Myth-busting in our 800whp Nissan R32 GTR!
You may have heard turbocharged vehicles make a whole lot of noise when the driver backs off the throttle, specifically those with large turbos, large intercoolers, no blow off valve, and non restrictive intakes. But exactly what is this sound, does it hurt your turbo, and how does it happen.
We take out the StreetFX R32 GTR, which has a particularly large GTX3582R turbo, and demonstrate the different types of flutter and chat about what is happening.
Essentially, the noise is caused by what is known as "reversion". When a turbocharged vehicle builds up boost, and the driver backs off the throttle, the throttle body snaps shut, but the turbo is still spinning, trying to compress air.
This traps a whole lot of boosted air between the turbocharger and the throttle body, and if you do not have a blow-off valve (a device designed specifically to eliminate this pressure), then the compressed air has nowhere to go, besides back through the turbocharger, and out into the intake.
When the compressed air has nowhere to go, it causes the turbo rotational speed to rapidly drop, and attempts to push against the wheel. This can cause premature wear on your turbo, however closed throttle flutter on modern turbochargers is unlikely to cause a noticeable drop in turbocharger lifespan.
Nearly all turbos made for car use in the past five years use "ball-bearing" centres, as opposed to the old style "bushed" turbos. Ball-bearing turbos are less impacted by this "push" movement, however it is true that older turbos can develop substantial play in the bushed centre.. That said, if you're still running a bushed turbo on a performance car - maybe it's time to upgrade anyway!
To explain the difference between bush (also known as sleeve or journal bearing) and ball bearing turbos, check the images below.
Above: Sleeve/Bush type turbocharger
Ball bearing turbocharger
Note: We accidentally say "Blow off valve" during the video at one point when meaning "Throttle body" closing, this is obviously an error. A blow off valve helps alleviate the pressure in the intake during closed throttle surge, "blowing off" excess boost.
Note 2: We touch on the difference between open and closed throttle flutter. As an additional bit of information here, it was pointed out to me that closed throttle flutter is actually known as "reversion", whereas open throttle flutter is more accurately called "compressor surge". Both are technically compressor surge, and both sound essentially the same, however the different conditions under which they occur are important to identify.