The fuel amount depends on your oxygen density, just like a gas there is a optimum AFR. Diesel's have a sweet spot for making power, but unlike gas Stoich is that theoretical sweet spot, which is 14.5:1. Unlike gas we don't care about getting the mixture richer to become more stable so we can advance timing, we want it to be well mixed and very unstable so it's rate of heat release is quick and then center peak pressure just after TDC. I can't give you a number even if i could each vehicle/engine is different, you need a dyno to find this number or a cylinder transducer. Also pilot amount and timing will change noise, say noise goes up but you're making more power/tq then start tweaking pilot mass and advance. BP ratio is back pressure ratio, or intake pressure to exhaust pressure. The way i find this is take manifold pressure minus current barometric = X, then take back pressure minus baro = Y. I end up with Y/X=BP ratio. This ratio matters to see how much we are constipating the engine, most gas cars don't care because they don't have a VGT to be able to fine tune it. So back pressure is fixed until the wastegate opens. For rail pressure Your pilot is changing as well which plays a big role on noise. I believe anything within the manufacture's spec is good, I've seen a few CP4.2's ran at 31kpsi that have had issues. Because of this i stay at the manufacture's spec. Here is a good read on Pilot inejction and noise. http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf