We usually try to avoid any inpingement whatsoever on the bore, piston crown or cylinder head. While we never truly get to zero, it's at least something to shoot for. Whenever liquid fuel hits a surface, it's generally bad for combustion quality since it starts causing rich spots in the chamber (likely accompanied by some other area of the chamber being the mirror image lean too!) and requires some compensation for impact and evaporation like we do in port fuel systems. Liquid gasoline also does bad things to port walls that depend upon an oil film to reduce friction.

This is usually done for cold start catalyst lightoff improvement, not necessarily "normal" operation at warm engine temps and a target of stoichiometric operation.

I'm not 100% certain yet as to how GM models the high pressure DI pump. I know that at VDO, I had two different models I could pick from that were either linear (two point) or progressive (five point). Each of these had the ability to learn a set of trim values to account for build tolerances and wear with age, but still required that the base (feed-forward) values were relatively close to what the pump actually did. I was also using a six piston crank driven HPDI pump with an inlet-side pressure regulator as opposed to what appears to be a Bosch single piston HPDI pump with an outlet side regulator running on a three lobe cam. Obviously, whatever cal values GM uses in the ECU will be tailored toward the hardware they spec from their suppliers. If you drop in a high volume aftermarket replacement, I would suspect it will need the matching calibration values remapped in the ECU.

 

I heard that GM is aware of the issues people have had modding DI engines - significant mods to fuel delivery system and lack of aftermarket parts to support that. Supposedly, because GM is aware that Corvette owners mod their engines, GM has stated that the stock fuel delivery system on the LT1 will support 800 hp. Now, it might take some time to figure out how to get that extra fuel out of the stock system.

 

We have been heavily modding DI engines for years and all this was new to us at the time and it took a learning curve to get to where we are.

Have developed a ton of products for the DI LLT & LFX engines in the Cadillac & Camaros.

Here is a link to videos of some the owners have posted on you-tube: http://www.camaro5.com/forums/showthread.php?t=259858

Heres the top mount on the Caddy SRX: