Finally I have come to a decision in this area. I know that somebody will say that this is overkill for this project, but I will have room to grow. After a lot of digging around on different sites and reading a lot of flow information I came up with the following. Keep in mind that as pressure rises in a fuel system that the flow rate of your pump will decrease. The inverse is also true. To make matters worse there is no standard that manufacturers use to test their pump output.
The stock Bosch replacement pump for a 944 is p/n 0 580 464 069. It is rated for 98 L/hr @ 4 bar.
The stock Bosch replacement pump for the S6 is p/n 0 580 31 068. It is rated for 120 L/hr @ 3 bar.
With the difference in tested pressure I would assume that these pumps are about equal in flow at the same pressure. That would be for a stock application.
Since my 944's pump is of an unknown age and I hope to eventually be at a level of about 400bhp I decided to err on the side of caution and replace it with Bosch p/n 0 580 254 044. This unit is rated for 200 L/hr @ 5 bar or approximately 270 L/hr @ 3 bar. It's currently very popular in cars making at least 500 bhp so I have every confidence that it will supply enough fuel for my needs. It is also almost a direct fit for the 944 application. Just for your information there are several other Bosch units that fall in between the 069 and 044 models, but to tell you the truth I got tired of trying to figure out how small is too small.
Side note: As far as just using the s6 pump on the 944 until I start raising power levels...well you can't. The s6 is in tank and the 944 is in line.
Another note: After writing this info I have continued to receive information from different people. They have informed me that there are a lot of people with modded 951's making 400-500hp that are using the stock pump. That may be true, but it makes me uneasy. That being the case I may rethink my decision on using the 044 and try to find out the part number for the Bosch HV pump. It has upgraded performance over the stock pump, but not as much as the 044. I could live with that.
There are three main reasons to not go too big when selecting a fuel pump.
1: A larger pump requires more power to operate. This is going to take hp to produce, plus your wiring may not be up to the task of the increased current requirements.
2: Something I was not aware of until researching this topic, but it makes sense. If you circulate way more fuel than you need it will begin to actually act like a cooling system and transfer heat from your engine bay into your fuel tank. That is not a good thing.
3: Oh and this one is important to everyone. A larger pump costs more.
Final thoughts:
Alright the mystery is solved. I'm now going to choose a different pump. This is the mysterious Bosch HV pump. I located someone who read the numbers off the box for me so that I could look up the flow rate and current draw. Supposedly it is good for 450 rwhp sustained. That certainly will supply what I need without getting overly large. Here is the number break down on the three models for this car.
0 580 464 069 - 98 L/hr @ 4 bars - max 7 amp draw - stock replacement
0 580 254 910 - 130L/hr @ 5 bars - max 10 amp draw - this is the HV
0 580 254 044 - 200L/hr @ 5 bars - max 15.5 amp draw - this is the 044
On to the next problem.
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Great job! This will be very helpful. I would take issue with the 'problem' of flowing too much fuel and transfering heat to the tank. Most fuel systems use this to actually cool the pump. You can boil fuel without it igniting so it's not dangerious. It could contribute to more vapor-lock, etc but I doubt the flow amount is going to add a lot of extra heat. . . .
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