The Impotent Turbine Shaft

Why is there a howling noise from my turbo when I accelerate?

There are a few possibilities such as a momentary lack of oil slightly wearing the bearings and causing imbalance or impact damage to one of the wheels, however the most common cause for a howling type noise is head-droop...

It is a problem most of us males have experienced at some point in our life due to excessive alcohol, however it differs in cause for the turbocharger. Head-droop in a turbocharger is when the head of the turbine wheel droops and is no longer perpendicular to the turbine shaft to which it was welded via a process called friction welding.

In nearly all cases this occurs due to repeated hot shut downs and extreme temperature, fatigue or combination of both. The turbine wheel head is made from a casting and is relatively heavy in comparison to the shaft. If it is repeatedly shut down when extremely hot it may drop slightly and it only need drop a few thousands of an inch to result in a very loud howling noise which is caused by the resulting imbalance. A turbocharger with head-droop will eventually fail.

If having your turbocharger repaired, remember to advise the turbocharger specialist of any noises as it is quite difficult to measure head-droop given there are few if any flat surfaces on a turbine wheel when trying to micrometer such a small amount of movement. If I find a turbine wheel is near or over 0.10g of imbalance I suspect head-droop as the most likely possible cause and contact the customer to enquire whether the turbocharger was noisy or not.

It is possible to balance a turbine wheel with head droop however the wheel will obviously still be offset and it is unlikely the noise will disappear hence its very wise to replace it. I always advise a customer of this circumstance and highly recommend replacing the turbine wheel as it will void any warranty if the customer insists on taking a chance with a suspect turbine wheel.

It is far cheaper to replace a turbine wheel rather than chance one with head-droop only to find yourself removing the turbocharger again to replace it anyway!