Depending on how it is accomplished, turbocharging can have three quite distinct effects on performance. If no or little additional fuel is supplied, power output remains static, but emissions go down. Surplus air lowers combustion temperatures and provides internal cooling. Such engines should be more durable than their naturally aspirated equivalents.
The high-boost/low-fuel approach to turbocharging is limited to large stationary and marine plants. Makers of small, high-speed engines are more concerned with maximum power or mid-range torque.
Supplying additional fuel in proportion to boost yields power, which can translate as fuel savings for engines that run under constant load. Some gains in fuel efficiency (calculated on a hp/hour basis) accrue from turbocharging, but the real advantage comes about when high supercharge pressures allow for lower piston speeds. Large marine engines, developing a 1000 hp and more per cylinder, use this approach to achieve thermal efficiencies of 50%. On a more familiar scale, the naturally aspirated International DT-414 truck engine produced 157 hp at 3000 rpm. The addition of a large turbocharger boosted output to 220 hp, for a gain of 40%. Once in the fairly narrow power band, the trucker could save fuel by selecting a higher gear.
The third approach is more characteristic of automotive and light truck engines, which operate under varying loads and rarely, if ever, develop full rated power. What is wanted is torque.
The Navistar 7.3L, developed for Ford pickups and Econoline vans, is perhaps the best demonstration of the way a turbocharger can be throttled for torque production. In its naturally aspirated form, the engine develops 185 hp at 3000 rpm and 360 ft./lb. of torque at 1400 rpm. A Garrett TC43 turbocharger boosts power output marginally to 190 hp; but torque goes up 17.8% to 385 ft/lb. In normal operation the wastegate remains closed, directing exhaust gases to the turbine, until 1400 rpm. At that point, which corresponds to the torque peak, the hydraulically actuated wastegate begins to open, shunting exhaust away from the turbine.
Serious applications of turbocharging, whether for peak power or mid-range torque, impose severe mechanical and thermal loads, which should be anticipated in the design stage. Insofar as they work as advertised, add-on turbocharger kits—which rarely involve more than a rearrangement of the plumbing—are a buyerbeware proposition. The 7.9L is considered a very rugged engine in its naturally aspirated form. But turbocharging called for hundreds of engineering changes, including shot-peened connecting rods, oversized piston pins, Inconel exhaust valves, and special Zollner pistons, with anodized crowns.