https://www.tytlabs.com/english/revi...3_009hotta.pdf
This paper has some interesting information. Fig. 7 concerns pilot timing under
light load and low RPM. I believe main injection timing is 4 deg ATDC. ISFC (indicated specific fuel consumption) goes down as pilot timing gets closer to main injection event. I don't suggest that the actual numbers in the graph will work for our engines. But it makes sense that having the pilot event closer to TDC when under light load conditions would result in better fuel economy when the main injection event is short and around TDC (to a point).
Also note the different injection quantities in Fig.7. Varying pilot injection quantity doesn't change fuel consumption all that much. Fig. 4 shows much the same, but doesn't go as low in pilot injection amount.
I've read other papers on pilot timing. One of them found that best fuel consumption for their small diesel engine was at 17 deg BTDC...very close to Jim's 15 deg number.
Pilot injection is something that I haven't touched yet. It is next on my list. I think to start off I'm going to modify the low load areas. I'm going with 10% of main injection quantity at 15deg before main injection.