Just for some clarification, the spring in the oil pump is the pressure relief spring. Changing this spring changes the peak oil pressure not the oil pressure over all. For example a stiffer spring (or shimmed spring) will increase the oil pressure when the engine is cold and at high rpm.

It will not increase the oil pressure at idle.

Also, measuring the volt drop at the pump is fairly pointless, the 'volt drop' will depend on the current being drawn by the pump which clearly changes with load. A better approach is to measure the resistance of the pump wiring. You can then predict the volt drop for a range of currents and hence pump loads.

To do this properly the resistance should be measured from the pump negative terminal to battery terminal, and from pump positive terminal to battery positive terminal obviously with the pump relay active.

Add the two resistances together (they are in series) and from here you can see if you have a problem.

V = IR, if you have a total resistance in the supply path of 0.5ohms and you predict your pump draws 10A a full load you would do the following:

V = 10*0.5 = 5V so the voltage at the pump would be battery voltage (14V) minus the voltage drop across the resistance = 9V.

It is likely that your fuel pump will not have a current draw of 10A, this is probably more like 6A (you can measure this with a multimeter) and the series resistance would ideally be less than 0.2 ohms.

Again - V=IR so V(drop) = 6*0.2 = 1.2V resulting in a pump voltage of 12.8V.

Interestingly a Bosch 044 pump is rated at 12.8V, with a flow of 4.6L/min at 3.0bar drawing 11amps equating to 700 ish bhp potential.

Either way, my point is that the voltage drop is dependent upon the current being drawn and total series resistance, not on some figure spouted on the internet. Measuring a voltage drop at idle on one car is totally not relevant. For that matter a 5V drop means nothing what so ever as long as the pump can still provide enough fuel to maintain the set pressure of 3-3.5 bar.

A better test would be to install a fuel pressure gauge and to make sure the pressure didn't drop at high loads, replacing the pump wiring for no reason is just stupid.

Motor speed is dictated by voltage, motor torque by current. Pump flow is proportional to pump speed and hence voltage. An 044 at half speed and half flow is still enough for 300bhp at 6.4V assuming enough current can be drawn to maintain pressure. So all this internet scare mungering about pump wiring is mostly rubbish on cars with lower power figures. Unless of course you are intending to make 400bhp with one pump, in which case you should be more concerned.

Rob,