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Why Diesel Turbo Loses Boost At Much Slower Rate Than Gasoline Turbo When Lifting Off Throttle?

Discussion in 'General Discussion' started by 440cid, Nov 2, 2018.

  1. 440cid

    440cid
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    I wonder why when we lift off the throttle, gasoline engine loses turbo boost instantly while diesel engine loses boost at much slower rate. I think I read somewhere it has something to do with exhaust back pressure but I am not too sure how it works and I'd like to know more about this. Please educate me ;)

    Turbocharged Gasoline:


    Turbocharged Diesel:
     
  2. SebastianJDM

    SebastianJDM
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    well, the gasoline turbo has a blowoff valve, which is a valve that opens to relieve the pressure in the intake. diesels do without. also, diesel engines will generally use larger turbos, so the inertia will be higher and the turbo will hold boost longer, even when comparing it with a gasoline engine w/o a blowoff valve.
     
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  3. 440cid

    440cid
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    Thanks for your explanation. I was messing with turbocharger jbeam and just found out this difference. I never really noticed or measure the boost using gauge/OBD driving while turbocharged gasoline & diesel cars.

    I guess this behavior is advantageous for turbocharged diesel engine then...
     
  4. SebastianJDM

    SebastianJDM
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    in real life it's possible to damage the turbo on a diesel engine due to not having a pressure relief, especially when running more boost or a larger turbo. that's why a lot of diesel guys use a blowoff valve on their setup, but it can't be vacuum controlled like in a gas engine, as there isn't a vacuum to pull from. they usually have them set up for throttle position iirc, which works but takes some tuning with to get right. watch this video for more information on why blowoff valves are important, and the effect of not having one (since everyone wants to be internet genius but never gets it right):
     
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  5. fufsgfen

    fufsgfen
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    Diesels don't have throttle butterfly, it is straight opening, with gasoline engine throttle butterfly closes and BOV makes pressure go away.

    As BOV relies on pressure difference between manifold and turbo piping/IC, there obviously is no such thing with diesels.
     
    • Informative Informative x 1
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