7.3L to 6.7L comparison
I replaced my '99 7.3L with a '15 6.7L recently. The two trucks are as identical as you can make from the order book: Supercab F350 dually 2WD Lariat 3.73 gears. For several years I have made the drive from home to Lake Tahoe, making this trip perhaps 40 times with the 7.3L, so I am quite familiar with the grades, fuel mileage expected, etc. The road climbs from sea level to 7200 ft Donner Summit and then drops into the 5900 ft Truckee valley, 165 miles each way. I set the cruise control on 63 when traffic allows. Hauling a 4300 lb Bigfoot 10.4E.
With the 7.3L, there were 13 grades which required a downshift to 4th. Practically this also meant dropping the speed to 55 due to the engine roar from the big gap between 4th and 5th. After crossing the flat Central Valley I would have about 15 mpg on the meter, maybe 16 if there was a tailwind. By the summit, this would have dropped to 12, sometimes 11.9, then picks up a little going down the other side. On the downgrade into Truckee, I had to downshift out of overdrive and ride the brakes a bit to keep the speed under control.
Today was the first opportunity with the 6.7L (also, the first trip with the air suspension - but that is another topic!). The gearing in top gear is pretty much identical to the 7.3L, but with 6 speeds spaced better in the transmission. It managed the whole climb in 6th on cruise control without apparent effort, no downshifting, no noise, no fuss. From the gages, it was working a bit, EGT peaked at 910 deg, oil temp at 242 (oil is designed to run hot in these engines), boost about 30 PSI on the steepest grades. Quite relaxing by comparison to the 7.3L. There was plenty of power to spare to pull back up to speed if I got caught behind a truck.
I had about 16 mpg on the meter when I hit the grade, it was down to 13.2 at the summit. Both trucks are about 1 mpg optimistic when checked by hand. On the downgrade into Truckee, the exhaust brake kept the speed at the cruise control set point though it did have to downshift to 5th to do it. No brakes required.
These new Superdutys are sure complicated and expensive - but they sure are nice!
With the 7.3L, there were 13 grades which required a downshift to 4th. Practically this also meant dropping the speed to 55 due to the engine roar from the big gap between 4th and 5th. After crossing the flat Central Valley I would have about 15 mpg on the meter, maybe 16 if there was a tailwind. By the summit, this would have dropped to 12, sometimes 11.9, then picks up a little going down the other side. On the downgrade into Truckee, I had to downshift out of overdrive and ride the brakes a bit to keep the speed under control.
Today was the first opportunity with the 6.7L (also, the first trip with the air suspension - but that is another topic!). The gearing in top gear is pretty much identical to the 7.3L, but with 6 speeds spaced better in the transmission. It managed the whole climb in 6th on cruise control without apparent effort, no downshifting, no noise, no fuss. From the gages, it was working a bit, EGT peaked at 910 deg, oil temp at 242 (oil is designed to run hot in these engines), boost about 30 PSI on the steepest grades. Quite relaxing by comparison to the 7.3L. There was plenty of power to spare to pull back up to speed if I got caught behind a truck.
I had about 16 mpg on the meter when I hit the grade, it was down to 13.2 at the summit. Both trucks are about 1 mpg optimistic when checked by hand. On the downgrade into Truckee, the exhaust brake kept the speed at the cruise control set point though it did have to downshift to 5th to do it. No brakes required.
These new Superdutys are sure complicated and expensive - but they sure are nice!
