How long should a turbo engine last?

Q: Back during the oil crisis of 1973, many automakers came out with small (1.8-liter and 2.0-liter) turbo engines. Many of these vehicles experienced massive engine failure around 70,000 miles due to bearing failure, a lack of lubricant, overheating, and various other issues.

Have these issues been addressed and corrected with the current crop of small turbo engines? How long should a newer turbo last? What does it cost to replace such an engine?

- Robert Garrett

A: Good stuff, Robert! Before answering your question, perhaps we should take a quick look at what engine turbochargers do and how they work.

Turbochargers pump air into an engine at a higher rate than would occur if the engine was naturally aspirated. Power can be boosted by about 25 percent on a gasoline engine and about 40 percent on a diesel, allowing a smaller, more efficient engine to be used in place of a larger one.

A turbo broadens the torque curve, meaning the engine doesn't have to be running at a high speed to generate good power. This improves drivability and allows more efficient low-speed engine operation.

Driven by hot and expanding exhaust gasses, a turbine wheel rotates at up to 200,000 r.p.m. depending on exhaust flow. At the opposite end of a connecting shaft, a compressor wheel draws in air and pumps it through a cooling radiator and then to the engine's intake manifold. Variable geometry and a waste gate may be used to adjust boost pressure to effective-yet-safe levels, maximizing performance while preventing engine damaging detonation (an explosion, rather than a burn of combustion gasses).

Another valve mitigates surge, which is a potentially harmful condition that can occur if the throttle is rapidly closed with the turbo spooled up. An attractive feature of turbochargers is they utilize wasted energy, rather than consuming it, like a belt-driven supercharger.

The turbos of the 1960s and '80s typically used journal bearings and did not utilize liquid cooling. This resulted in notable friction and operating temperatures frequently above the point at which lubricating oil begins to coke (solidify into charcoal-like particles). Many of the engines were carbureted and lacked smart computer controls to manage operation and protect the turbo and engine from damage.

Modern turbos utilize ball bearings, liquid cooling, better metallurgy, and in many cases variable geometry, managed by computer smarts. A movable vane or nozzle allows the turbo to adapt to low and high engine speeds, reducing lag on acceleration and maximizing engine torque across the board.

Modern turbocharged engines are often equipped with direct fuel injection, which allows a higher compression ratio to be used, without damaging detonation, and variable valve timing, taking better advantage of the turbo's benefits.

A sophisticated engine management system monitors intake pressure, listens for detonation, and does whatever it takes for ignition timing, EGR flow, and to boost pressure to keep things safe for the pistons. According to a Ford spokesman, their turbo engines are designed to be equally as reliable as a naturally aspirated engine. I think it's safe to say a more highly stressed turbo engine deserves more diligent maintenance than a larger, lazier, normally aspirated one. My best guess for the cost to replace a failed turbo would be about $1,500 to 2,000. Some engines have two, and there are additional parts and complexities to be considered with this system.