As you think about building a fleet, a common "spec" for all you units will cross your mind. Here are a few powertrain thoughts to consider...
In researching my first "truly-new" D-Unit truck designed for Expedited/Express operations, I have done quite a bit of research, investigation and study. What follows are some of my findings and supporting analysis in drivetrain specification and selection.
Spec'ing an efficient, cost-effective engine/transmission/axle combination for D-Unit operations with an eye toward profitability can be a challenge. Good power and torque are essential, but the fact remains that the largest non-payroll line item on an Owner-Operator’s expense sheet is fuel! Even a slight improvement in fuel efficiency can make a big difference in operator’s bottom-line.
Most D-Unit operators I spoke to and have heard from tell me the average freight weight they carry per trip is about 6,000 pounds. Rarely, are "D" loads weighing in over 10,000 pounds. This means that on your average Freightliner FL70 sleeper-equipped platform, 260-300 HP is more than enough to sustain 65 MPH all day long with the cruise-control on and keep fuel consumption hovering around 10 MPG or better. Any more power beyond 300 HP is nice, but will consume as much as 10% more fuel on any given trip. Given the percentage of deadhead we run (as much as 40% and more), excessive power does nothing but hurt our profit margin. As long as a D-Unit’s average loaded weight scale’s to 27,500 pounds or less, there is no practical need for power beyond 300 HP and torque in excess of 860 Foot Pounds.
Engine Choices - down to two:
Caterpillar 3126B: 300-HT HP ("High Torque"); 860 Ft Lbs torque at 1440 RPM
Cummins ISC 300: 300 HP; 860 Ft Lbs torque at 1300 RPM
The CAT 3126 series had a number of mechanical rough edges several years ago but most of those have been worked out now. Today, a new 3126 should be very reliable. CAT’s have always had a reputation for pulling well up a grade under load and that seems consistent with the 3126B reports I have heard. The reputation for out-of-warranty repairs being the most expensive in the industry also seems well deserved.
The Cummins ISC 300 is a newer engine, out only a two years or so, but boasts the latest technology. The problems with ISC fuel pumps and fuel lines associated with the engine's introduction was solved way back in the late 1900s, back last century (ha!). The Cummins has a better power/weight ratio, is quieter (better balanced) and more fuel-efficient. With the ISC, oil drain and filter changes are scheduled on 15,000-mile intervals, vs. the 3126’s 8,000 mile interval.
Transmission: Eaton-Fuller’s FSO-8406A 6-speed seems to be the industry standard -- tough, proven, fully synchronized and easy to shift. An overdrive box with a final output ratio of 0.78, the FSO-8406A can handle the 860 foot pounds of torque delivered by either of the engines listed above. Any engine producing more power/torque (such as the Cat C-10 or the Cummins ISC 315) will put you into a “heavy-duty” Class 8 type 9-speed box at a minimum. This means the box will be much heavier and require "range shifting" (between 4th and 5th, this is much less desirable for newer, less experienced O-Os). Behind engine with less than 250 HP, 6 gears may not be enough (the range between gears to too great), but with 860 Ft Lbs of torque, this is just fine. Even when running at a GVW of 33,000 pounds.
Drive Axle Ratio: Get this wrong, and the whole package will either be a performance dog, or drink fuel like a logging truck straining to run 75 MPH! It is important to note that professionally, I very, very rarely drive faster than 65 MPH - speeds above 65 just don’t seem worth the risk (tickets and accidents) and equipment stress. I set my cruise-control at 64 90% of the time. Given my preference of using Low Profile 275/80R/22.5 "Low Pro” Rubber (at 516 revolutions / mile), the FSO-8406A 6-speed’s OD final ratio of 0.78, and a desire to position the engine’s best power and economy RPM setting to run at 65 MPH, the final axle ratio can now be calculated.
For the CAT 3126B, the best power/economy cruise RPM setting is 1,950 RPM. For the Cummins ISC 300, it’s 300 RPM less; 1,650 RPM. This means the best ratio for the CAT 3126 is a 4.33 or 4.56. For the Cummins ISC, because its “sweet spot” is 300 RPM lower (and much quieter, I might ad) it’s a 3.90. How are these ratios calculated? I used the Eaton-Fuller On-Line Road Speed Calculator found at
http://truck.eaton.com/na/spec_products/road_speed_calc/.
My Conclusion: The final purchase order hasn’t been cut yet, but as for my operation as of this writing, because of the better fuel efficiency, extended oil/filter drain intervals and newer electronic controls, I’m strongly leaning toward the Cummins ISC 300 driving a standard Eaton-Fuller FSO-8406A (.78 OD) six-speed turning a 3.90 axle. This specification puts running at 65 MPH right in the middle of the engines "sweet spot," 1650 RPM, allowing me to lug down to 1350 to 1400 RPM or so, where the torque really builds, before needing to grab 5th when pulling a steep grade under load. Although yet unproved, on an FL70 platform with a 70’ sleeper, this combination should average at least 10 MPG at 27,500 Lbs while averaging 65 MPH, and nearly 11 or 12 MPG-plus when deadheading empty.
Hope this helps...
Rams Fan