apalrd wrote:
It's a trade off.
The fuel with a PFI engine will always take some heat from the valve and port. You are right and valve timing can improve this. However at high speed/load you would have to keep injector duty cycle below ~25% which might not be possible.
If you are very knock limited, an extra degree of advance can result in a % or more indicated torque improvement and DI can allow a few degrees.
We have tested upstream (top of the runner) and downstream (directly at the port) injector locations and made slightly more power with upstream injectors than any calibration of the downstream injector (on E85). We have not tested DI yet, but I'm looking at it.
Air is flowing on my engine for 240 degrees so I can be spraying fuel into the moving air with a 33% duty cycle.....which depending where the HP comes out is just about exactly where I'll be. I've never tried this before so no data I've personally collected, but my motec dealer told me he does all his engines this way and they all make more power from a V8 at 6k to sportbikes that came with high/low injection, always a 3-5% increase...or so he says.
As for knock limited I guess the key is don't go there. It just works out better to go low enough compression to allow spark to be optimized for power, not optimized for knock avoidance. Going from 9:1 to 13:1 only adds about 8% more hp IIRC which is roughly what you lose with a 2-3 deg timing retard so unless it's a combination you know well I've always found its just best to be conservative on CR. I've never even installed a knock sensor on anything....playing with fire in my mind.