otrfun wrote:
4x4ord wrote:
According to the BSFC map for the 6.7 Cummins that Shiner posted on another thread the 3.42 pulling in 5th vs 4.10 in sixth yield very similar MPG while towing. 3.42's should yield close to 10% better fuel economy while running empty at virtually any highway speed. Again, according to the same BSFC map, towing with 3.73 gears at 65 mph should offer a slight advantage in MPG over either 3.42's or 4.10's. Empty the 3.73 will fall half way between 3.42's and 4.10's . . .
"3.42's should yield close to 10% better fuel economy while running empty at virtually any highway speed" vs. a 4.10. Interesting. Not what I've experienced.
Are you making these observations based on BSFC maps of the crankshaft output of a Cummins in conjunction with gearing charts?
Or, are your observations based on BSFC maps formulated and based on on-the-road tests of production-ready, OEM Ram trucks? If so, I'd be very interested in seeing these. Could you supply a link? Thanks.
I think part of the discrepancy here is emissions controls. Looking back at the report from where I got the BSFC map from, it does not mention emissions so the map may be without them since they are somewhat of an unknown variable. I do know that the EGR will open more under moderate load at low rpms as the combustion temps rise(which correlates to higher EGT's) since NOx creation rises significantly at 2,800F.
The EGR introduces spent exhaust gases that is already deprived of oxygen which has a major impact on the engines efficiency. It does not do this at higher rpms because the engine is receiving more air for the amount of fuel being injected to make the same horsepower and the higher rpms also allow more air to flow through the engine at a faster rate. Hence the reason why EGT's decreases when you shift into a lower gear putting you at a higher rpm.
Another factor is that the more cooled exhaust gases introduced, the more soot it creates which clogs up the DPF quicker. Clogging up the DPF quicker means it will need to regen sooner than normal. If the DPF is not hot enough to regen then it will perform an active regen and inject extra fuel on the exhaust stroke to heat up the DPF(at least for Ford and Cummins engines) so it is using more fuel than normal.
This may be a factor into why the BSFC maps may not align with what is happening in the real world. Especially when outside air temps are in or near triple digits since the temperature of the ambient temps have a significant impact on combustion temps and EGT's. As ambient air goes up, so will EGT's/combustion temps and the EGR will activate accordingly to keep NOx generation low. The more it activates, the less efficient the engine is because it is receiving less clean air with less oxygen. Our northern friends in cooler climates may not have to deal with this as much.
2014 Ram 2500 6.7L CTD
2016 BMW 2.0L diesel (work and back car)
2023 Jeep Wrangler Rubicon 3.0L Ecodiesel
Highland Ridge Silverstar 378RBS
Gas will kill the TV.
