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Thread: Boost drops above 4500 RPM

  1. #1
    Join Date
    Oct 2016
    Location
    Bay Area, CA
    Posts
    21
    My Cars
    1987 325is (S50 Swap)

    Boost drops above 4500 RPM

    Hey guys!

    The car is running much better! I popped open the AFM to investigate a lean issue and it turns out the previous owner had marked the correct position of the AFM wheel. After adjusting it to that, the car feels much better. It looks like the previous owner also plugged the holes on the factory wastegate raising boost to 9psi. The car feels great below ~4000 - 4500 RPM, but after that boost starts to fall to ~5PSI.

    Any ideas what would cause this? Knock box?

    I have a wideband on the way and the exhaust shop installed a o2 sensor bung when they installed the cat.

    Thanks!

  2. #2
    Join Date
    Jun 2010
    Location
    TX
    Posts
    4,487
    My Cars
    1983/84/85 745i
    Sounds like stock turbocharger performance to me. It starts dropping boost up top.

  3. #3
    Join Date
    Oct 2016
    Location
    Bay Area, CA
    Posts
    21
    My Cars
    1987 325is (S50 Swap)
    So it is normal for boost to be only 5psi or so at 5,000 RPM? a 50% drop seems excessive, but I trust your knowledge regarding these cars/turbos.

    Thanks for the insight!!! I really appreciate you being so responsive and willing to help.

    Quote Originally Posted by Hit Man X View Post
    Sounds like stock turbocharger performance to me. It starts dropping boost up top.
    Last edited by thecarousel; 04-18-2018 at 10:58 AM.

  4. #4
    Join Date
    Jun 2010
    Location
    TX
    Posts
    4,487
    My Cars
    1983/84/85 745i

    Thumbs up

    Stock boost on the Motronic was just under 6psi. Even that tapers off on the high end, past about 5200rpm from what I remember. You are also fighting stock engine management to maintain that 0.4 bar the best it can with a worn out wastegate. 4psi drop seems pretty severe that quick.

    You swapped a plugged cat. Is your exhaust the stock style with dual 2" pipes? Remember, compression bent pipes are not true 2"

    Knock and boost box will just keep pulling timing to reduce knock and keep up boost. If the knock becomes so severe or overboost sensed, it will shut down the boost system...which is power to the solenoid. It also can cut boost off totally if the AFM swings open too fast.

    In any event, try unpugging the wastegate holes and test driving it for a few days. Maybe you have a boost leak that appears at higher engine speeds/boost levels. Engine torque and could reveal a small crack in a charge coupling. The coupling right at the turbo takes the most abuse. I took one off and it fell apart in my hand. The hump hose is the next abused one due to flex and heat from manifold.
    I make E23 parts.
    09/1983 745i (stolen spring '13 around Houston, TX Achatgruen on nutria buffalo. 8481080)
    10/1984 745i
    11/1984 745i
    11/1984 735i (10:1-265/6)
    Ford, MB, and GM round out the pack.



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