Fuel Pressure and Flow

[TOL-S911]

Expert advise is required with regards to MFI fuel releif valve on fuel filter. According to previous threads I understand that the fuel pressure to the MFI pump is regulated by the releif valve attached to the fuel filter. My releif valve holds 9.5 psi so I desided to change it out. My local Porsche dealer says he can get a new one from Germany for about $200.00. When asked is 9 psi is good enough the parts guy checked with their semi-retired air engine mechanic who has over 40 years experience and the reply was that in all his years he never had to change out the relief valve. They did not say if 9.5 psi was okay or to low. My valve has 09 stamped on the side and I am assuming that this means 0.9 atmosphere or 0.9 x 14.2 = 12.7 psi which equates to the tunning specification.

Is my calculation correct?

I have replaced the releif valve with a used one which has the same 09 stamped on the side and it gave me a pressure of 9.5 psi also. The pump has plenty of pressure. When the discharge line behind the releif valve is pinched the pressue increases to 30 psi.

I am also wondering if the version of MFI makes a difference. My system for the 73 Carrera RS and has the last version of MFI pump Bosch Number 0408 126 019. All the lines and filters including the fuel tank strainer are new this year due to rot of original fuel lines. My engine runs good.

Could it be that the pressure was lowered to avoid fuel seeping into the oil or oil seaping into the fuel causing the oil burn off on start up which most operators say is normal?

Am I okay with 9.5 psi?

All opinions expert or otherwise are most appreciated.

TOL-S911
New member this year

[304065]

No, pressure in the loop between the outlet of the fuel filter on the bottom and the inlet on the MFI pump should be 1 bar. The relief valve is in the fuel pump, not in the filter console.

http://forums.pelicanparts.com/showt...t=ultimate+MFI

[TOL-S911]

There is another relief valve on the filter console which regulates the pressure to the MFI pump. This is the valve that I am questioning? My releif valve on the fuel pump is working at 2 bar (28 psi).

I had a long look at the Grady Clay "Replacing Fuel Line " thread and am greatful to all who have contributed to the thread.

Not much was stated about the specification of the fuel console relief valve. Are different specification valves available? I now have two with the same relief pressure and am not sure if I should order the third.

TOL-S911

[jcge]

Some references on the filter unit


Filter unit

Translation : Overflow valve, opening pressure 0,6kp/cm2 (= 0,6 bar)

Assembly drawing

Parts list

Pressure valve is #93 in the drawing (Bosch part number 1 417 413 009)

The marking "09" on your valve is the abbreviation (last 2 digits) of the part number.

Bosch list the following OE number for Porsche 901 110 915 00 - according to their online catalogue, it's still available.

Regards,
John

[jcge]

Here's the reference on the electric pump (Bosch part 0 580 970 001)

0 580 970 001

Capacity = 1 bar

Note that there is (sometimes) a restriction orifice inside one of the return fuel lines (near the "T" - see this post on Pelican

7.5mm OD x 10mm L x 1mm ID

As you have observed, restricting the return line will increase the system pressure to the point where the electric pump (internal) relief valve operates.

Regards
John

[jcge]

Testing the pressure (bottom picture posted on Pelican by Grady Clay)