Simple Question, Difficult answer.

When trying to pump efficiently and as much volume as one mechanic pump can muster, is it is a good thing to have another pump BEFORE this pump to feed it? Or is it better to have this pump draw directly from whatever reservior it is getting its fluid from?

I am upgrading my fuel pump on my 89 so that I don't have to go back in when I do fit an SC. The 89 has an external and internal pump. The internal pump, to be removed, must be pretty difficult without removing the tank.

I will probably be getting the Aeromotive pump that is sold by places like Jegs and summit.

So should I leave the internal pump there or make the higher volume pump pull the petrol on its own?

Thanks.

 

Cant help you there, but i do know that a 12 pack of beer ran through a 3/4 garden hose moves pretty quick when poured from the 3rd floor of your college dorm room

My extent of fluid dynamics.

 

pissing into the wind can get pretty ugly...

 

Well rember, fluids in a roughly horizontal pipe are going to flow because of pressure deffernetial, and not much else.

The only reason to add a feeder pump is if your not getting enough pump head.

OK, here's some more information.

If the pressure on a liquid gets too low, it can vaporize. Doing so will cause a pump to cavatate, and basicly destory it's self. Bad. As a result there is a limit to how low the pressure feeding the pump can go. Too low, the liquid vaporizes in the pump, cavatates and dies. As a result there is what's called a minimum pump head.

OK, so you know how much pressure the pump needs to work, now add the head lost to firiction in the tubeing leading up to the pump. The diffrence between the pressure in the tank, which is probably atmopsheric, and that head lose is all you've got to make the liquid flow through the pipe. If that differential in pressure is too low, the resulting velosity of the fluid is too low, and you don't get a high enough flow rate.

As you can see, becuase of the above limits, even a higher volume pump wouldn't move any more fluid before trying to destory it's self.

OK, my best guess, if Porsche added an internal tank pump, then they at last suspected that there wouldn't be enough head for the pump. I really wouldn't want to add the internal pump if I didn't have too. If you remove it an try to use a higher flow pump, I'm willing to bet you'd run into trouble.

Safest bet, leave the internal pump in there, but if you upgrade the flow rate of the other pump, be sure to upgrade the flow rade of the internal pump.

 

It's early, and I'm having slight anxiety over my new car, and my new governor. Having said that...

I took fluid dynamics in college. That will not help here. But a little logic can.

If the first pump is slower than the following pump, then it's worse than having one pump. And I believe the opposite is true. So it seems that two pumps is not a good solution. Altho, we have a pump in the tank, and a pump outside the tank. So maybe I'm full of it.
My approach would be to go to the next configuration of pump. Like a positive displacement. Actually, our vane pumps are positive displacement. Look at what the high performance race cars are using. I thought they had the same pumps we do.
Maybe your accumulator is bad. That's really part of the job that it performs.
If that was worthless, try thinking about this. Have you ever seen the amount of fuel a good vane pump puts out? It's way more than your engine can burn. I think there's a problem somewhere else.

 

It pumps way more fuel at atmopsheric pressure, but how about at the required 60 psi, or whatever is required, I forget now. As the pressure goes up, the flow goes down, sigificantly.

While the pump maybe positive displacement, the electric motor is not. As you load the said pump up, it's going to slow down. Result is the pump curve. The two pumps will sorta influance how fast the other pumps, and if everything is with in acceptable specs, then there's are no worrys.

You probably should have two pumps to help prevent any vapor lock or pick up issues, and to give you that little bot of margion to make sure the main fuel pump alwalys had enough head, but if you increase the flow on one, you should increase the flow on the other. Thats kind a duh right? If you increase the flow rateing on one, the other will still serve as a bottle neck, and that is bad.

 

Brendan, it isn't necessary to drop the tank to replace the in-tank pump. You need to drain the tank, remove the rear-most tank cradle bolts, and loosen (only) the front-most bolts. At that point you can swing the cradle down far enough to get at the in-tank pump. Be careful when removing the electrical connectors from the pump - they have a barb which you need to disengage before you yank or you'll pull the spade connector from the pump body (not a big deal if you know that your in-tank pump is dead but a big deal on a new pump.)

 

Thanks Worf. That may make things easier. I can put in an 88 module, which is just an orifice, if I do remove the first pump.

Gregg, Units - How would I increase the flow of the internal pump? Increase the voltage?

 

New, higher capasaty pump.

Incerase the voltage, and your likely to burn it out.

Because you've got two pumps, to really do it right, you'll probably need to buy two new pumps. Darn uh?

This all said, you might beable to get away with just one pump, but it basicly need to be right underneath the fuel tank with as short a length of hsoe as possible. This is where my stock fuel pump is located, but I don't know if they modifyed it for the S4. I also belive I only have one fuel pump, but I can't rember right now for sure.

 

Brendan,

What you need to look at is the flow rate you need.

The two-pump system was used to have the in-tank pump prime the second pump. It's not that the flow of two pumps in series was needed. For more flow it's easier and cheaper to use a single, bigger pump.

If I was doing some serious mods myself, I'd remove the in-tank pump and put in a new pump. Figuring out which that is is the trick. Maybe ping a S/C guy for a specific model.

 

Look at what the guys use who are making 1500 horsepower. That will give one a clue which direction to go.

To increase flow, go with parallel pumps. That's just a thought. I've never seen it done, and wouldn't think it neccesary.

And as for increasing voltage. We only have 12 volts.

 

With respect to the voltage issues, Aeromotive supplies a converter that can change the voltage to the pump with reference to load on the engine, to suit the use of a LARGER pump for street use.

In this instance, if you have a large pump operating at full capacity ALL the time, it will start to run TOO much fuel through the system, over pumping it, and heating it up. This is bad. Therefore at cruise, the pump will recieve less voltage, and at accelleration, it will recieve spikes to keep up with the requirements the injectors are asking for.

The hose/tank arrangement is the same from old to new 928s. I would imagine that there are no real higher volume replacements for the intake pump.

 

Units - Tell me more about this pressure differential.

If I have 5 gallons of gas sitting pretty much on top of a screw drive pump, pushing 50psi of gas into a system, will I really need to worry about a pressure differential?

 

Brendan,

If the internal pump is a centrifugal model, then you would want to leave it there unless the current draw of both pumps exceeds the circuit capacity. If the current draw proves to be too high, then you could just disconnect the wiring from the first pump. There would be no need to physically remove it since that type of pump will flow through when not energized.

If they are both vane pumps (which I doubt), then the first pump should be removed.

 

I think its flow through, as when you drain the tank it comes out pretty fast.