by the plunger or operating arm against the cam's lobe makes the design positive displacement....spring pressure combined with total diaphragm movement (area) determines both volume and pressure parameters....the output is anything but smooth.
BOB RENTON
BSB67
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No.
Just because a pump is not a centrifugal pump does not make it a positive displacement pump. A positive displacement pump discharges a fixed volume per revolution or per stroke.
lewtot184
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i bought a new stock replacement hipo 440 pump for my '69 coronet stocker. bench tested it and it was 16psi!!!, not the 8-9 it should have been. basically worthless. put a carter 6903 on it and forget about it. those aftermarket korean made pumps are junk.
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We'll just have to disagree.....in this case, the pump discharges a fixed volume per pump stroke, or partial stroke, depending if the carb or downstream device, can accept the pump's discharge, there by making it a positive displacement device.
BOB RENTON
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To BSP67 and to further the discussion about whether the fuel pump is a positive displacement pump, i offer the following:
The two most widely used fuel feed pumps are diaphragm and plunger-type mechanical pumps. Diaphragm pumps are a type of positive displacement pump. Diaphragm pumps contain a pump chamber whose volume is increased or decreased by the flexing of a flexible diaphragm, similar to the action of a piston pump.
And
Positive displacement pumps, which lift a given volume for each cycle of operation, can be divided into two main classes, reciprocating and rotary. Reciprocating pumps include piston, plunger, and diaphragm types; rotary pumps include gear, lobe, screw, vane, and cam pumps.
Both descriptions apply to the standard automotive, engine mounted and driven fuel pumps. In tank electric pumps are usually a vane pump design but the old Stewart-Warner 240A was a plunger operated design, ehich could be heard ticking when energized. Just thought you might like to know.....
BOB RENTON
The two most widely used fuel feed pumps are diaphragm and plunger-type mechanical pumps. Diaphragm pumps are a type of positive displacement pump. Diaphragm pumps contain a pump chamber whose volume is increased or decreased by the flexing of a flexible diaphragm, similar to the action of a piston pump.
And
Positive displacement pumps, which lift a given volume for each cycle of operation, can be divided into two main classes, reciprocating and rotary. Reciprocating pumps include piston, plunger, and diaphragm types; rotary pumps include gear, lobe, screw, vane, and cam pumps.
Both descriptions apply to the standard automotive, engine mounted and driven fuel pumps. In tank electric pumps are usually a vane pump design but the old Stewart-Warner 240A was a plunger operated design, ehich could be heard ticking when energized. Just thought you might like to know.....
BOB RENTON
BSB67
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From your quote:
Positive displacement pumps, which lift a given volume for each cycle of operation, can be divided into two main classes, reciprocating and rotary. Reciprocating pumps include piston, plunger, and diaphragm types; rotary pumps include gear, lobe, screw, vane, and cam pumps.
"A positive displacement pump produces the same flow at a given speed (in revolutions per minute--RPM) no matter what the discharge pressure."
You are confusing types of pumps with pump function. "Positive displacement" is the term used to define the function of the pump. Most diaphragm pumps are positive displacement. However, the fuel pumps we are discussing are not, obviously. Otherwise the pressure would continue to rise with engine speed. Instead, our fuel pumps deliver only the needed voulme at a relatively constant pressure irrespective of the rpm or drive speed. Based on its function, the fuel pump is not positive displacement.
Just thought you might like to know.....
Positive displacement pumps, which lift a given volume for each cycle of operation, can be divided into two main classes, reciprocating and rotary. Reciprocating pumps include piston, plunger, and diaphragm types; rotary pumps include gear, lobe, screw, vane, and cam pumps.
"A positive displacement pump produces the same flow at a given speed (in revolutions per minute--RPM) no matter what the discharge pressure."
You are confusing types of pumps with pump function. "Positive displacement" is the term used to define the function of the pump. Most diaphragm pumps are positive displacement. However, the fuel pumps we are discussing are not, obviously. Otherwise the pressure would continue to rise with engine speed. Instead, our fuel pumps deliver only the needed voulme at a relatively constant pressure irrespective of the rpm or drive speed. Based on its function, the fuel pump is not positive displacement.
Just thought you might like to know.....
Curious what the pressure was on your new 6903? The last 3 that I bought were 10-12 psi. That was last summer, maybe they addressed the problem?
lewtot184
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i've had 5 of them thru the decades; all were 5-6psi. some years back a friend put one on his 440, using one of those small pressure gauges by the carb, and it read 10psi running. i told him to change the pump thinking he had a 4862 on it. long story short he had the correct pump just a goofy gauge. my '69 r/t has a 6903 on it that i bought in either 1985 or 1992, don't remember which; still going. my '65 coronet has one on it installed in 2003; still going, even with mods. they're my go to pump and they're the spares i keep if ever i need a pump. both of my cars use carter carbs.
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As I mentioned previously.....we'll just have to agree to disagree. Interpretation of the method is subject to one's total understanding or lack of it. BTW...are you a pump designer? Just curious....
What happens to the delivery pressure IF, the volume goes to zero with the engine ideling or during a coast down from a high speed endeavour.....does the diaphragm continue to flex or does the overtravel spring allows the pump arm to move without flexing the pumping diaphragm against the main spring? Remember that a liquid cannot be compressed, so what is occuring?
BOB RENTON
BSB67
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This is the source of your confusion. Maybe take apart a pump.
The engine actuates the pump arm, which does one thing, suck the fuel from the tank. The spring, and only the spring, pushes the fuel to the carb. This is why at idle, when nearly no meaningful fuel is used (i.e. near zero volume), the pressure does not skyrocket.
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This is how I described the spare pump for my 440 engine operates: The main or pressure spring will operate or push the diaphragm, when and if, the down stream or exit pressure will allow fuel to flow. However, the pump's actuating arm will continue to operate, driven by the push rod and cam to the extent determined by cam profile and the pump's arm or leverage ratio or in the case of the LA engine, the cam lobe and pump arm directly. In the case of fuel demand, the diaphragm will flex, driven by the main spring only as much as required (positive displacement) to maintain a constant volume and pressure limited to displaced volume of the diaphragm and the pressure exerted by the main or pressure spring. Perhaps this can best described as a variable positive displacement pump, subject to the speed of the of the pump's actuating arm or mechanism and volume requirements. Just my opinion of course.
BOB RENTON
BSB67
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I do understand you really want to be right.
To the OP and everyone else, I appoligize for the off topic side track.
Daves69
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If it helps ya', I just put one on in line with a Holley blue with cheap gauge. Not that the gauge is calibrated and/or certified by the bureau of weights and measures, but this is what the M6903 by itself is showing....
After about 5 mins of idle time............
Great, they must have got the issue resolved. Good to know.
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