Wow! What a lot to digest
This is a great thread. TTT for those of us who haven't read it ALL the way through.
I wanted to clarify something re: pressure vs flow. Some are saying that higher pressure= more flow. Other are saying that higher pressure= less flow. They seem to be arguing against each other.
Truth is, BOTH are true, and I am going to explain HOW this can be. First, we have to understand that pressure can be STATIC or DYNAMIC, while flow is ONLY DYNAMIC. If we have no motion (static) then we by definition have no flow.
If you have 35 psi in your tires, then pressure is static. If you are filling a tire with a nail in it, and the two cancel each other out to maintain 35psi in the tire, then this is a TOTALLY DIFFERENT case! Now you are dealing with DYNAMIC pressure.
Pressure cannot form without restriction, I. E. combustion pressure only builds when both intake and exhaust valves are closed.
That bears repeating:
***pressure cannot form without restriction***
This is because of the laws of physics. "nature abhors a vacuum". Pressure differentials ALWAYS tend to equalize themselves. You puncture a spray can, gases will escape until the pressure IN the can equals the pressure OUTSIDE the can.
Thus, the only way we can build pressure is with something to stop nature from taking its course- a container of some kind. This could be a cylinder or a line, a tank, anything that will seal.
[[The perfect analogy is electricity. Current (flow) is dynamic, while voltage (pressure) is static. If we have a simple circuit of a resistor and battery, we can measure the voltage drop ACROSS the resistor and find the current that is running through it. In other words, we can take a know pressure and a known restriction and calculate the FLOW going through it. Ever wonder why you can measure voltage (pressure) in parallel, but you HAVE to measure current (flow) in SERIES- that is, make it a part of the circuit?]]
Let's apply this to the VP44. When you step on the pedal, the fuel demand of the VP goes up (more fuel, more rpm, etc). Thus, the fuel in the line has some place to go, and the "restriction" to being pumped is less. Since we have to have restriction to have pressure, the less of one we have, the less of the other. So when the restriction at the VP goes down, the fuel pressure goes down as well. In this case, less restriction caused the outflow (of the fuel supply to the VP) to increase and therefore a pressure drop. Let's be clear on cause and effect: the lower restriction caused the higher flow and lower pressure. The lower pressure did NOT "cause" higher flow. The lower pressure was AN INDICATOR of higher flow.
On the other hand, an increase in pressure CAN cause an increase in flow ONLY IF THE RESTRICTION IS CONSTANT. If you have an orifice of fixed size, increasing the pressure will cause an increase in flow. This is because the pressures will try to equalize (as above) but the RATE that they will try to equalize is proportional to the pressure differential. It's also exponential-- you won't get double the flow by doubling the pressure-- you'll get a lot less than double the flow.
IN a nutshell, higher pressure can CAUSE higher flow, but lower pressure is AN INDICATOR of higher flow. These seem to be mutually exclusive, but they are apples and oranges, so one doesn't rule out the other.
So with this as a background, let's walk through the requirements of the "ideal" fuel system to the VP44:
1) Pressure would never exceed 14 psi (or whatever the pressure is of the internal bypass in the VP44 that sends fuel right back to the tank. This is easier on the VP, and more efficient because we aren't building pressure we don't need.
2) very high VOLUME. Ideally it would be large enough that we have minimal pressure variance as the VP varies it's fuel demand.
3) The largest fuel lines possible, especially before the pump. Why? We seek pressure stability, so having the largest volume of pressurized fuel gives us a large reservoir from which to draw. Thus, an increase in fuel demand has a smaller effect on the pressure level of the fuel. On the pre-pump side, the larger line means less restriction. Is it easier to drink through a McDonald's straw or a coffee stirrer?
4) Pump is close as possible to tank (and below the lowest fuel level). Since this, too, increases the volume of pressurized fuel (longer line from pump to filter) we have more stable fuel pressures. Not to mention the obvious about the pump preferring to push instead of suck. There's no need to put a bulkhead fitting on the bottom-- the stock design is fine, but maybe a larger line up to and including the fuel pickup would help a lot. Racers will also tell you to make sure your vent hole is large enough. You can starve your engine if it's too small.
Anyway, that's my input into this fray.
HOHN