Diesels are most commonly setup with regulator on the return. This provides the maximum amount of fuel for lubrication, cooling and burn. It should also cut down on the amount of introduced bubbles into the flow for a more steady a/f mixture. Our Army's combat equipment uses a small clacker style pump (sometimes dual pumps) to feed the high pressure pump which feeds the injectors that, in a Cat 3126 or bigger, fires at 3500psi. Where you measure fuel pressure is just as important as where your regulator is I would say. Keeping tabs on inlet AND outlet pressure may be a good idea.
3126 uses a "HEUI" injector that fires the injector based on ECM input and uses oil pressure to actually make injector start (on a C7 the injection event is commanded on and off, 3126 is more like a PowerStroke or Navistar injector where the injection event is stopped by return spring pressure on the oil side), not fuel pressure. A 3126/C7 should run a consistent fuel pressure and also uses a regulator on the return on most applications. The ECM will change HEUI pressure based on temp, load, RPM, etc. etc. However, a 3126/C7 will run anywhere from about 650psi to 5000psi of engine oil pressure and still fire the injector.
a 3116 Cat uses a mechanical unit injector that is VERY much like an old skool 2 stroke Detroit injector. In this case the injection starts based on cam timing and what pressure the nozzle valve opens at depends on
This is slightly different than in a pump and nozzle diesel injection system. In this case, the nozzle only cares about what pressure it sees from the distributor head on the pump and when the pressure rise over comes nozzle spring pressure (commonly called the pop pressure in the fuel shop)
Eletronic unit injectors or high pressure common rail stuff is sort of a combo of both of the above.
Also on a lot of diesel's, the fixed orifice on the return side does a couple things. In a mechanical pump is may be the pressure regulator to control supply pressure. In a lot of newer stuff (last 25 years or so) that orifice is used for erosion control due to the ever increasing pressures.
A regulated return helps maintain consistent fuel temperature and helps keep aeration down (this is more so a problem with diesel and derivates). A dead end system tends to heat soak the fuel, although in new production cars composites and better heat management have helped mitigate the problem. If you have a small fuel cell and want to do a lot of street miles I've run a regulated return with a cooler on the return to keep from boiling the gas. Works with carbs, although you can't do much if it's the fuel bowls themselves that are getting heat soaked.
