[diesel_john]

diesel timing lights use a small piezo- electric ring which clamps around the injector line and actually outputs a charge signal during the expansion of the injector line when the high pressure builds up prior to injector pop off. I was able to view the entire injection pulse on a scope. gas grill igniters, knock sensors all use the same technology.

But the the pump rpm and throttle position is the best for a non turbo'd, non governed diesel.
The turbo'd and speed governed have mechanisms within the pump that change fuel delivery independant of accelerator position. on the turbo if you disconnect the intake manifold pressure feedback to the injector pump the pump won't know that the turbo is producing more pressure and it will not increase the fuel above the accelerator positon.

An interesting footnote to the differences in gas and diesel was observed the other day. When i go down a hill to pick up speed with the diesel, i have to let off the accel. pedal or the engine uses more fuel as the rpm increases. Mileage does not increase like a gas engine, during vacuum increase. And in fact decreases because of more aero loss.