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Help me understand fuel pump flow/pressure ratings and returnless systems


mindspin311
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We made some mistakes when we put the fuel cell in our Mini. Didn't fully understand how the stock system worked. It finally bit us at Daytona as we blew up 2 pumps and we were unprepared to replace them.

 

After some research, I see where most of the mistakes have been made, but sizing of pumps still confuses me some. So this is what I know...

 

  • Mini is a returnless system, in-tank pump feeds the HPFP that's mechanically driven by the motor
  • The returnless guts are all in the stock tank with an internal 5.1 bar (~75psi) pressure regulator
    • The regulator will recirculate fuel back in to the thank when it opens (I assume...)
  • Stock pump is supposedly rated at a nominal 3.6 bar (~52psi) with a supposed max of 5 bar (~72psi)
  • Stock pump min/nominal flow rate is 39gph (~147lph)

 

Our failure mode seems to be that we ran the cell without a regulator on the low pressure side and eventually the pumps we used blew open the pressure relief to the point where they stuck open and no long pumped out of the cell.

 

We are using the DW Micro pumps as both lift (cell to surge) and a main pumps (surge to HPFP).

image.png.b38b97362fb2be9e65fbc3a707c6ae9d.png

 

What immediately confuses me is that the 100psi pressure relief valve was stuck open, but the above graph only goes to 70psi. Seems the pump is capable of more than that given the failed relief valve. So now I am trying to figure out in what situations is this happening?

 

When looking at the above specs and comparing to the stock specs I have found there is a mismatch. The DW pump operating at the stock pump's pressure of 52psi does not meet the flow rate. That requires a lower pressure. Oh, and then how does all this affect the regulator? If its rated at 75psi and and the stock pump 70, then when does it ever come in to affect?


Damn I'm confused 🤣

Edited by mindspin311
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18 minutes ago, mindspin311 said:

We made some mistakes when we put the fuel cell in our Mini. Didn't fully understand how the stock system worked. It finally bit us at Daytona as we blew up 2 pumps and we were unprepared to replace them.

 

After some research, I see where most of the mistakes have been made, but sizing of pumps still confuses me some. So this is what I know...

 

  • Mini is a returnless system, in-tank pump feeds the HPFP that's mechanically driven by the motor
  • The returnless guts are all in the stock tank with an internal 5.1 bar (~75psi) pressure regulator
    • The regulator will recirculate fuel back in to the thank when it opens (I assume...)
  • Stock pump is supposedly rated at a nominal 3.6 bar (~52psi) with a supposed max of 5 bar (~72psi)
  • Stock pump min/nominal flow rate is 39gph (~147lph)

 

Our failure mode seems to be that we ran the cell without a regulator on the low pressure side and eventually the pumps we used blew open the pressure relief to the point where they stuck open and no long pumped out of the cell.

 

We are using the DW Micro pumps

image.png.b38b97362fb2be9e65fbc3a707c6ae9d.png

 

What immediately confuses me is that the 100psi pressure relief valve was stuck open, but the above graph only goes to 70psi. Seems the pump is capable of more than that given the failed relief valve. So now I am trying to figure out in what situations is this happening?

 

When looking at the above specs and comparing to the stock specs I have found there is a mismatch. The DW pump operating at the stock pump's pressure of 52psi does not meet the flow rate. That requires a lower pressure. Oh, and then how does all this affect the regulator? If its rated at 75psi and and the stock pump 70, then when does it ever come in to affect?


Damn I'm confused 🤣

So your using that DW pump under steady pressure?   
My understanding is that a lift pump should only operate under little to no pressure.  It’s just meant for moving fuel from one place to the other.  Maybe a slight bit of pressure from head, or small plumbing restrictions but that’s it.  The graph is a bit puzzling though. Not sure why that pump should ever see that much pressure as a “lift” pump.  
 

Our car uses a returnless system, but only has the one pump.  I chose to build a 1/2 Gallon surge tank that the factory pump/regulator etc fits into.   The lift pump (same DW one) moves fuel from the cell to the surge tank. There’s an overflow at the top of the surge tank back to the cell.  

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20 minutes ago, petawawarace said:

So your using that DW pump under steady pressure?   
My understanding is that a lift pump should only operate under little to no pressure.  It’s just meant for moving fuel from one place to the other.  Maybe a slight bit of pressure from head, or small plumbing restrictions but that’s it.  The graph is a bit puzzling though. Not sure why that pump should ever see that much pressure as a “lift” pump.  
 

Our car uses a returnless system, but only has the one pump.  I chose to build a 1/2 Gallon surge tank that the factory pump/regulator etc fits into.   The lift pump (same DW one) moves fuel from the cell to the surge tank. There’s an overflow at the top of the surge tank back to the cell.  

Yes, we are using them as both lift (cell to surge) and a main pump (surge to HPFP). I will edit the original post.


Given that the Mini uses a lower pressure pump to feed the HPFP, we assumed that a lift style pump would get the job done. So it is "deadheading" in to the HPFP. We made the mistake to delete the pressure regulator in the tank which is probably there to relieve this condition in stock form.

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6 minutes ago, mindspin311 said:

Yes, we are using them as both lift (cell to surge) and a main pump (surge to HPFP). I will edit the original post.


Given that the Mini uses a lower pressure pump to feed the HPFP, we assumed that a lift style pump would get the job done. So it is "deadheading" in to the HPFP. We made the mistake to delete the pressure regulator in the tank which is probably there to relieve this condition in stock form.

Those DW lift pumps are made specifically for applications where they can be run dry as a lift pump under little to no pressure. I’m guessing that while the chart shows them capable of generating that pressure, they are not made to do that.   I think you would require a proper low pressure fuel pump for that application.  Even at 50psi your looking for, that’s likely too much for that pump for extended periods.  

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24 minutes ago, petawawarace said:

Those DW lift pumps are made specifically for applications where they can be run dry as a lift pump under little to no pressure. I’m guessing that while the chart shows them capable of generating that pressure, they are not made to do that.   I think you would require a proper low pressure fuel pump for that application.  Even at 50psi your looking for, that’s likely too much for that pump for extended periods.  

Its a fair assessment. I do know that when the car was running, we had no issue with fueling. Maybe we were just on ticking timebombs for when they finally said we can no longer do this and gave up at Daytona.

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You can run those pumps from cell to surge, assuming that the overflow of surge returns to the tank.  

 

Your second pump that feeds the hpfp should probably run something like the corvette fpr and have the return from that also go to the surge tank.  I would think a 200 lph pump would be fine for this application.

 

Also, if you are deadheading the pump against the hpfp, it will most likely get hot and fail quickly. The fpr allows fuel to still flow through the pump.  It may NOT be flowing if just deadhead against the hpfp.

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3 minutes ago, wvumtnbkr said:

You can run those pumps from cell to surge, assuming that the overflow of surge returns to the tank.  

 

Your second pump that feeds the hpfp should probably run something like the corvette fpr and have the return from that also go to the surge tank.  I would think a 200 lph pump would be fine for this application.

 

Also, if you are deadheading the pump against the hpfp, it will most likely get hot and fail quickly. The fpr allows fuel to still flow through the pump.  It may NOT be flowing if just deadhead against the hpfp.

I have seen the Corvette filter/FPR combo, but it is rated less than the internal Mini FPR (58 for the Vette, about 75 for the Mini)

 

What I haven't figured out yet is if the HPFP needs to see that 75psi to operate properly or not. Trying to find some sort of Mini/BMW document with that info.

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4 hours ago, ABR-Glen said:

Just put the stock 75psi regulator back in the line between the pump and the HPFP.

The "returnless" idea is getting you into trouble, they didn't eliminate the return, just kept it in the tank (as you noted).

 

Right, thats the plan, but do I need to size a pump properly? If I choose a pump thats putting out 100psi or greater the entire time and I'm relying on the FPR to do its job 100% of the time, is that good for the FPR?

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1 hour ago, mindspin311 said:

Right, thats the plan, but do I need to size a pump properly? If I choose a pump thats putting out 100psi or greater the entire time and I'm relying on the FPR to do its job 100% of the time, is that good for the FPR?

A mechanical FPR is just a diagram and a spring.  Not much to fail there.  Your pump will never see more than the set point of the FPR.  
 

I would think almost any proper fuel pump would work.  Your racing a mini. It’s not got that much fuel consumption compared to say an LS motor.  But the fuel pump sees long continuous duty in endurance racing so don’t cheap out on a no name one.  

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Another factor to consider is the OE design of the MINI fuel system is the in tank lift pump operation is pulse width modulated (PWM). The amount of on time of the lift pump is reduced at low rpms. Logically I would think that racing at wide open any pump would be running at 100%.  How are you powering up the each on of the pumps? Might be worth looking into. 

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22 minutes ago, misshift said:

Another factor to consider is the OE design of the MINI fuel system is the in tank lift pump operation is pulse width modulated (PWM). The amount of on time of the lift pump is reduced at low rpms. Logically I would think that racing at wide open any pump would be running at 100%.  How are you powering up the each on of the pumps? Might be worth looking into. 

I'm not sure its running off PWM. I don't see anything in the wiring diagrams that suggest that. There's just a relay that controls power to the pump. I'm of the assumption that the pump is running wide open and the 5.1 bar regulator keeps things in check. Its probably more active at idle and part throttle vs full throttle/boost.

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A pump will only put out as much pressure as the restriction it sees. 

 

The spring in the fpr sets that pressure if you have an fpr. The excess flow is returned to the tank by the fpr eliminating a possible deadhead of the pump.

 

If you don't have an fpr, there is no return to tank and the variable demand of the hpfp will create a bigger restriction than an fpr.  Lower fuel consumption (low loads, low engine rpm) will create more fuel pressure at the pump.

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13 hours ago, mindspin311 said:

Right, thats the plan, but do I need to size a pump properly? If I choose a pump thats putting out 100psi or greater the entire time and I'm relying on the FPR to do its job 100% of the time, is that good for the FPR?

Volume/flow is more important than the max pressure. You need to be able to flow enough volume of fuel for the engine HP. Return less type system normally has the regulator, that maintains the system pressure, located at/in the tank so the fuel that is not needed is returned directly to the tank by the regulator. If you are going to redesign your fuel system you need a fuel pump capable of fuel flow for your HP and a fuel pressure regulator capable of the needed flow and required pressure. Using a surge tank just requires a pump to provide the needed flow. Pressure is not regulated as you should have an open path from the surge tank to the tank. 

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4 minutes ago, hcsi99 said:

I have seen that. BMW has a few similar items. My concern is the regulator pressure. I figure the Mini regulator is set at it's pressure for a reason. The Vette piece is a lower pressure by about 15psi.

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40 minutes ago, wvumtnbkr said:

You can buy an aftermarket fpr and install that.  Most of them are adjustable.

 

Or, just install the stock mini one.

The OEM Mini is built in to the fuel filter assembly that goes in the tank. I have discovered that the E46 M3 FPR is rated at 5.0 bar vs 5.1 bar for the Mini. So I may be able to retrofit that filter/FPR combo as it is designed to be external to the tank. I can't imagine a 0.1bar different will cause much issue.

22 minutes ago, TimS said:

You said you were using Radium products. They have it integrated into the surge tank. Not a bargain piece but all in one spot. 
http://www.radiumauto.com/FST-R-Fuel-Surge-Tank-with-Integrated-FPR-P348.aspx

We have the radium surge that goes internal to the fuel cell. Slick setup that really simplifies things, with a co$t haha

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21 minutes ago, mindspin311 said:

The OEM Mini is built in to the fuel filter assembly that goes in the tank. I have discovered that the E46 M3 FPR is rated at 5.0 bar vs 5.1 bar for the Mini. So I may be able to retrofit that filter/FPR combo as it is designed to be external to the tank. I can't imagine a 0.1bar different will cause much issue.

If you have the radium surge, just buy the FPR for it.  Nice and tidy and you can set to whatever pressure you want.  Do you monitor FP?  How does excess fuel return from surge side?  Where is your filter/filters?

Edited by Hurljohn
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Answering some of these questions in no particular order.  If I forgot any that you want answered, ask again and I'll get to them.

 

1) Yes, you can use the E46 regulator with your system, the ECM will adjust the inlet opening of the HP pump to account for the 0.1 bar difference.

2) If you deadhead a pump, it will put out more than the "rated" pressure, and it will not be happy.

3) PWM is used on the low side pump in a returnless system to reduce heat buildup in the tank.  Heating up the fuel creates excess fuel vapors that have to be handled by the EVAP emissions system, so anything that can be done to reduce the heat is a really good thing.  It also reduces the electrical load of the system which is a bonus.

4) Feeding the HP pump with something close to the OEM pressure will make it happy.  Feeding 15psi to a HP pump wanting 75psi will cause it to be unable to deliver the required fuel flow under higher load, higher RPM conditions.

5) just because the graph ends at 70, doesn't mean that the pump will not put out more pressure, it just does not like to. 

6) Do not use the pressure relief as a regulator.  It is not designed to do that.  It is strictly there to protect the rest of the fuel system from a failed regulator.

 

 

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34 minutes ago, mindspin311 said:

The OEM Mini is built in to the fuel filter assembly that goes in the tank. I have discovered that the E46 M3 FPR is rated at 5.0 bar vs 5.1 bar for the Mini. So I may be able to retrofit that filter/FPR combo as it is designed to be external to the tank. I can't imagine a 0.1bar different will cause much issue.

We have the radium surge that goes internal to the fuel cell. Slick setup that really simplifies things, with a co$t haha

I have a 5 bar m3 FPR assembly that came off when I replaced it with a 3 bar one for my car.

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1 hour ago, Robmink said:

Answering some of these questions in no particular order.  If I forgot any that you want answered, ask again and I'll get to them.

 

 

3) PWM is used on the low side pump in a returnless system to reduce heat buildup in the tank.  Heating up the fuel creates excess fuel vapors that have to be handled by the EVAP emissions system, so anything that can be done to reduce the heat is a really good thing.  It also reduces the electrical load of the system which is a bonus.

Is there a way to definitively confirm whether the pump is PWM controlled? I can't find anything that say it is, also can't find anything that says it isn't.

 

All I know is that there's a relay in the main fuse panel that provides power to the pump.

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