What the Flow table is
For each injection stage, the Flow table holds the flow rate of the injector (or injectors) on each output. If you have two injectors sharing one output, enter their combined flow, because that's what the ECU is actually switching.
The numbers usually come from the injector's data sheet or supplier. NSP shows flow in cc/min by default. If you'd rather work in lb/hr, change the Fuel Volume unit under Tools > Units Options.
Why one flow number isn't enough
An injector's rating is only true at one pressure. A spec like "1000 cc/min at 3 bar (43.5 psi)" means that's what it flows when the fuel pressure is 3 bar higher than the pressure in the manifold it sprays into. That difference is the pressure differential, and when it changes, so does flow. As a rule of thumb, flow rises and falls with the square root of the differential, so doubling the pressure gives roughly 40% more flow, not twice as much.
That's why we enter flow at several differential pressures, not just one. Plenty of free online calculators will work out the extra points from a single known rating. With the table populated this way, the ECU can keep fueling accurate as manifold pressure and fuel pressure move around.
How the ECU works out the differential
NSP calculates it differently depending on whether you have a fuel pressure sensor and how Fuel Pressure Input Type is set:
| Fuel pressure sensor? | Input type | Differential used | Typical system |
|---|---|---|---|
| No | MAP Reference | Base Fuel Pressure | Return system with a manifold-referenced regulator, so the differential stays constant |
| No | Constant | Base Fuel Pressure minus manifold pressure | Returnless or unreferenced regulator, where rail pressure is fixed |
| Yes | Either | Measured fuel pressure minus manifold pressure | Any system with a sensor |
Fit a fuel pressure sensor if you can. The ECU then uses the real differential instead of assuming it, and you can log and set up protection for the moment the pump or regulator starts to fall behind.
Common mistakes
- Entering a single injector's flow when two share an output.
- Using a flow rating measured at a different pressure from the one your system runs.
- Setting the input type to MAP Reference on a returnless system, which makes the ECU assume a constant differential that doesn't exist and leaves the engine lean on boost.
Flow works hand in hand with dead time and the short pulse width adder. Get all three right and your fuel table stays close to the truth. See also flow percentage and injection stages.
