Fuel Pressure for a 3.5L MKV

Hello friends,

Would someone be able to tell me what the ideal fuel pressure ought to be for a standard set up 3.5L 1951 MKV, please? The carburettors are in good order and have the mixtures accurately set.



Hi Tim,

I don’t know the answer to your question but as I understand it SU made two types of pump.

  1. A low pressure one for use when mounted in the engine compartment sucking fuel from the tank.
  2. A high pressure pump for use when mounted near the tank and pushing fuel through the fuel line.

The only difference between the LP and HP pumps is the strength of the diaphragm spring so if you have the wrong type it is very easy to convert it to the right type. My car should have an LP but I converted an HP by this means so as to get the later style double contacts that are more reliable. It works very well.


Hello Peter,
It’s the original square bodied type pump mounted to the chassis under the car. It’s the high pressure version. I’ll call the SU distributor tomorrow and ask.
I’m going to fit a fuel pressure regulator to the car which incorporates a filter. It has a gauge which can be fitted to show the pressure which might be handy. It also keeps the correct pressure in the line to the carburettors.

Best wishes,

You’re looking around pressures of 2 to 3 psi.

You don’t need a pressure regulator. In the SU carb system, all you care about is volume, i.e. gallons per hour or cc’s per minute.
The square body pump is known as the PP31 LCS or Large Capacity Single.
The correct volume measurement is given in the Mark V Service Manual on page B40.
That’s 947 cc’s or almost a liter in a minute, or fill a coffee cup in 10 seconds.
There is a filter inside the pump, and another in the fuel tank drain plug, and two more inside the carb float bowl inlets.
I usually just drain the tank once a year and clean out that drain plug filter.

Hi Rob,

I’ve cleaned all those brass filters but will still fit the regulator to monitor the flow and have the paper filter as added protection. As present there is an unsightly inline filter so this unit will look much better and serve a purpose as I can see what’s in the glass bowl.


The Mark V has the fuel pump on the frame some distance, h, vertically below the carbs. The minimum pump pressure must lift the fuel up that vertical distance. The minimum fuel pressure differential is the mass density of the fuel, rho, times the acceleration of gravity at earth’s surface, g, times the vertical lift differential, h. I am away from documents with specific manufacturer numbers but supposing rho=750 kg per cubic meter, g=9.8 meters per second squared, h=0.4 m then minimum fuel differential at pump output is a bit under 0.5 psi. A bit more pressure is required to achieve needed flow at carbs.

My recollection for the Mark V is total pump output pressure differential should be about 2 psi and never go over 3 psi. Pressures a little over 2 psi will cause the needle valves on the float input to jiggle free from sealing causing too rich low engine rpms.

If one puts a pressure gauge up carb height it should read over a calibrated 0 to 2 psi range.

Also it is good to have extra fuel filter in the line. My car has it between fuel tank and pump on the frame under car to keep any sludge or grit from getting into the pump.