Two-stage injector's fueling theory...
I have found that the operation of the unit PD injector and the mechanical VE share a similar fueling characteristic. Both injectors receive all of the needed fuel from the pilot pop (NOP1) injection for idling the engine and only require the second stage for performance levels about 2x idle speed.
How I deduced this starts with problem with second stage for Pumpe Duese injectors, '04-'06, BEW, BRM and BHW in the US. These nozzles have a bad habit of losing the main stage (NOP2) fueling caused by the injector's solenoid device failing.
One way the fueling can fail is if the injectors are left out of the engine for an extended period of time, especially for a couple of hot summer weeks, the solenoid will glue shut if not stored submersed in diesel. This can be a very expensive mistake, with little recourse, but to have the injector rebuilt. But the point is, when the injector's second stage fails, the engine will start easily, idle well, but run very poorly... little better than high idle.
This is what we theorized is happening. The pilot pop (NOP1) in the PD injectors is mechanically provided, just like a VE injector. The main injection for PD nozzles is electronically controlled by the solenoid. So, the injector, if damaged in this manner, will start the engine instantly and idles very smoothly. By that effect, we can determine NOP1 is sufficient to run a PD motor at idle, plus a little bit more. But when the engine gets above around 1500 rpm, the second stage must provide a volume of fuel or the engine will not perform above about 25-30mph.
For two-stage VE injectors (AHU-ALH), the same theory applies. NOP1 is what makes your car start and idle. There are two separate shims for each stage of the injector. The setting of NOP1 is relatively easy to achieve and can get the engine to start and idle smoothly. Years ago, we proved the effectiveness of using the VCDS and idle balance in block 13 to perform a test, comparing one injector's values against another. As long as the pop pressures were in the realm of OEM stock pressure settings (220 bar), this test would show differences in idle fuel volume between injectors. It turns out, we find VCDS can also be manipulated to read NOP2.
About 2 years ago, we received a set of injectors where the fuel economy (FE) dropped dramatically on a set of 764 nozzles. The idle balance numbers at 903-907 rpm looked good. But when the engine speed was increased, the idle balance numbers ran out of range, with one injector showing -1.6 and the rest of them slightly positive reads. The injector reading the negative amount is over-fueling and the idle balance program is attempting to decrease the fuel to that injector. The pop pressure MUST be adjusted to provide the correct fuel flow amount. It is not pop pressure, but FLOW VOLUME that is most important. We see some companies that provide information for NOP1 and NOP2 values, but 1) the test is done into atmospheric pressure; not cylinder pressure and 2) the company specifically states FLOW VOLUME is NOT warranted. That has become a very serious issue with some of our customers. With NOP2 poorly set, we have seen pistons melted or cracked from NOP2 overfueling.
So, what we found using VCDS for block 13 (Idle balance compensation) and block 15 (liter per hour fuel usage..), the idle balance can be raised somewhere between 1500-1600 rpm and the idle balance program in VCDS will still operate and display balance numbers. Above that rpm, the injectors better be set for the correct flow, or at very least, the engine is wasting fuel; at performance levels, melting a piston. The method allows a peek at the NOP2 numbers. If the high idle numbers change dramatically compared to 903-907 rpm idle speed, the NOP2 imbalance is the cause.
Our method is to install newly shimmed injectors in our operating engine with cylinder compression 500 psi across the board, with our ported cylinder head, our Stage II cam, VNT-17 Garret turbo and RC 4. We think pushing the injectors a bit more will exaggerate any variation between injectors.
Using the high idle setting, we re-shim the nozzles second stage for fuel volume, based on reported high- idle balance numbers in Block 13. This may take some iterations, as changing one nozzle’s flow will affect the relative value of all four.
By doing this process, we have seen FE numbers climb, smoother injector fuel delivery and performance improve, sometimes dramatically. The end result is nozzle balanced in actual operation, rather than emulations.