Gearing Canadian Kenworth & Differences in Detroit and Cummins Aftertreatment Systems

2021, Emissions, Gear Ratio -

Gearing Canadian Kenworth & Differences in Detroit and Cummins Aftertreatment Systems

What were they thinking, maybe the problem is they do not think? A client of ours is purchasing a new W900-L or W990 in the Toronto Canada area, the truck will be powered with an X-15 Cummins and the 18-speed transmission, which has a final drive of .73 thus making it a double overdrive transmission. The dealer has a 500 horsepower, 1750 torque at 950 RPM, however the rear gears are 3:70 with low pro 22.5 tires. Really? Are you kidding me? This spec dates back 30 years ago. With a peak torque at 950 RPM the best operating range for this engine is 1150 to 1250 RPM.  A 3:08 rear gear at 65 miles per hour is 1250 RPM, Canadian speed limit is 65 miles per hour, so why would the dealership order the truck with a 3:70 rear gear?     

The next Kenworth they have on the lot is a W990 powered with an X-15 565 horsepower, 1850 torque at 1,000 RPM.  This truck has 3:58 rear gears with the double overdrive 18 speed.  Again, this was the common gear back in the mid 1990’s for N-14 Cummins, ISX, and 3406-E Caterpillar engines.  Back then owner operators were driving 5 to 10 miles per hour slower, they were not fighting the 14-hour clock, and peak torque on those engines were around 1200 RPM.  The 14-hour clock is the one law responsible for the higher speeds of semi-trucks.  With peak torque being 200 to 300 RPM less, the rear gears must be higher.  People not familiar with gearing, the higher the gear, the faster the speed at the same RPM, so a 3:55 is a higher gear than a 4:10, sometimes an owner operator will call and say he wants to lower his gears to gain more speed, what he is saying he wants to lower the numeric number to gain more speed which is a higher gear ratio.

When you are purchasing a new or used truck you MUST know the rear gear ratios, the speeds you are going to cruise, your top speed, and the transmission and the amount of gears and the ratio of the top gear. Some top gear ratios can be 1 to 1 direct. .85 is a single over 13 speed (I’m not sure if this transmission is available in anything newer than 1995) and .73 high gear which is a 13 or 18 speed double overdrive. The 10 speed, which most company trucks are equipped with, is a double overdrive because the final drive ratio is .73. The problem with this transmission is the large gap in RPM from 9th gear to 10th gear, it can be 400 to 500 RPM.  

When purchasing a new Kenworth or Peterbilt powered with the X15 Cummins, 13 or 18 speed transmission, the 3:08 gear ratio is recommended.  At 60 mph the RPM is 1170, 65 mph the RPM is 1250, 71 mph the RPM is 1360 and 75 mph the RPM will be 1440.  These speeds are in the highest gear.  Do not order the economical version of this great engine, always order the performance series, and get the 565 to 605 horsepower version.  Higher horsepower engines driven properly will render better fuel mileage and longer engine life.  Always order the truck with a turbo boost or manifold pressure gauge. 

We currently have a DD15 Cascadia in the shop with a dead One Box aftertreatment system. The truck only had 600,000 miles and that’s a $12,000 repair. If you calculate the cost per mile for the life of that DPF it works out to $0.02 cents per mile. The cost of Max Mileage is 7 cents per gallon which if you get about 7 mpg that’s 1 cent per mile. So in other words, half the cost and the DPF would have been spared. 

This got us thinking about the differences between the Detroit One Box and the Single Module system from Cummins. The One Box is a switchback system which means the exhaust flow needs to take a series of turns before it exits out the same side of the box as the inlet. The Cummins Single Module system has a straight through design which not only improves flow, but makes the unit smaller, lighter, and less complex. As we know from designing and building our performance mufflers, a straight through design is always the best way to reduce back pressure increasing performance and fuel mileage. The name of the One Box is a little misleading because it does not come as a complete system. The cost to replace a One Box is about $8,000, but including the DPFs and sensors it can be about $12,000 for the complete unit. The cost to replace a Single Module is about the same. However, in our experience the SCR system on the One Box is more likely to fail first, whereas on the Cummins it’s usually just the DPF. The SCR is part of the One Box and you cannot replace just the SCR without having to replace the whole $8,000 One Box. Having to replace the DPF on a Cummins is only about $2,000 in comparison. The Cummins engines do produce a fair amount of soot, so regular DPF cleaning is very important. No matter what aftertreatment system you have, if you keep the DPF clean using the catalyst combined with cleaning from DPF Alternatives, you should have a nice reliable truck. In fact, DPF Alternatives will give your DPF a lifetime warranty if you use their services and run Max Mileage. 

Written by;

Bruce Mallinson & Andrew Wilson

Pittsburgh Power Inc.

3600 S. Noah Dr., Saxonburg, Pa. 16056, 724-360-4080