Whether you drive a Chevy, Dodge, Ford, or GMC diesel, if it was built after 1995, it has a turbocharger under the hood-perhaps two if you've already picked up the '08 Super Duty. Ford has stepped up to the dual-turbo design by BorgWarner after using Garrett's variable-geometry turbo on the 6.0L Power Stroke and a fixed Garrett turbo on the 7.3L. Stock Dodge 5.9L trucks come with a fixed Cummins Holset turbo under the hood, a titanium-aluminum-alloy compressor wheel was added in the '97 model, and an electrically variable nozzle turbo is hooked to the 6.7L Cummins. Chevy and GMC trucks come with IHI (LB7) or Garrett turbos from the factory. The 6.5L from the '90s and the LB7 Duramax use fixed turbos, and the LLY-LMM engines use variable vanes in the turbine to keep boost on tap.
Each design uses the same basic principle: exhaust gases are routed into the turbine housing and spin the turbine wheel, which turns a shaft attached to the compressor wheel then sucks air from the intake and pressurizes it using the compressor housing, pushing more air into the cylinders than a nonturbo diesel. More air means more fuel can be used, which increases power output and efficiency. Using a bigger turbocharger or stacking multiple turbos is an easy way to get more power out of your stock diesel, but without proper planning, you could build an engine that will only achieve boost at high rpm-not much fun on the street. Before you upgrade your stocker, get familiar with the internal workings of these power-adders and some of the turbo terminology you may hear in the local performance shop.
Aftercharger: A radiator placed between the turbo-compressor outlet and the engine intake manifold used to cool the intake charge, which is heated by the pressurization of the turbo (also called an intercooler or charge air cooler).
Airflow (cfm): A measurement of how much air/exhaust is able to flow through the turbo. Airflow is measured in cubic feet per minute.
Area/radius (A/R): The ratio of the cross-sectional area of the exhaust-turbine inlet/compressor outlet divided by the radius from the center of the turbo wheel to the center of the cross-section (right). Differences in compressor A/R do not affect performance very much, but a large turbine A/R will allow big power gains at high engine rpm-this will cause turbo lag at low speeds.
Backplate: Located behind the compressor wheel, this supports the compressor housing, attaches to the turbo centersection, and routes air into the compressor housing.
Backpressure: A buildup of pressure in the exhaust that prevents the free flow of new exhaust gases and slows the speed of the turbo wheel. Backpressure that builds after the compressor in the intake can cause the wheel to suddenly stop spinning (surge).
Ball bearings: Steel or ceramic bearings mounted inside a circular housing that surround the turbo shaft, used in place of less expensive brass-sleeve bearings (journal bearings). Ball bearings are thought to provide about 25 percent faster turbo spool-up and require less oiling pressure than stock journal bearings.
Blow-off valve: A valve between the turbo and the intake manifold that vents air to avoid turbo surge when a preset pressure limit (boost) is surpassed.
Boost: The intake pressure created by the spinning of the compressor wheel inside the housing. Measured in pounds per square inch over the normal atmospheric pressure (14.7:1).
Boost controller: A mechanical or electrical device that alters the boost-pressure signal sent to actuate the wastegate, allowing higher boost pressures than would normally be permitted.
Boost creep: When the boost rises past the set limit. This is often caused by a wastegate that cannot handle the exhaust flow.
Boost spike: A period of uncontrolled boost when the wastegate and/or blow-off valve cannot act fast enough because of sudden changes in the engine load.
Boost threshold: When engine conditions provide enough exhaust pressure to create boost in the intake manifold.
Boreless turbo: A turbo that uses a compressor wheel that doesn't have a hole drilled through it. This design increases the strength of the compressor wheel in its highest stress area.