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Vehicle: 2012 Subaru Sti

I have been driving turbocharged cars for a while now and I have blatantly accepted that before shutting down you must let your car idle for a few minutes after driving hard to allow the turbo to cool down. This process will allow you to extend the life of your turbo, however, I must say I have not heard of any scenarios where the turbo on these cars actually gets damaged by not following this process.

Are there any set rules on how long you should let your car idle after driving hard (i.e., keeping the turbo in boost, revving up passed 6000 rpm)? Is letting your car idle before shutting down an effective method for cooling the turbo? I have also used a turbo timer at one point, it may be useful to point out where this device has strengths in this matter.

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Yes, you have to do that. Your turbo heats up to around 900 degrees Celsius under hard driving conditions. When you shut off the car, You suddenly take away the heat source and the ambient temperature falls to around 95 degrees Celsius (the normal operating temperature for a Subaru that has been driven hard). This causes the turbine housing to experience thermal shock. Now granted, it would take a long time for this thermal shock to actually cause damage, but you're probably reducing the turbo's life by putting it under stress unnecessarily. If you let the car idle for a minute or so, you allow the exhaust gas temperatures to gradually go down to 700, 600, 500, etc.

An example to support what I said: 1. When a cast iron manifold is welded (turbine housing is cast iron too), it is recommended to toss the whole thing into a fire and let it burn out overnight so that the manifold cools down gradually to prevent stress points from forming.

From experience, it's not necessary to let the car idle if you simply drive normally for the last 2 or 3 miles before you shut off the engine. Just never shut the car off immediately after doing a hard pull. E.g. when the cops pull you over on the highway.

Have a look at this video to see how hot a manifold (and by extension the turbo can get): vxr glowing manifold

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    I know this is pretty naive of me to ask, but does a turbo really heat up to 900 degree Celsius???
    – Dan
    Commented Jul 22, 2015 at 11:05
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    Yes. It's around 1500F under open loop, high boost conditions. That's the temperature at the manifold, which is the thing the turbo connects to, so not much time for it to cool down. That's only the temperature of the turbine side though, the compression side rarely goes above 80 - 90 degrees unless you're running insanely high boost.
    – Captain Kenpachi
    Commented Jul 22, 2015 at 11:28
  • Yikes, that's hot! So in a turbo charged engine, the engine will still remain around 80 degree Celsius (because it is constantly being cooled). How come the radiator/fan which cools the engine, can't keep the turbo just as cool as well?
    – Dan
    Commented Jul 22, 2015 at 12:19
  • The turbine side needs hot gases in order to make it spin really fast. If you cool down the turbine, you cool down the exhaust gases and that means you lose boost pressure and increase turbo lag. Heat is a good thing here.
    – Captain Kenpachi
    Commented Jul 22, 2015 at 12:29
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    I can't speak to the OPs Subaru but I owned a 1984 Ford Escort GT Turbo. I bought it super cheap with a blown turbo. I had the turbo rebuilt, bolted it back on and drove it like I stole it. After about a year I had to rebuild it again. That's when then turbo guys taught me about thermal soak and letting it idle before shutting it off. The key is keeping oil circulating through the turbo until the housing has cooled enough that it won't superheat the bearings, scorching the oil that's on them.
    – Matt Simerson
    Commented Jun 21, 2016 at 22:15
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As they say, you need to let the turbo spool down as it spins excess of 100,000 RPM while doing this oil is used to lubricate it, if you shut it down immediately there are high chances of oil residue forming on the turbo lines and clogging it up.

That said, most modern turbochargers do not need(while driving in the city) such cooling down period as such since this was true in case of old cars since they didn't have water cooled turbos.

I would still not advice shutting down a car after a drag race immediately , idle 15 seconds or so might do the trick

In everyday driving most people don't rev their engines so hard to create maximum boost all the time , so you don't need to wait for 15 seconds every time you have a short drive.

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Yes, otherwise parts of the turbo such as bearings can go bust and will need replacing. After hard driving, just drive your car slowly/calmly for a few miles. This will let the turbo cool down and because the engine is still on, the various turbo parts will still be lubricated whilst the turbo is cooling down.

The following post may be of some use: https://mechanics.stackexchange.com/a/5212/11074

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