Theory Question: Higher Low Fuel Pressure equates to higher High Fuel Pressure?

proboner

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Sep 13, 2020
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So I'm in the process of trying to extract every last bit out of the DI system. Drilling out the fuel rail, removing the HPFP filter, completely replacing the lines from the tank up to the HPFP.

It has me wondering. I currently run a Walbro 450 and at high RPM in 3rd gear the Low Pressure side is dropping down to about 65psi average. Saw a recent post in here making great power on DI with same blend I plan to run (e30) and when I look at those logs I see the Low Pressure side is up closer to 85psi. Is that providing an advantage on the high pressure side? Would upgrading to something like a Walbro 535 achieve an actual increase in pressure, or would it just raise my flow headroom on the low side?
 

Torgus

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Personally I think maxing out what the DI system can do is a bad idea. You always want to leave headroom on the table, especially with fueling or one hiccup and your cylinder(s) goes lean and you pop the engine. That being said...

If you want more pressure you need to add an external fuel pressure regulator. I would go boost referenced but as you are not using PI you don't need it per se. Then set it for 80psi for instance. However most fuel pumps flow less as pressure increases(and amperage draw increases) you will want to keep this in mind as you raise the pressure:

F90000285-Flow-Chart.jpg



My vote would be to replace the driver's side top hat with somethin like the PR top hat for starters to remove that restriction. It also makes adding in a FPR and fuel lines easier. Depending on your HP goals I would not replace the fuel pump, but add in a second. As you are going to increase the pressure your flow will drop. Two pumps should give you all the fueling you will likely ever need. Or just stick with your current pump until you run out of flow/pressure and then add in a second.

You are better starting out with higher fuel pressure hitting the HPFP than lower. I don't think it will make a large difference stock is 72psi you are just going up 8psi to 80 for instance or 13psi @ 85. They key is being able to keep this exact fuel pressure up the entire RPM band under full load. Ideally you don't want it dropping much if you can avoid it. So to your question will it increase the HPFP's pressure, not likely in any meaningful way that will effect performance. However, keeping the fuel pressure up and stable the entire run is optimal.


Edit: I think all of the work and $$$ spent will not be worth the relatively small gains(if any) you will see. But you do you. I would just add PI and leave HPFP & DI headroom on the table. Safer and it will also clean your valves and allow you to run full E85.
 
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proboner

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Sep 13, 2020
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Appreciate the response!

To note, I'm trying to extract most out of the DI system, not push it to the ragged edge or reliability. I'd generally consider extracting the most as making as much power as possible while still maintaining at least 1500psi rail pressure across the rev range. Obviously much more power can be made in other ways, but it's just opening a whole new can of worms (especially from a cost perspective) that I'm trying not to get into. Maybe in the future I'll want to go there, but for today I'm trying to limit myself.

Given the context you've provided, I think I'll just stick with my 450 for now... sounds like we're already at diminishing returns trying to go beyond this. I wish I could use the PR top hat, but I run a Z4 and it's not compatible. Sigh.
 

martymil

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The top hat is not the issue unless your aiming for 800hp plus.

The biggest problems of the fueling system are between the top hat and the hpfp inc removing the inlet filter on the hpfp.

Plenty of nice upgrades from many manufacturers, take your pick.

You will never max out your di system as you dont run a double barrel and even then.

Get a POD from PFS for your hpfp and it will serve you well and beyond of what your block will handle whilst keeping your di
system happy and within range.
 

proboner

Specialist
Sep 13, 2020
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The top hat is not the issue unless your aiming for 800hp plus.

The biggest problems of the fueling system are between the top hat and the hpfp inc removing the inlet filter on the hpfp.

Plenty of nice upgrades from many manufacturers, take your pick.

You will never max out your di system as you dont run a double barrel and even then.

Get a POD from PFS for your hpfp and it will serve you well and beyond of what your block will handle whilst keeping your di
system happy and within range.
Yup, I'm already solving for those problems and building a custom setup. No one builds something out of the box for the Z4, as far as I'm aware.

Not looking to go POD. Already afraid of my crank hub spinning as it is, let alone putting extra load on it by add a gear multiplier at the HPFP. And then I need to start looking for a new clutch as well, to hold the power potential there. It's all just way more time and money, that I'm not (yet) willing to spend.
 
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martymil

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Just get a spline lock and be done with it, at that power level its only a matter of time not if, if you run a pod or not.

Not one failure to date here in oz and every engine build here gets one.