SeanR

I understand that the housing isn't paper, thanks.

GregBBRD

Sweet.

If Todd designed and made it....it will work!

I've found that the '89 and later pumps "like" having fuel fed to them...they get noisy and hot when they have to "suck".

I've run the 044 pumps without a pump feedling them, and they seem to work fine..so this should work good, with that pump.

ROG100

Fred,
I was comparing the stock strainer (not the intank pump strainer) to the new one and still think it looks more restrictive than the stock strainer as the stock is a single thin membrane. Does it matter that it is more restrictive - probably not when tied to an 044 pump. At the point I made the comment I did not know it was more specific to the 044 pump.
Roger

dr bob

The issue that caused the factory (and other Bosch customers) to use a two-stage pump system was fuel boiling in the inlet of the pump in hot weather. As Fred R stated, the net pump suction head needs to be higher than the boiling pressure of the fuel at whatever the temperature might be. On a hot day as the temperature increases, so too does the suction pressure requirement. It's not enough that the fuel is liquid as it enters the pump. It must remain as liquid as it is drawn into the first stages of the pump. With just the external pump, that means that there is pressure lost in the screen, the inlet tubing, the connecting hose, the external pump inlet fittings, and finally the inlet of the first stage of the pump itself.

Still with me? The casing temperature of the external pump is another variable, where 'a bit hotter' is the difference between pumping liquid and trying and failing to pump vapor. So we add a priming pump inside the tank. It's a small pump made to offer a fairly high volume with a small pressure difference between inlet and outlet. It sits in the liquid fuel in the tank so it doesn't get a lot hotter than the fuel. It's at the bottom of the tank so it starts out with tank pressure plus the head from whatever fuel happens to be in there. The head pressure isn't a lot-- it takes almost 3 feet of fuel to generate 1 PSI at the pump suction at 70ºF, and the hotter it gets the more head it takes to have the same suction pressure.

So the low pressure differential in that in-tank pump means there is less chance of boiling in the inlet bell and the first stage. It does increase the pressure enough to eliminate boiling in the inlet and first stage of the main pump.

Back the the proposal that using a higher-pressure higher-flow 044 pump means you won't need the in-tank pump. I propose exactly the opposite. If you drive in conditions where the fuel temp gets high, like driving in the hot desert, extended hot idling in stop-and-go traffic with the AC adding under-hood heat, these are times when the in-tank pump is needed.

It should be noted that the AC system actually cools fuel returning from the engine. That system was added to help with fuel boiling problems starting with the later CIS cars, and it continued to the end of production. Bosch initially supplied higher-pressure pumps for many CIS cars, but eventually the core problem was identified as pump suction boiling as much as it was a problem with boiling in the fuel distributor.

My too sense.

IcemanG17

The "Widow" had nothing but problems from the intank pump.....once it was removed and replaced with the strainer ZERO issues.....My old 88 had only the strainer and ran fine in hot weather and on track....same for the 84 Estate.....

soontobered84

I had my GTS on a dyno running a "base" line and it was detonating. I ended the dyno run and began searching for my issues. Found out that my intake pump was seized and the external pump was having to draw fuel past the seized turbine fins of the internal pump. I replaced the internal pump with a strainer and new fitting (which, by the way, has about a half again larger opening and requires a larger fuel hose). I put the car back on the dyno, and had no detonation with the single stock external pump.

I drove the car all last summer here in the Texas heat, and experienced no problems or difficulties with fuel delivery.

I believe I understand what the internal pump was designed to do, and the reasons behind it, but I personally have experienced no difficulties with just the external pump.

YMMV

FredR

Dear Hacker,

My observations are not about perceived power gains using a [bigger] 044 pump, but all about limiting hydraulics. The 044 pump is only required when extended power delivery options have been fitted thus requiring more fuel delivery. Our fuel pumps operate at more or less a constant operating point on the pump curve and fuel that is not consumed in the engine simply returns to the fuel tank in what we call a "spillback loop" controlled by pressure regulator that sets a constant pressure in the fuel header so that when the injector opens for a specified time interval a relatively precise amount of fuel is metered.

Sticking a 044 pump behind a stock in tank pump is asking for trouble if the in tank pump is not big enough to serve the 044 pump- it can only make things worse and certainly not better because the hydraulic losses in the stock plumbing will induce ever more losses when the fuel flow is increased [losses are proportional to the square of the flow- i.e. double the flow = 4 times the hydraulic losses as petrol invariably is pumped in a turbulent flow condition- Reynolds number >4k]. Therefore it makes a lot of sense to improve the system hydraulically [bigger mesh area- bigger diameter piping etc right up to the eye of the impeller as Dr Bob correctly states.

The 64 dollar question is whether there is sufficent "head" [ no-not the type dished out by Baywatch lifeguards you pervents!] to prevent cavitation. The onset of Cavitation is actually very difficult to detect and one of the clues is a sudden drop in pump discharge pressure of about 5%. Thus- if this happens there may be a slight [otherwise inexplicable] drop in detectable HP to give a clue as to what may be happening. Far better to monitor fuel header pressure of course.

By not fitting an in tank pump one is effectvely removing "a blockage" if the system is not working correctly [or not working at all] hence potential improvement. In a system that is designed and working correctly the opposite may happen.

The pumping systems I design and work on from time to time are slightly bigger than this- [about 5MW to be precise!] but the design principles are exactly the same. If one is operating in an environment where an in tank pump is needed, the chances are that a 044 pump will need an appropriate booster pump to cover all contingencies.

Multistage pumps [our fuel pumps probably have either 3 or 4 stages] or high speed pumps that develop high efficiency head performance are typically characterised by poor suction performance, single stage booster pumps are typically characterised by good suction performance but poor overall efficency.

If you can get away without the in tank pump so much the better but I can assure you the system hydraulic design at best is marginal [maybe inadequate] when the fuel level is very low. If you look at the mid lower section of the firewall you will see in the a/c system plumbing a small heat exchanger- that cools fuel using the a/c system. I presume this is a stock feature [i.e. not just for hot climes models] and you can be sure that Porsche did not fit this simply to increase the cost of the car when new.

Regards

Fred R

hacker-pschorr

I was simply trying pointing out these new parts are not designed to simply replace the sill available in-tank strainer in a stock 928.

Weather or not cavitation has ever been detected with an 044 pump is irrelevant, this project started when I learned (from Todd) the stock fuel line is below the minimum standard for an 044 pump. He already addressed this years ago with his car. I approached him to make some custom fittings for my car and Tim's, and it turned into a small project to make a few more.

Todd never approved of how we were installing these pumps. In his opinion too many fittings and not enough hose, which in his opinion is a safety issue (as I mentioned before). He feels reducing the amount of hose can increase the chances of fuel leakage in the event of a rear end collision due to the multiple fittings becoming damaged.
Also some of the adapters I've seen people use have an ID smaller than the stock fittings.

Is there a market for this kit? I have no idea, I just wanted one for myself and the first batch is already spoken for. Tim will be starting a new thread soon with specifications and better photos.

This all started because none of us could find a 5/8 barb fitting with the threads for an 044 pump. Todd drew up the design and off to the machine shop we went.

He's not physically making these himself, he designed the pump fitting and co-designed the in-tank fitting. My father-in-law is the foreman at the machine shop producing these.

Jim R.

As a data point for why this piece is needed for higher output modified cars, my car has had fuel pressure "issues" noted while datalogging.

With both a new in-tank and a new stock external pump we had to shut my car down at 5500 rpm's on the dyno with fuel pressure dropping into the mid 20psi range.

Next up was the new stock internal pump feeding a new bosch 044 with a heavy duty relay and wiring...by redline about 5 psi drop in fuel pressure. Pressure measured with innovate electronic fuel pressure sender to datalogger.

This should flow enough to not "choke" the 044 pump and be a nice clean install.

JR

dprantl

Just the other day I was comparing the size of my GT's now removed in-tank pump (and replaced with a stock strainer behind a 044) with a Walbro from a SAAB 9000. The diameter and length of the pump was identical, and the Walbro sure as hell flows a lot more. Food for thought?

Dan
'91 928GT S/C 475hp/460lb.ft

Brent

I don't have any hard facts/data to backup the above statement, but this is all starting to make sense now. Looking foreword to hearing more.

Z

That's exactly what happened on more than one car when using the stock in tank pump.

Even if the in tank pump is capable of flowing enough to supply an external pump of whatever kind, the strainer on the in tank pump is more restrictive than the one that Andrew now has in his car. The new one is significantly larger, and has more pleats as well. It has a much larger surface area than either the strainer on the in tank pump, or the stock plastic strainer that's in cars which came without the in tank pumps.

When asked about the in tank pump in the past, I've always recommened removing it and replacing it with just the strainers. The in tank pump configuration is just a bad idea in my opinion. My car is one of those that didn't come with an in tank pump. If I ever do buy a 928 that has one, removing it will be one of the first things that I do.

The in tank pump is suspended horizontally by that rubber hose which connects it to the fitting that screws into the tank. Does anyone really think that it's a good idea to use a piece of rubber hose to hold much of anything horizontally by just one end, especially in a vehicle? The weight of the in tank pump is constantly pulling downwards on the hose. Every bump that you hit causes more stress on that hose and more flexing of it, as does every time that fuel pushes against it from sloshing around. Andrew isn't the only one to have replaced an in tank pump with a broken hose and have the new one fail the same way in a very short time. If there was any real need for an in tank pump, hanging it horizontally by only holding it by one end on a piece of rubber hose is not the way to do it. Do those of you who've had issues related to fuel heating think it's a good idea to submerge a constantly running electric pump, that generates some degree of heat, in your fuel? I'd say that this is at or very near the top of the list for really dumb things done in the design of these cars.

If you're using a stock external fuel pump, replace the in tank pump with one of the strainers that were used on the cars that came without the in tank pumps. If you're using a higher flowing fuel pump, use one of the strainers like the one that Andrew now has. If you're determined to continue using a stock in tank pump, ask your favorite 928 parts vendor if you can get a discount for buying them in quantity.

AO

I'm starting to wonder if the only reason why my car didn't loose pressure was BECAUSE of the split hose. Worst design EVER!

dcrasta

Again, brilliant explanation sir. Also, double points for using 'Head' and 'Suction' .

BC

I don't think the strainer likes actually straining much. Especially with particulates that may drop out from sitting.