Re: DP Tuner/Injector question?
If what you're hearing is timing advance...
The sound you hear is caused by injection delay actually. If you realize that a "diesel" engine is a compression ignition engine, then you see that the pressure/heat from the compression of the intake air charge approaching TDC is what ignites the fuel when the injection event takes place. Well, the pressure/heat can be expected to be less and less the farther and farther in advance of TDC you are. Meaning the cylinder is colder.
Is your truck louder when it's cold as hell and you just cranked it and started driving off, or when it's fully up to temp? Obviously when the cylinders are cold.
This is because when the diesel is injected, the cylinder environment is too cold to rapidly ignite the fuel. And while the fuel does burn, it does so at a very slow rate, allowing more diesel to be injected per time, than fuel being burned. It then fires off, making your engine sound noisy, and even emitting blue smoke when the ultimate temperature never gets high enough for complete combustion, such as on a cold engine, or one running severely reduced compression. What you hear, is the result of a cold cylinder not immediately burning off the fuel at or near the rate at which it's being injected. Because of this, you get a pocket of unburned fuel developing that will finally tend to "flash" off as you approach TDC and the pressure/temperature finally gets this pocket of fuel to "pop". And you hear it. This is not the injectors you're hearing, although they do make their own noise, no doubt.
When you inject the fuel at increasing amounts of advance, you are doing so into a colder and colder cylinder. Effectively increasing the delay between start of injection and rapid combustion. And in so doing, creating a larger and larger volume of fuel as of yet unburned.
Long story short, the sound you hear is the rapid combustion of a pocket of fuel. If advanced far enough, a drop in cylinder pressure will be noted, followed by a VERY large rise in cylinder pressure accompanying the "flash" of burning diesel that had collected when the cylinder was too cold to rapidly burn it.
Maybe that makes sense to you, or maybe not. But I can see no better explanation for the data I have seen and gathered myself.