Before turbocharged diesels hit the truck market, you had few choices. Either you bought a 6.9L IDI F-series Ford (rated at 170 hp or 180 hp) or the 130hp, 6.2L Chevrolet C/K-series. As of 2009, Chevy, Dodge, and Ford all offer packages with 350 hp or more, while meeting much stricter emissions standards. The aftermarket has embraced turbocharging as well, with 500-plus-horsepower becoming an everyday figure. Turbocharging is the biggest single reason why today's diesels can achieve these power levels, so with that history in mind, let's take a closer look at how the single most important part on your engine works.
The Basics
On an average day, air pressure at sea level is about 14.7 pounds per square inch (psi). When an engine is turbocharged, the turbocharger acts like a very high speed fan, which forces more air into the engine. The amount of pressure a turbo can generate is measured in pounds per square inch above atmospheric pressure. So a turbocharged engine with 15 pounds of boost would be moving roughly twice the air of a naturally aspirated engine, and all things being equal, would make roughly twice the power. With newer diesels, boost pressures can be as high as 40 psi, yet the engine will remain reliable, and make three to four times the power of a naturally aspirated diesel.
How A Turbo Works
A turbocharger in its most basic form consists of only a few parts: a frame, shaft, compressor, turbine, and compressor and exhaust housings. Exhaust gases from the engine are used to spin the turbine, which in turn drives the compressor through a shared shaft, which generates boost pressure to be sent to the engine. These types of turbos have been used successfully since the 1920s in racing and diesel engines.
Here, a BD Super B turbo is...
Here, a BD Super B turbo is seen next to a stock HX35 (found on '94-'981⁄2 Cummins engines). While they may look similarly sized, there are several subtle differences. The compressor housing is larger to achieve a higher maximum airflow, while the exhaust housing is smaller for better spool-up characteristics. Internal differences in oiling, bearings, and turbine and compressor wheels are present as well.
Boost And Drive Pressures
While we've already introduced boost pressure, another important aspect of turbocharging is drive pressure. Drive pressure is the amount of force (in pounds per square inch) that is being used to spin the turbocharger. A drive-to-boost pressure ratio of 1:1 is ideal, although in reality, drive pressure is usually a bit higher than boost pressure. If a situation occurs where drive pressure far exceeds boost pressure (say, 35 psi boost, 65 psi drive pressure) then you might be looking at trouble. To imitate a high drive pressure situation, try breathing in a normal breath, then cover your mouth with your hand and exhale. That is what you're doing to your engine. High drive pressures are hard on parts and make your turbocharger much less efficient.
Too much boost can also be a problem for turbochargers. To produce more boost, turbos will spin faster, and every turbocharger has a spot where it just can't spin any faster. If you have an HX35 (found on '94-'98 1/2 Dodges) for example, it can only produce about 40 psi before overspeeding becomes a threat. If you run 45 psi of boost or more on an HX35 for any extended period of time, your turbocharger is almost sure to fail.