Why are there so many injector timing settings h gas l gas


The injector pushrod rides on the cam lobe all of the time, there is no clearance the way there is with a valve pushrod. When you set the timing on the injector you are setting the static height of a plunger inside the injector. The plunger is in a chamber that is full of fuel. When the cam lobe pushes the plunger down, it covers up a fuel port and now the injector can develop pressure to fire the fuel into the chamber. When you set the timing height you move the plunger closer or farther from the fuel port, so you are setting how fast the injector begins to develop pressure to fire after the cam lobe starts to move the plunger. A smaller number on the timing gauge will fire faster than a larger number, in other words it will be more advanced and a larger timing gauge number will be more retarded. As a point of interest my manual has two different settings for brown tag N65’s – 1.484 for advanced timing on the cam gear and 1.460 for standard timing on the cam gear. As you would expect, with standard timing they add in more injector advance with a tighter static height.

Each type and specification of injector will have different internal gubbins that control fuel volume, where the fuel ports are, the profile of the plunger, the needle valve spring setting, the size of the spray tip. As the control rod moves it rotates the plunger and changes where the plunger covers the port and so changes the volume of fuel and the timing of when the injector fires. By changing the static timing height you can change or optimize the injection event for different engine operating conditions. If you take TomC’s case the engine was timed – both static timing and internal injector settings, I have no doubt – so that the engine ran as efficiently as possible at 1800 rpm. You could have a slightly different setting and get optimum performance at 1300 rpm, or at 2200 rpm, and you would want to change it for turbo pressure, etc. There are also settings that minimize pollution, soot, smoke, and those would almost certainly be different than settings for maximum power or fuel efficiency. Advancing or retarding the injection event changes cylinder pressure, heat production, when the fuel burn starts and finishes. Other changes to the injectors change the volume of fuel and the duration of the injection event – all have different effects. Setting the cam gears to advanced or standard timing not only changes the base timing of the injector, it also changes the timing of the exhaust valves. Advancing the exhaust valve timing opens the exhaust valve sooner, possibly robbing the cylinder of some useful pressure to contribute to the crankshaft as power, and closes the exhaust valve sooner, probably raising the dynamic compression ratio and allowing more power to be developed, but again potentially increasing cylinder heat. It’s all a balance and there are many many variables.

When the rack moves the control rack of the injector it rotates the plunger, which has a profile built into it so that it can vary the volume of fuel that it moves in each stroke. It does that by first covering the lower fuel port so pressure can build, then when it covers the upper fuel port it locks the volume of fuel that it has captured and is going to inject. When the rack is set to no-fuel, it basically leaves the lower port open and so never captures any fuel at all. When an injector sticks, what usually happens is the plunger sticks so it can’t rotate and so will freeze the control rack. While the static timing height controls the range of timing that the injector can have, the actual timing is set by when the plunger actually closes off both ports and develops the pressure to open the needle valve and actually inject the fuel. The profile of the plunger can be varies so that as the control rack is moved and the fuel volume changes the injection start time is constant and the stop time varies, the start time varies and the stop time is constant, or both the start and stop times vary. What you can’t have is both the start and stop times constant because then there would be no way to vary the fuel volume. Unlike a gas engine, which wants an air\fuel ratio to be kept very close to 14:1, a diesel engine can have widely varying air/fuel ratios. You basically adjust it’s power output by varying the amount of fuel injected from very little to a lot, while the air volume stays relatively constant subject to blower and turbo boost changing a bit. If you get more fuel into the engine than can be burned with the amount of air you get black smoke from unburned fuel. You then have choices – you can de-fuel a bit to eliminate the smoke, you can advance the injection timing to give the fuel more time to burn at the risk of melting a piston, or you can add more air with a bigger blower, or more turbo boost.

If I had to hazard a guess, I would say that the 1.484 in 1982 was a pollution control setting, 1.460 was a efficient power setting, and 1.470 moved the power band slightly lower and reduced soot a bit. But I really don’t know why the different settings, I am just guessing. It also points out that the engine will run fine with a variety of different settings, what is more important is to have them all the same, and that since the actual effective timing is highly dependent on the control rack position having all the control racks synchronized ("running the rack") is of significant importance.

BTW the reason the setting is given as 1.460" rather than 1.46 or 1.460000 has to do with precision. 1.460" is precise to 1/1000" while 1.46 is only precise to 1/100". In other words, 1.46 can be equally accurate at 1.455 or 1.465". 1.460000" would need to be measured with a laser and at a set temperature – varying the temperature a few degrees would change the measurement. So by saying 1.460" they are telling you that you need to measure with a technique capable of being accurate to one thou, which is quite achievable with a go/no-go gauge like the timing pin. Quite hard with a dial caliper or a depth micrometer, to be honest.