# 750HO E85 Fuel System - Are my calculations and ideas sound? Hope you can help.....



## 6east (Jun 29, 2013)

*750HP E85 Fuel System - Are my calculations and ideas sound? Hope you can help.....*

Hey ,

Okay so one of the upgrades I am currently performing on my 05 '6 is the fuel system so I can if decided tune with E85.

I've pretty much worked out what flow my pump(s) will need to maintain for the target horsepower and I will provide this information later but the question I have is about reducing the base fuel pressure and if its viable?

The base pressure on this and I believe other FI Audis is 4.0Bar (58psi), as you may know when calculating fuel needs pressure plays a big part as the pumps must flow the required fuel at the base pressure plus any boost which in this case will be 1.5-1.8ar so I have rounded up to 2.0Bar as I have with all calculations to allow headroom. 

The question I have is why is the bad pressure 4.0Bar instead of 3.0Bar? Most if not all efi parts are rated at 3.0Bar.

If I was able to reduce the base pressure to 3.0Bar by changing to a 3.0Bar FPR this would allow me to simply the fuel pumps greatly as the flow required would need to be achieved at 5Bar (72.5psi) instead of 6Bar (87psi) which my calculations and research have shown is right on the boarder.

Can anyone please advise if lowering the base fuel pressure will result in any problems? I understand the tune will need to be modified which is fine as it was going to be extensively tuned anyway but I am more concerned about any detrimental effects this would have in terms or power, reliability etc.

Based on the target horsepower of 750hp and 900bp while running E85 and the following specs:

4.2Lt 40V (5V Per) 
7000rpm redline
Base FP 4.0Bar
Boost 2.0 (1.5-1.8 Avg)
880cc Siemens Injectors 

I used the calculators here: http://www.deatschwerks.com/resources/fuel-calculators/ and came up with the following:

E85 Needed = 6400cc/min (20% Headroom)
injector = 800cc/min @3Bar (80% DC, 20% Headroom)
Fuel Pump = 360LPH @6Bar (20% Headroom, 4bar FP plus 2Bar boost)

Now looking at the flow chart for the Walbro 465 shown here: http://3.bp.blogspot.com/-NJWhwL9iI...I/Ki-1sVY9nww/s1600/DCSS-405-vs-465-13.5v.jpg 

If I was to lower the base pressure this pump can supply more than enough fuel to satisfy my calculations, that is based on the assumption they are correct.

If I was to remain the base pressure this pump would fall short of satisfying my calculated flow requirements, I will be able to increas its flow by using a Fuel Pump Booster by running the pump at 17.5V.

So sorry for the long post but I was hoping that I would find some help and advise to determine if either of the following are viable:

1) Reduce base pressure to 3.0bar and install single Walbro DCSS465.
2) Retain base pressure at 4.0bar and install single a Walbro DCSS465 and voltage booster to increase flow.

I also assume I will need to upgrade the feed and return lines, I was planning to using a -6 (3/8") FEED and -8 (1/2") RETURN.

Thanks in advance!


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