Originally Posted by
CamTom
Hey there!
Thought I'd share my limited experiences with ignition timing in the cruise cells (which appears contrary to what I've been reading, but makes sense in my head) and ask some theory/experience questions as I move into the higher load parts of the map.
So I started off reading a fair amount of folks talking about bumping timing up slightly in the cruise cells to get better fuel efficiency. Granted, most of the info I've read is from folks tuning Cummins motors, but that's all I could find.
So I tried that and didn't get any better economy. Then I saw a thread where JaegerWrenching posted his map and he had LESS timing than stock at low loads, so I gave that a shot. I picked up some economy and the driveability got a little nicer with the timing between -4 and -1 in my cruise cells.
This made sense to me because if we're targeting ~12-14 degrees ATDC for peak cylinder pressure, and we've already "preheated" the airmass with a couple of pilot shots, I can't imagine ignition delay (ID) being any more than ~10-15 degrees at those rpms. I read an article that found calculated ID at 25% load to be around 1 +/-0.05ms at 1800rpm using a bunch of different calculating methods. They were using with a slightly smaller bore/stroke but a higher compression ratio and no pilot injections. At the bottom end (0.95ms) at 1800rpm is about 10 degrees and the top end of that time range (15ms) at 1800rpm is about 16 degrees of ID. So my findings made pretty good sense to me.
For example, I'm currently running -2.5 degrees at 1800rpm (~74mph in my truck) and 369lbft of requested torque. I think that should put my peak cylinder pressure at ~13.2 degrees ATDC. Maybe.
I think it makes real good sense at lower loads, but here's where I get doubtful. As the saying goes, all models are wrong and some are useful...
They continued the calculations out to 100% load at 1800 and 2100rpm and found ID to be around 0.5ms (~5.4 and ~6.3 degrees respectively).
Does that mean I should be looking at -7 degrees of timing at 2100rpm and WOT? The stock map has 5.2 degrees there. I think the model breaks down at the edges.
The study did note that advancing the start of injection lowered cylinder temps/pressure at TDC, which increased ID. I guess that's what's probably happening here.
Does anyone else have any experiences with better fuel economy at lower load at reduced timing from stock? I'm starting to move out to the 443 and 516lbft requested torque cells at 1800rpm (I'm currently near stock but smoothed, around 4.5 degrees) and am calculating that timing still wants to be around -2.5 degrees , but I'm wondering if that's far enough out on the model to where the math will start to be less useful. How's everyone else looking in that range?
My truck does have dual fuelers, but I'm not sure how much of a difference that makes at cruise other than being a horsepower/fuel suck. I haven't seen where I can log rail pressure yet.