I thought I would start a thread on VP injectors, instead of cluttering up the thread in the S03 forum.
There are two distinct types of injectors that are offered for these trucks. VCO and SAC. The original injectors were a VCO injector. In this type of injector the needle blocks all the holes in the nozzle to stop the flow. With a SAC injector, the needle blocks a passage to a small chamber (or sac), that has the injector holes in it.
There are advantages and disadvantages for each type of injector. With the original VCO style injector because the needle has to block all the holes, both on the opening and closing of the injector the start and stop of the injector flow is less precise. Any contamination in the nozzle makes the problem worse, and may keep the injector from sealing completely. The VCO style injector also opens slower, than a SAC injector. As the needle unseats, the pressure in the chamber does not increase as rapidly as it does in a SAC. This is the reason that with the same hole size and count a SAC injector will make more hp then a VCO. The affect of the needle not unseating/seating evenly is called needle drift. The fuel passage in the injector enters the fuel chamber from one side. This cause the needle to drift to one side as it open, which in a VCO causes some of the nozzle holes to unseat before others. VCO nozzles are generally less durable than a SAC nozzle. Some of the advantages of a VCO, are that it doesn't have the little sac of fuel in the nozzle, when the injector is closed. This fuel can cause idle haze in a SAC injector. The VCO nozzles are typically less expensive to produce.
SAC injectors... Originally these were mainly used in marine applications, and the previously mentioned left over fuel in the sac was not considered an issue. The later micro-blind SAC's it really isn't an issue either. Just as a side note, the 6.7 injectors are SAC. Advantages of SAC... The nozzles are typically more durable because the needle does not contact the area of the nozzle where the holes are. As holes get more numerous and larger, the VCO nozzle gets weaker. This durability allows for higher needle lift, and more tolerance for pop pressures. With higher needle lifts, when the injector closes the needle contacts the seat with more force. With a SAC injector, the seat area can handle this pressure better. A SAC injector will also deliver a shorter more concentrated spray pattern, which keeps the fuel more in the bowl of the piston. The piston bowls are smaller in a VP truck then a CR, and the angle is greater. The injection also starts quicker, and ends faster then a VCO injector. The advantage to this is that the SAC nozzle inherently has more advance (less ID), and flows more fuel for the same size of holes. That's why with everything else being equal, a SAC nozzle will make more hp, for the same size. While a VCO may idle with less smoke, because of the fuel in the sac, a SAC nozzle will typically run cleaner every place else.
Needle lift... There are two basic ways to get more fuel flow from a VP injector. Increasing the number and size of the holes, and increasing the needle lift. I personally like running injectors with more needle lift. You can keep the number and size of the holes smaller to get better fuel atomization and a more responsive opening and close. For example a company that has been making VP injectors since they first came out, has a 150hp, 200hp, and a 220hp injector that uses the same 7 hole, by .0985 orifice nozzle. The way they get the extra flow is by increasing needle lift (and a few other mods...). For a 220hp injector you get a very clean burn, easily tune able injector. These are SAC injectors. There is about a .010 difference in lift, between the low hp and high hp injector. If I were looking for a 220hp to 250hp injector I would much rather have one that has .0985 holes than one that has .014. Just my preference...
I hope this helps clear some things up, and I didn't just make things more confusing...