Gentlemen, read this carefully.
Bill, don't buy into that higher pressure, lower flow BS! For a given size of a restriction, raising the pressure on the inlet side of a restriction *will* increase the flow rate through that restriction. Set up a simple test with a pump and try it. That Bernoulli stuff will just confuse the issue. We must ensure that the VP-44 is the largest 'restriction' in its feed system. If it is, then most of the pressure that is generated by the lift pump (wherever it is) will be present at the VP-44. There will be some pressure lost at any given flow rate because of parasitic restrictions in the filters, fittings, lines, etc. That is why we try to use less restrictive components, so that the downstream pressure losses do not reduce the available pressure at the VP-44 to less than desired. Lowering the restriction is better than raising the pressure, because it gives a more stable pressure at the VP-44 with varying flow rates, obviously. A higher-pressure lift pump could be used to overcome system pressure loss at high flow values, but that would require a regulator feeding the VP-44 to limit the pressure at the VP-44 inlet to a safe level when the flow rate, and, consequently, the system losses are lower.
Consider this: I'll use MM's figures (DD2, DDTTPM) from his post of 04-27-2001 05:05PM. At WOT he's seeing 30 gph flowing back from the injector/overflow combination. Assume that 15 gph is injected and burned. If this is the case, there *must* be 45 gph flowing into the VP-44. The pressure at the VP-44 is 4 PSI. If his lift pump is capable of delivering 45 gph at 15 psi (and we realize it can, because it's allegedly capable of delivering 110 gph @ 15 psi), that means that at 45 gph, there is enough restriction in his filters, fittings and lines (the system) to reduce the pressure by 11 psi. There is 11 psi loss from the outlet of the lift pump to the inlet of the VP-44, when the system is flowing 45 gph. Here's where it gets tricky. If MM installs a pump that is only capable of 7 psi, and the system loss is 11 psi at 45 gph, what the VP-44 creates when it is demanding 45 gph is -4 psi. Huh? Yes, *IF* the VP-44 is capable of pulling a vacuum, that's what will be seen on a pressure (vacuum) gauge on the inlet fitting on the VP-44. That's because our lift pump can only generate 7 psi. We know that our loss is 11 psi through the system at 45 gph which is set by the demand of the VP-44. 7-11=-4.
Here is THE important things to remember:
1. GALLONS PER HOUR IN THE FUEL DELIVERY SYSTEM IS DETERMINED BY THE DEMAND (THE REQUIREMENT) OF THE VP-44, NOT THE LIFT PUMP.
2. THE LIFT PUMP MUST BE ***CAPABLE*** OF DELIVERING MORE GPH THAN THE VP-44 DEMANDS UNDER ALL CIRCUMSTANCES.
3. PRESSURE AT THE LIFT PUMP ONLY ENSURES THAT THE REQUIRED GPH IS AVAILABLE TO THE VP-44 AT SOME DESIRED POSITIVE PRESSURE AFTER ALL THE SYSTEM PRESSURE LOSS (FILTERS, FITTINGS AND LINES) IS TAKEN INTO ACCOUNT.
4. THE LIFT PUMP WILL ONLY DELIVER WHAT IS DEMANDED OF IT, UP TO THE LIMIT OF ITS GPH CAPACITY.
MM, have you plumbed the flow meter into the return port of the VP-44 Yet? If not, then I don't believe that your data is valid. It is still my contention that most of the fuel you are seeing in the return line to the tank (after the 'T') is actually coming from the injector return manifold at the back of the head. 30 gph sounds like a lot of fuel until you realize that it is only 2 quarts a minute. In reality that's not much fuel. I will be plenty embarrassed and apologetic if this is not the case. I will eat a suitable size of crow; they're plentiful around here and nobody will miss just one! Plumb that sensor! Prove me wrong!
Respectively submitted for your consideration.
Tom
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Tom Glover - VE7DQ, C-GWTG
99 QC SLT, LWB 4x2, ETC/DGP, Wht/Agate, etc.
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