Originally Posted by Warpick
With out overextending too much the DI means that you're not taking air and fuel on the intake stroke. It's just air. So carbs are like air/fuel mixing centers port injection is shooting atomized fuel with the air and compressing it all on the compression stroke.
Fuel is a liquid and can't be compressed as much as air. So with direct injection. You have nothing but air being compressed and as the air is compressed you have all that pressure in the chamber and THEN the fuel is injected and ignited.
Compression needs to be payed attention to, but it's not as important on carb or port injection methods. Which is why you can run such a high compression. It's all air being compressed. The only thing you really need to worry about is can the internals handle all the energy happening in the combustion chamber.
This seems to be a very common misconception. Fast foward to 0:25. You will notice that fuel is sprayed into the cylinder on the intake stroke. If it were sprayed at the top of the compression stroke, it wouldn't be very effective in cooling the intake charge. That and it would probably spontaneously ignite, causing the engine to "diesel."
EDIT: Why can't I ever get these damn videos to embed?!! Here's the link: