Dave928S

^^ Good decision. The temps you get in summer in Adelaide definitely justify the in tank pump ... IMO.

FredR

I have done several autocross type events on a tight and twisty low speed course on very hot days and took out as much weight as I could to help matters including running with a low fuel load. The crap I removed amounted to close on 80kg or an extra "me". A full tank of fuel amounts to about 90 litres or 70kg so with about 1/4 tank of fuel on board another significant weight saving advantage- or so I thought.

What I found in practice was that on one particular right hand hairpin bend at the bottom of a hill, as I floored it out of the exit the damm thing was fuel starved and I got nowhere quickly. There were two possibilities for this- either the cornering forces I was generating [over 1g] were uncovering the suction or the fuel pump was cavitating even with an in-tank pump fitted and operational. Immediately after the run, off circuit and whilst still heat soaked I tried a series of hard straight line accelerations and same thing happened thus concluded the in-tank pump was necessary and then some as without the pump it could only have been worse. A later run with a full tank of fuel and the problem was gone.

Another point I have wondered about is whether the stock in-tank pump is adequately sized to put before a 044 pump. Unfortunately I have not been able to find pump curves for either the 044 or the stock in tank pump but for the in tank pump to work [i.e. achieve the design intent] it needs to generate a specific amount of head at the rated flow conditions. With two pumps in series the 044 will control the flowrate and the in tank pump will try to flow an amount of fuel to keep up with the bigger pump. As the flow increases through these pumps the head generated drops off rapidly and it is quite conceivable that at some operating point the in tank pump will end up doing nothing and beyond that become a restriction leading to cavitation. If practical operating experience in hot conditions suggests they work well together fair enough but I wonder if anyone has tested the operating envelope.

Either way I would go with the in tank pump until such time as I had found its limitations then perhaps try the same without the pump to test for any differences. Even better would be to do some flow tests to get some numbers but that is not as easy as it sounds.

Good luck with the build- do keep us posted.

If there is such a thing as an in tank pump model that is balanced to the needs of the 044 I have never come across it albeit I did look for such some time ago.

ramcram

Thanks Fred,
I think I'm going to try and find an aftermarket in tank pump, something a bit 'newer'.
It is only 32mm diam so me thinks it will be a challenge.

ramcram

I have been grinding my mind away on a new water bridge design.
I don’t need the complex one Porsche used, with water direction changes at thermostat opening, because I won’t be using a thermostat at all.
I’m using an electric water pump [Davies Craig ewp150]. This unit is controlled by a digital box which pulses the coolant at various rates dependant on the coolant temperature, which has a selectable set point.
I only need an input from the pump to the big hole above the original unit, a bridge between the 2 head output holes, an outlet to the radiator top inlet, an air bleed back to the header tank and fittings for the various temperature senders.
My thoughts are to use a length of rectangular tube, on edge, straight from one head to the other, welded to the existing little plates that bolt on. This would give me a good flat front surface to outlet to the top of the radiator, a good flat top surface for the temperature senders [EMS, water pump, gauge/light] and it would provide a convenient lock to keep the pump supply pipe plugged into the block.
If I make this from steel, I can do it in house but if I do it in aluminium, then I have to send the bits out to my professional welder. I prefer aluminium for looks and weight and I’m not concerned about dissimilar metals. A, because there are plenty in the stock system and B, because I will be using ‘Liquid intelligence 115’ waterless synthetic coolant. This stuff doesn’t corrode, cavitate, freeze until -56C, boil until 190C, so it doesn’t need system pressure or artificial boiling point additives or antifreeze additives or anticorrosion additives because it’s all in there. It looks expensive until you consider the money usually spent on regular system flushes with this stuff that is ‘for life’. Even if a motor rebuild is necessary it can be drained, saved and poured back in.
My worry is thermal expansion. The heads are set at 90° to one another, which means that they move apart when the engine heats up and come back home when it cools down again. The stock water bridge must expand with the rest??? When Porsche moved to a fixed unit inlet manifold in 87-95, they sat it on a very thick gasket with the mounting bolts within a thick rubber grommet, so they were expecting things to move independently. The inlet system is different I think because and engine idling at the traffic lights will heat soak up with the throttle closed there is a the minimum of air running in through it, so it will likely expand. The lights go green and the throttle is opened and there is a sudden inrush of potentially very cool air and it will likely expand.
The water in the water bridge will remain the same temperature as the rest of the system, so the worry is, will it grow/shrink at an acceptable rate during warm up and cooling?
I’m also thinking of doing a small version at the back of the heads to feed the heater. I like the thought of coolant not being trapped in a dead end.

ramcram

I've currently got my spare complete in-tank pump [which was seized] in a bucket of kero. it's working well, quietly and drawing about 1 amp.
I'm going to leave it running for a few hours to clean and lubricate itself?

ramcram

Update on in-tank pump. . . . dead.
Turned it off then back on and it sometimes won't start until it is tapped with something!
Took the top piece, with the mounting thread and hose off it. the wires and hose are in very good condition.
Got my other in-tank pump that came to pieces when it was removed, put power to it and beauty, it ran well.
Fitted the top from the dead one and soldered the wires, put it in the kerosene tank and it pumps 10X the volume of other one.
Pulled the dead one apart to see what's in there and found a carbon brush commutator motor that uses a vane pump similar to a water pump that draws the fuel up over the brushes, that are right next to the outlet.
Interestingly, there are 2 small holes on opposite sides, just above the inlet screen, that spray fuel out while the pump is running. They agitate the fuel around the pump quite a bit.
I wonder if they are a pressure relief bypass or something?

ptuomov

Can you guys help me out with the physics of this in-tank pump issue? If the pickup uncovers, then it doesn't really matter whether the pump is inside the tank or outside the tank. Temperature only really matters because of cavitation at the pump intake side. Tank fill level matters if the pickup uncovers, but it also matters because of the thermal inertia and because of the increased pressure at the pump inlet. Problem with tank venting (vacuum pulled in the tank) also matters.

Long story short, I don't see any obvious reason why an in-tank pump would help with the main pump in the stock S4 location if the pickup diameter is sized right. What will help greatly is much bigger than stock passage from the tank to the main pump. I had various pumps cavitating until the latest (and final) revision of the passage between the tank and pump, and now two Bosch 044s in parallel are pulling from the tank without any cavitation issues.

hacker-pschorr

I've pondered this myself. Todd us running dual 044's without an in-take pump and with e85 his requirements for fuel volume are even higher.

I've seen many setups on 911 Turbo race cars with a single or double 044 without a "feeder" pump at the track not having fuel issues. Most of these are in the 500-600hp range.

ptuomov

I think that with the correctly sized pickup line between the stock strainer tube and the 044 pump(s), there's no need for an in-tank pump. The stock location S4 for the external pump is just right.

I wouldn't attempt the fenderwell pump location without an in-tank pump, of course.

ramcram

Interesting the twin pump system. I've had 2 5.6 litre Mercedes and they had twin pumps at the tank.
I will be starting of with the stock in-tank pump feeding an 044 pump in the stock 928 holder.
I've just tested my repaired in-tank pump and it flows 1 litre in 12 seconds, drawing less than 1 amp.

worf928

IIRC, the in-tank pump topic was debated-to-unconsciousness in a Jim Coreman thread circa 2011: The in-tank pump is in place to deal with the slight potential for vapor lock in a fuel system with a busted check valve.

John Speake

The in tank pump is designed to prime the external pump, which I understand is good at pushing fuel, but not so clever at pulling it in.

ptuomov

As you know, vacuum can't pull a fluid. It can only be pulled by gravity and pushed by pressure (in this case the tank absolute pressure).

Because the stock pump is placed the way it is in S4, the pump is fed with a relatively large positive pressure. That is, if the tank is correctly vented to about 14.7 psia ambient pressure and if the feed line is large enough (for the flow rate), the combination of ambient pressure and gravity pushes the fuel into the pump pretty damn hard. I don't see any reason why an in-tank pump would be needed if the rest of the system is functioning correctly.

Noting the earlier comment, it's possible that if the check valve downstream of the pump is broken and if somehow there's an air bubble in the fuel system that makes itself just right inside the pump with equal fuel pressure on both sides of the air bubble, then theoretically I guess the pump could be spinning in air and fuel not moving. This would require a broken check valve and the stars randomly aligning in perfect order, so I personally wouldn't add an in-tank pump into the system with all the things that can go wrong with a second pump just to mitigate the risk of a very unlikely scenario that includes a failed check valve.

GregBBRD

Apparently, Porsche didn't agree with your theory....

The first models had in tank pumps, then they removed them, only to re-install them in the later cars.

Pretty obvious that they had some issue in those middle production vehicles, which they solved with an in tank pump.

.....and check valve failure in the 928 model is extremely rare....I haven't seem a bad one in the past 20 years....maybe longer.

ptuomov

For the early cars, the in-tank pump makes sense to me. For the later cars, it doesn't make sense to me. Check valve is nearly infinitely more reliable than an additional pump. If the tank venting system fails, then one has bigger problems, too. Maybe the main pump wires draw less current with the in-tank pump installed?

The one scenario in which the in-tank pump might help is the following. You run the fuel tank completely empty, so empty that there's air in the fuel lines somehow, including in the pump. Then, you only have small emergency container of fuel, perhaps retrieved from a gas station by foot. Pouring that small amount of fuel into the tank after running the car completely out of gas may result in a situation in which the fuel lines including the pump have air in them. Because there's only a small amount of fuel in the tank, the pressure due to gravity is not enough to compress the air in the lines like a full tank would. If in addition the air bubble doesn't burb from the pump (maybe car parked in an unusual angle), then maybe you get the pump vapor locked. Highly unlikely, all things considered.

In practice, it seems that dual pump cars have a lot more problems than single pump cars, so even if someone can figure out a reason why they added the in-tank pump back, that decision to add it back looks like a mistake with hindsight.