MAHLE's damage brochure says the parts inside a turbo are designed for a defined speed range, and that if it is exceeded, "major turbocharger damage can occur within seconds." Tuning is one cause, but not the only one.
What overspeed does to a compressor wheel
Compressor wheels are usually aluminium. MAHLE explains that at overspeed, centrifugal force deforms the metal permanently: the wheel "begins to creep, and the outer diameter increases", leaving small dents on its back face. If the speed rises further, the wheel can touch its housing, break apart, or both.
Fatigue works more quietly. A paper for the Institution of Mechanical Engineers (IMechE), by engineers from BorgWarner Turbo Systems, Renault and the Institut français du pétrole, notes that fatigue fractures are a frequent cause of compressor wheel failure. It treats each rise to high turbo speed and back as a load cycle that uses up part of the wheel's life, and adds the damage up over time. That is why a turbo surviving one dyno session proves little about how long it will last.
What drives a turbo past its limit
- Tuning that pushes the turbo beyond its maximum permitted speed, which MAHLE lists first.
- Jammed vanes. On a variable-geometry turbo, carbon can jam the vanes in their low-speed position, and MAHLE says the turbo then overspeeds as the revs rise. See our guide to sticking turbo vanes.
- Faulty boost control. MAHLE names a defective or leaking pneumatic or electronic control.
- Running past the choke line. On Garrett's compressor maps, the choke line is typically where efficiency drops below 58%. Beyond it the speed lines crowd together, and Garrett says "turbo speed increases very quickly and a turbo over-speed condition is very likely." Its advice is that if operation goes beyond this limit, "a larger compressor is necessary."
- Altitude. Garrett's worked example shows the same engine needing a different pressure ratio and airflow to make the same power at different altitudes, so the match has to allow for where the car will be used.
Matching an upgraded or hybrid turbo
The aim is a compressor whose map covers every point the engine will run at, inside the choke line and the turbo's speed limit. Garrett describes a turbo speed sensor as a good accessory for validating the match and making sure the turbo is not over-speeding, and the IMechE paper notes that speed sensors are commonly used to limit overspeed.
For a given blade design, a billet wheel changes durability rather than airflow. Garrett states that "No noticeable gains in performance can be seen when testing a machined wheel vs. a cast wheel of the same design." The advantage is strength and fatigue life where a wheel is cycled hard; Garrett's example is a city bus switching constantly between full throttle and idle. BorgWarner says forging makes the grain finer and "improves low cycle fatigue, which directly determines turbo life", while milling gives a more consistent blade shape than casting. The IMechE paper concludes that milled aluminium wheels last longer than cast ones.
GP Turbos builds hybrid turbos and supplies billet compressor wheels. What any upgraded turbo delivers depends on the wheel design, the match to the engine, the supporting modifications and a matching calibration.
Heat damage
MAHLE links excessive exhaust temperature to cracked turbine housings and coked oil lines, and names tuning as one cause. Garrett adds that excessive exhaust temperature can collapse the turbine piston ring, one of the causes in our guide to a turbo leaking oil.
Signs to look for
MAHLE's signs of overspeed are small dents on the back face of the compressor wheel, a wheel that has grown in diameter and, later, contact with the housing. Holset warns never to keep running a turbo with damaged blades, because the rotor balance will be affected.
After a wheel failure
A wheel that has crept, rubbed or burst has to be replaced and the housings checked. Garrett's training notes warn that fragments can pass into the air pipework, charge air cooler and exhaust, and MAHLE says debris left in an old charge air cooler usually reaches the engine later, which is why it recommends a new charge air cooler and air filter with any replacement turbo. Our turbo rebuilds page covers the rebuild, and turbo diagnostics covers how the cause is traced.