Defi's control unit II has an option to read "Differential pressure" with the use of both boost and fuel gauges. What is this info usefull for? Is it at all like an A/F? Thanks for the help!

 

This is the reply I got from Defi on the question:

"Differential pressure" is a value which subtracted the value of pressure in
manifold from that of fuel pressure.
It is useful to know how the condition of fuel pump is.

Does this make sense to anyone?

 

So nobody uses this function? My guess then would be that its not usefull information if nobody uses it.

 

When you raise the boost, the fuel pressure should also increase at the same rate, maintaining the pressure differential.

Most fuel pressure regulators are designed to maintain a differential supply of 3 bar (43.5 psi) of fuel pressure.

In my car, when it's idling, the fuel pressure shows 2.5 bar. Boost gauge is -0.5 bar (vacuum). Thus the pressure differential is maintained (2.5 - (-0.5) = 3 bar). If I'm making 1 bar of boost, then my fuel pressure should be 4.0 bar (3 bar pressure differential + 1 bar of boost). If I'm making 2 bar of boost, then it means the fuel pressure should show 5 bar. Hope that makes sense.

 

That makes perfect sense, thanks for the reply!

 

In my experience, fuel pressure can rise faster than boost pressure, so differential pressure can spike. For example, with my Walbro, there is a spike in fuel pressure from 43psi to 51psi even under moderate loads with little to no boost. From what I have gathered from others, this is from the ECU raising the voltage to the fuel pump?

So fuel pressure may not always rise 1:1, but the important thing is probably that differential pressure not fall while load is high/RPM is increasing.