Engines 5. Because of their constant horsepower over a wide speed range, Detroit Diesel engines can be oper- ated at full throttle at lower rpm than other engines. This offers benefits in fuel economy and engine life. And more than likely, it will be possible to top most grades without downshifting. The top road speed possible in a gear would be the shift point for that gear. The results should be re- corded in the proper order of shifting and displayed inside the cab. 7. To slow the vehicle on downgrades and curves (us- ing the engine), shift to a lower gear and allow the vehicle to decelerate in that gear.
The engine pro- vides maximum braking effect when running at the top end of the operating range, but it must not be allowed to exceed its full-load rated rpm. Continue to downshift as further reduction in vehicle speed is required. If the vehicle is above the allowable maxi- mum speed of a lower gear, use the service brakes to slow the vehicle to an acceptable speed where the transmission may be downshifted safely. Again, the importance of knowing the shift points is dem- onstrated. When the vehicle starts into a grade, allow the en- gine to lug down to maximum torque rpm before downshifting.
Downshift, if required, at maximum torque rpm also or at the predetermined road speed for the next lower gear. Do not be afraid to lug the engine down. It has more than enough torque at low rpm to keep a loaded vehicle moving against a grade and it won’t harm the engine. 6. The driver who is not familiar with the vehicle’s shift points can greatly improve driving skill by learning them for all gears. By knowing rather than guessing where the shift points are, it is possible to avoid overspeeding the engine by downshifting too soon or missing the full use of a gear by downshifting too late. The shift points of any vehicle can be deter- mined by a simple road-test method.
Run the ve- hicle and determine the maximum road speed pos- sible in every gear at the engine governed full-load speed setting. CAUTION Do not allow the engine to exceed its governed speed, or serious engine damage could result. IMPORTANT: The engine governor has no control over engine rpm when the engine is being pushed by a loaded vehicle down a grade. Use service brakes and gears in combination on long grades to keep the vehicle speed under control and the engine rpm below full-load rated governed speed. 4.15