The sound of a turbo fluttering can be alarming for people used to naturally aspirated engines. Turbo flutter is sometimes described as “pigeon noises” or written out as “stu-tu-tu.” Both of these are pretty good descriptions of the sound, but what actually is turbo flutter?
Turbo flutter is another name for compressor surge.
So, Is Turbo Flutter Bad?
Compressor surge occurs when a turbocharger is operating at peak boost and then the gas is let off, closing the throttle body. This traps air, which then exits back out through the turbocharger causing the compressor to spin. If the gas is released at the redline, you’ll hear crackles from the exhaust as well.
This sound is unique to turbocharged engines, and a lot of people love the sound. Unfortunately, compressor surge can damage your turbocharger. Turbo flutter is also unrelated to speed or performance.
Understanding how a turbocharger operates can help to understand the ways that turbocharged engines create unique sounds, like turbo flutter.
This is the normal pathway of air through an engine.
- First, air enters through the intake manifold.
- This air is used in the cylinders to fuel combustion.
- The air that remains after combustion is expelled through the exhaust manifold.
In a turbocharged engine, instead of simply exiting as described above, the exhaust air is used to create additional power.
- This air exiting through the exhaust turns the turbine.
- The turbine spins, which causes the compressor to spin.
- The compressor spins, pressurizing and cooling air.
- The pressurized, cool air enters the cylinders to fuel combustion.
A turbocharger is comprised of two primary components: The compressor and the turbine. The compressor and turbine are mounted on a common shaft. This means that when one spins, so does the other.
The compressor’s spinning pressurizes air before it enters the engine’s throttle body. Denser, cooler air produces more power in the combustion chambers. The exhaust gases created from the combustion are fed through the turbine, causing it to spin and thus spin the compressor.
Turbochargers don’t create power until the turbine is turned by the exhaust. This means that the power isn’t instantaneous. A turbocharger's power is determined by boost threshold and turbo lag. The boost threshold is the amount of pressure that’s necessary to turn the compressor. This gap between when you start operating your car and when the turbo starts creating more power is called “turbo lag.”
Turbo lag is the biggest disadvantage to turbochargers. It’s no wonder that manufacturers have developed ways to limit turbo lag. One of those ways is a twin-scroll design for turbochargers.
Twin-scroll turbochargers are designed to mirror the advantages of having two turbochargers.
In a twin-scroll turbocharger setup, the exhaust manifold separates the exhaust gas based on pairs of cylinders. It then routes those exhaust gases in such a way that they hit the turbine in an alternating fashion. This means that the turbine doesn’t lag the way it would if it were getting exhaust in bursts.
The result is more consistent power.
Inside a Twin Scroll Turbocharger
Peak Boost and Overboosting
Boost references how much the intake manifold’s internal pressure exceeds the external pressure. Peak boost occurs when the internal vs external pressure is optimized. A turbocharger creates the most power at peak boost.
Too much boost creates overboost, and overboost is bad for your engine. Engines can only contain so much power, and more than that can cause serious problems.
The line between peak boost and overboost would be challenging to manually find. Fortunately, safety measures exist to prevent turbochargers from creating overboost. One of these measures is the wastegate.
The wastegate is a valve that redirects exhaust gases away from the turbine. This prevents the turbine from getting more exhaust than it would be able to safely use. When peak internal pressure has been reached, a spring is actuated and the wastegate is opened.
As long as peak boost is held, the wastegate will be the preferred route for exhaust since it will have a lower external pressure. Some believe that turbo flutter is actually caused by “wastegate chatter.”
The “wastegate chatter” rumor would lend credence to the opinion that turbo flutter indicates high performance. While wastegate chatter is a turbo-specific noise, it is different than turbo flutter.
Wastegates can either be internal or external to the turbocharger. Most production turbochargers will have an internal wastegate. This is partially because they’re more compact, and also partially because internal wastegates are easier to install. External wastegates are better for performance but are noisier and not always street legal.
What Causes Compressor Surge?
Compressor surge, or turbo flutter, doesn’t occur on the turbine side of the turbocharger, but rather on the compressor side. When your throttle is suddenly closed, the turbocharger lags behind in response time. The turbine has exhaust gas that it’s trying to use but can’t. Since the gas can’t go through the throttle, it exits the same way it came in. As it exits, the compressor blades chop through the air. This is what creates the famous “stu-stu-stu” noise.
Unfortunately, this noise could be bad for your engine. How bad it’s likely to be is dependent on a few factors. Some factors, like the construction of your turbocharger, are beyond your control. Others, like the speed you were traveling when your throttle closed, are sometimes controllable. How often you’ve had compressor surge is another factor that determines how badly your engine may be damaged.
Much like overrevving your engine, the results of compressor surge are difficult to predict. The results can range from “almost unnoticeable” to “catastrophic failure.” Fortunately, there are ways to make sure that compressor surge doesn’t happen while still adjusting your throttle pressure as needed.
How To Fix Compressor Surge
Blow Off Valve
Blow Off Valve
A blow off valve is like a wastegate, but while the wastegate reroutes exhaust gases, a blow off valve redirects air from the compressor. Like a wastegate, the blow off valve operates automatically based on pressure differences.
The sound of air escaping the blow off valve has been described as turbo flutter by some, but is different in a few key ways. The first is the actual sound. The blow off valve will make a “psssh” sound, only louder. This is the sound of air harmlessly escaping your turbo. Unlike actual turbo flutter, this sound isn’t indicative of any kind of harsh or overly heavy use for your engine.
If this is the noise that you associate as turbo flutter, then you’re in luck. Blow off release isn’t harmful to your engine or turbocharger in any way. This satisfying, turbocharger-specific noise should dispel any lingering doubts your friends had about what you have under your hood.
- Acts as a wastegate, but on the compressor side.
- Creates a pleasing noise.
- Operates automatically.
Another way of addressing compressor surge without a blow off valve is using an anti-lag system. Anti-lag systems address both compressor surge and turbo lag. Since they handle the two main turbocharger troubles, these are an especially elegant solution.
Anti-lag systems reroute air before it enters the turbine, causing a spike in the pressure of the exhaust manifold. This keeps the turbocharger spooled, so there’s no lag between when you first hit the gas and when you feel the turbo power kick in.
- Reroute air before it enters turbine.
- Keeps turbocharger spooled.
- Addresses compressor surge and turbo lag.
If you love the unique noises of turbocharged engines, you’re definitely not alone. Turbo flutter is one noise we can all do without though. Adjusting throttle pressure is an important part of driving, and engine braking shouldn’t damage your turbocharger.
Blow off valves are an easy fix for turbo flutter. Most turbochargers on the market today come with a blow off valve installed. If you’re worried the factory blow off valve isn’t enough protection, there are aftermarket blow off valves available. For drivers who are frequently at peak boost, these blow off valves can be a good investment.
Anti-lag systems are another way to protect your turbocharged engine from compressor surge. In addition to compressor surge protection, these systems can eliminate turbo lag.
Image Credit: Images licensed via Creative Commons