CowpokeJ":3nrsnj2o said:
What kinda modifications will get you 24mpg?
Cowpoke, that 24 mpg figure is only possible with the 1996 - 1998 5 speed equipped truck. You bump the pump timing to 15 degrees, disable the turbo wastegate, and set the AFC to be a little more aggressive than stock. Assuming the injectors don't have 150,000 miles on them, you'll be in the 22 mpg average range. A few of my customers managed 24 (hand calculated) at sea level unloaded, but it wasn't the most common result.
Other engines (94 and 95 5 speeds, 94, 95 autos, 96-98 autos) utilized a different pump, camshaft and injectors, so 24 mpg wasn't possible except in very rare circumstances. 20 - 21 was often attainable on older injectors though. I did install a few sets of F1 diesel injectors on the "off model pumps" and managed to get as high as 24 mpg, while dynoing 350 HP.
Even the 24 valve trucks were able to hit 24 mpg. You needed a nicely balanced set of injectors (stock wouldn't normally do it), and a timing/fueling box that was very aggressive on timing. I preferred Edge with my fuel economy customers, but the TST often worked well. My 2001 cab and chassis dynoed a consistent 450 HP, but at 70 mph cruising speeds I would get 22 - 24 mpg in the US. Canadian economy was worse, even with our larger gallon, due to the poorer quality diesel up here.
If you have a 1998.5 - 2002 Cummins and your economy is consistently below 18 mpg unloaded while at or near sea level, have your lift pump fuel pressure checked. Once you dropped below 10 or 11 PSI at idle, your mileage would begin taking a hit. I'm talking summer fuel. Winter fuel will automatically mean a drop in economy.
Rod