Often when people think of turbocharged cars, the characteristic noise that comes to mind is the turbo flutter, or the ‘tsu-tu-tu’ that happens when the driver backs off the throttle in a hurry, such as when changing gear. That noise may tell the world that you have a big ol’ turbo, but the truth is, that noise can be doing more harm than good.
Compressor surge is a phenomenon where the compressor part of the turbocharger cannot increase the pressure of the air it is pushing and results in the reversal flow of air through the compressor.
The fluttering noise is the sound of the compressor “chopping” through the air rather than pushing the air into the engine, as the air has nowhere to go when the throttle is suddenly closed. The most common time which compressor surge occurs is during gear changes on a manual transmission car. Under acceleration, the turbocharger is flowing air and the engine is ingesting the air which means the air that the turbocharger is pushing is going somewhere at a certain pressure. When a gear change occurs, the throttle which allows air to flow into the engine is closed.
This results in a large pressure spike as the turbocharger is still trying to flow air due to the inertia of the compressor and turbine, but there is nowhere for the air to go. The pressure within the intercooler piping continues to increase until the compressor reaches its pressure limit. When it reaches this limit, it cannot flow the air any more and the built up air pressure inside the intercooler and pipe begins to flow backwards through the compressor which is trying to flow air forwards.
The result of this reversal flow of air is the immediate deceleration of the turbocharger and a high load on the bearings which support the compressor/turbine shaft. At low turbo speeds and low pressures, the deceleration of the turbo and the load on the bearings is low, i.e. a small amount of fluttering at low engine speeds and throttle movements is negligible.
At high turbo speeds and high pressures, compressor surge during a gear change can be damaging to the bearings of the turbocharger as the deceleration rate of the turbocharger is high and the reversal airflow through the compressor is high.
This deceleration will also reduce the boost response of the turbocharger when the throttle is reopened as the engine will need to work harder to increase the turbo speed back up to operating RPM.
A Turbosmart blow-off valve (BOV) is designed to maximize boost response and eliminate the problems associated with compressor surge. During a gear change the BOV will open up due to vacuum and boost pressure. This will vent the excess pressure build up and allow the turbocharger to continue to flow air.
This will also reduce the deceleration of the turbocharger as the turbo does not need to work against a closed throttle – the excess air pressure will be vented rather than forcing itself through the blades of the compressor, slowing it down. Flow is an important specification of a BOV. If the BOV cannot flow enough air, the pressure increase in the intercooler piping when the throttle is closed can still be high enough to cause compressor surge.
Worn bearings can result in play in the compressor blade, which can get so bad it eventually strikes the walls of the compressor housing, causing expensive damage to the fins of the compressor. In a worst-case scenario, this can result in broken bits of turbo making their way into the engine’s intake, potentially resulting in severe engine damage.
Blades damaged by compressor surge
All Turbosmart BOVs are designed for maximum flow and can flow enough air to eliminate compressor surge, meaning the bearings of your expensive turbo will last much longer, and you won’t loose as much turbo speed between gear changes, reducing lag and improving acceleration.
A vent-to-atmosphere blow-off valve has its own distinct sound, giving off a ‘pshh’ sound between gear changes, unlike the fluttering sound caused by compressor surge.
Turbosmart’s blow-off valves are available in vent-to-atmosphere, recirculating and dual port configurations. A vent-to-atmosphere valve provides the noise that tells the world you’re running boost, while a recirculating valve reroutes the vented air back into the car’s intake, meaning the operation is quieter and friendlier to the vehicle’s sensors. A dual port valve offers the best of both worlds, giving quiet operation on mild boost, and the distinctive BOV noise when boosting hard.
So, next time you hear turbo flutter, consider how much that is wearing the bearings in that turbo, and how much boost it’s wasting between gear changes. Suddenly, it doesn’t seem nearly as cool!
Have any more questions about turbo flutter or boost control? Ask the experts in the comments below!