Originally Posted by Shabodah
I think you're confusing diesel direct injection (16-24:1 compression) to gasoline direct injection (8.5-11:1).
Edit: but looking at how the duramax has been tuned is a good idea to determine where to go with this.
Nope, diesel direct injection and gasoline direct injection share the same type of mechanics in fuel/air injection. In both, the fuel injectors are have to feed the fuel directly in the cylinders, where air is already shoved in with a pretty high pressure. In order to overcome that pressure and shoot the fuel where it needs to go, a much greater pressure on the magnitudes of 1000s bars is needed.
I'm not exactly sure when the fuel is injected in this Ecotec, whether its simultaneously injected when the intake valves are open and air is rushing in, or if it's right after the valves close and the piston compression stroke is already coming up. Either way, the fuel has to have enough of a pressure and trajectory to aim at the different type of piston head in this engine so that it can mix with the air properly in the cylinder (like a diesel, a bowl shaped head).
That brings me to my next point with your compression ratios. Yes you are right with the gasoline (8.5:1-11:1) and diesel (16:1-24:1), those ranges can very with different engines too. But the compression ratio doesn't determine the fuel pressure, the type of mixing and air/fuel ratio does. You see, you're used to our pre-mixed port injection Ecotecs where the fuel and air mix together BEFORE they enter the cylinders. This makes the air/fuel mixture homogeneous (a constant, even mixture) all throughout the cylinder.
Direct injection (in both gasoline and diesel) have a stratified charge of fuel/air in the cylinder, which is differing fuel/air ratios all throughout the cylinder. The mixture will be near the 14.6:1 near the spark plug for proper ignition, but since the rest of the fuel and air mix in the cylinder during the piston's upward compression stroke, the mixture will not be even like that of our Ecotecs.
THIS is the reason for the different piston head design, so that the injected fuel aimed at the piston head bounces off and mixes properly near the spark plug for proper ignition. THIS is the reason the fuel injected needs to be at such a high pressure, so that the fuel is in a turbulent state which allows for better mixing with the air. And THIS is the reason why tuning will be harder: any change in the fuel flow will cause the mixing to be a lot different than designed for, and can cause improper ignition. So people really have to know what they're controlling in order for the tuning to work right. A lot of light will have to come from diesel tuning if it is to be done properly because this is a much different field of engines that most people are used to.
Wow sorry for the long explanation, but a lot of people aren't going to understand this direct injection like this. I figure I'd spread a little bit of what I've been learning in my internal combustion engines class