Quote:
Originally Posted by Kingtal0n
We are not increasing exhaust velocity through the turbine- thats been the point the entire time. To hold wheel speed constant the force on the turbine must be reduced to maintain the similar wheel speeds. Meaning we have the identical amount of exhaust gas volume to wastegate as before, and the same exact wastegate as before, therefore wastegate efficiency remains constant since it didn't get any larger and there is no additional exhaust gas that needs to flow through it.
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..except we are and that's why the OP is experiencing boost creep? Just Google "boost creep backpressure" and you'll find numerous articles about why reducing exhaust backpressure (like your downpipe solution) leads to increased flow through the turbine wheel (mind you a larger one like a 2871r) resulting in a boost creep situation. As the RPM's rise there's additional exhaust flowing through the turbine in extremely high flaming velocity, like a jet engine's exhaust (I say this tongue in cheek) and the stock t25 wastegate just sucks for bypassing this when there's less backpressure.
https://cobbtuning.atlassian.net/wik...GR+Boost+Creep
Quote:
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"Boost creep is a condition of rising boost levels past what the predetermined level has been set at. Boost creep is caused by a fully opened Wastegate(s) not being able to flow enough exhaust to bypass the housing via the Wastegate(s) itself. For example, if your boost is set to 12psi, and you go into full boost, you will see a quick rise to 12 or 13psi, but as the rpm's increase, the boost levels also increase beyond what the boost controller or stock settings were. Boost creep is typically more pronounced at higher rpm's since there is more exhaust flow present for the Wastegate(s) to bypass. Effective methods of avoiding or eliminating boost creep include porting the internal Wastegate(s) opening to allow more airflow out of the turbine, or to use an external Wastegate(s)."
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https://www.bimmerforums.com/forum/s...h-backpressure
You're thinking in hypotheticals. Throw a full catless turboback on a stock FD3S and watch this not happen:
"So when the downpipe volume grows larger, and the turbine isn't using any of the extra space (same wheel speed with even less pressure) all that extra volume can be used by the wastegate."