Another good video Gary! This has been a good discussion and it has led me to do a little testing of my own, however... I haven't mastered the technology required to get a video out of the shop and onto the internet! That's going to be my next project!!
I totally agree with CIWYM, something is happening at 15psi. That's the bottom line!
What I tried to do is separate the relief valve from the cavitation issue. My thinking is that if I could take take the relief valve out of the equation, I could more accurately evaluate the cavitation.
Here is what I did:
My set up today did not include gages. Tomorrow's set up will include a pressure gage on the output and a vacuum gage on the input (very important, as I will explain later).
1. Duplicated Gary's set-up w/o gages. Fuel on the floor below the pump, cranked down on the output and sure enough get the sudden drop in output and pump speeds up. Cloudy fuel in the line.
2. Removed the input fitting and inserted the clear tubing into the pump so that it sealed inside the pump, past the bleed port for the relief valve. Repeated the test, cranked down on the output and sure enough got the sudden pressure drop. Only this time fuel is dumping out of the input, indicating that the pressure drop was due to the relief valve opening.
This seems to indicate that the sudden pressure drop is caused by the relief valve opening and the cloudy fuel is from the aggitation in the pump.
3. Now to take the relief valve out of the equation, I cut a short (~3/8" long) piece of clear tubing and wrapped it with electrical tape. My goal was to press this into the input and effectively seal up the bleed port for the relief valve. If it didn't seal, at least restrict the flow enough to reduce the effect of the relief valve. Put the short piece of tubing in place and put the fitting back into the inlet. Fired up the pump, cranked down on the output, no sudden pressure drop, cranked some more, pump working harder and harder. Finally stopped the flow all together with the pump still working hard. Never did experience the sudden pressure drop.
My guess is that tomorrow when I put the gage on the output, I will see that with the relief valve sealed off, I will get higher than 15 psi without the drop in pressure.
This test leads me to believe that what Gary demonstrated in his first video is really the relief valve opening at 15 psi.
So Greg... how do you explain the second video? INQUIRING MINDS WANT TO KNOW!
Here is my theory... it all has to do with pressure drop across the pump. This is why my test tomorrow will include a pressure/vaccum gage on the inlet. In Gary's first video he was sucking fuel off the floor up to the pump and then generating 15 psi outlet pressure. That means that the top of the ball in the relief valve in the inlet of the pump is actually experiencing vacuum equal to the amount required to lift the fuel from the floor to the pump. When you combine that with the 15 psi outlet pressure pushing up on the ball from below, you have a situation where the relief valve will open easier.
So what about the second video... what was different? Placement of the fuel tank. What did this do?
Now the fuel is actually a little above the level of the pump so it is flowing into the pump with a little pressure behind it due to gravity. Now, think about how this affects the ball in the relief valve in the inlet of the pump. Instead of having a little vacuum pulling up on it, there is a little pressure pushing down on it. That means it is going to be a little harder to push open, which means in this configuration, you will need more than the demonstrated outlet pressure of 15 psi to open it, just as Gary demonstrated.
I couldn't figure out for the life of me why moving the pump to the rail would help the situation if the pump always cavitates at 15 psi and drops the pressure. But now after seeing Gary's second video and having to think through how the relief valve reacts to the pressure differential across the pump, it makes perfect sense!
In Gary's first video the pressure differential is greater, therefore the relief valve opens at a lower pressure. In the second video, the pressure differential is lower, therefore the relief valve opens at a higher pressure. Higher pressure than the pump will generate with the truck idling, meaning not more sudden pressure drop due to relief valve opening. This is a good thing!
I would be interested to know what would have happened if Gary had kept cranking on the clamp until near zero flow?
Now I am convinced that moving the pump down closer to, or below the level of the fuel is a good idea... especially if you stay with the OEM fuel pump! I would say by the same logic any pump will do better down there. Hey... isn't that what Gary said?
I do want to throw another little twist into the debate and it came from my own test today. I would submit that what you are seeing in the clear tube when you clamp down on it is the fuel going from laminar flow to turbulent flow. Fluid will flow very smoothly up to a certain velocity, but when it speeds up too much, it breaks up, loses those nice flow lines and becomes turbulent. As the clamp tightens up, it decreases the area that the fluid is flowing through. The only way to flow the same amount of fuel through a smaller opening is for it to go faster. That's what you are seeing in the tube as the fuel speeds up at the pinch point and breaks up into cloudy, turbulent flow. A little ways past the pinch point the fuel slows back down, goes back to the smooth laminar flow, but still carries the residue or cloudiness from breaking up at the high velocity.
Respectfully submitted.
Greg
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