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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > N54 Turbo Engine / Drivetrain / Exhaust Modifications - 335i > Technical Question



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      07-11-2009, 01:37 PM   #1
Presidio335xi
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Technical Question

I understand the BMW has tapered the psi at the top of the RPM range to save turbo life.

My question is, is that standard for all turbos or just in this case?

Why do they do it if that isn't standard?

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      07-11-2009, 01:44 PM   #2
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I am curious to this as well, why is 9psi "harder on the turbos" @ 6k RPM as opposed to 4K RPM? I mean unless they are just saying by the time you go from 3K to 6K you have been maxing the turbos for a few seconds already, thus building up too much heat? It almost seems like it would be easier to hit high boost at higher RPM's due to increased exhaust pressure/heat/speed? That plus wouldn't the engine be "wanting" more air at higher RPM?
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      07-11-2009, 02:16 PM   #3
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boost tapering is pretty common on all cars with factory turbos.... my wrx had the same taper. It's usually more of a problem with the stock turbos being too small, therefore tapering in psi as the rpms increase towards redline.

If you upgraded to a larger turbo along with supporting modifications(intake/exhaust/fuel/ecu) you can eliminate this by keeping the psi constant once spooled...

Maybe someone can explain in more detail? i'm no expert.
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      07-11-2009, 02:22 PM   #4
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Quote:
Originally Posted by Zeph View Post
I am curious to this as well, why is 9psi "harder on the turbos" @ 6k RPM as opposed to 4K RPM? I mean unless they are just saying by the time you go from 3K to 6K you have been maxing the turbos for a few seconds already, thus building up too much heat? It almost seems like it would be easier to hit high boost at higher RPM's due to increased exhaust pressure/heat/speed? That plus wouldn't the engine be "wanting" more air at higher RPM?
Yes...the engine wants more air at higher rpms.

The pistons are moving much faster and sucking in more air as a result.

Therefore, the turbos have to compress the air much faster just to keep the boost at a constant pressure.

Seems to me the only way they can do that is by spinning faster and faster as rpms climb.

Same boost = higher turbine speeds as rpms climb.

Pretty soon you max out the turbos ability to move air efficiently and they create friction which heats up the intake charge.......not to mention that shaft speeds go through the roof.
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      07-11-2009, 02:25 PM   #5
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^ awesome, that really does make sense. Thanks!

So where is this GT35R kit we have been waiting for?
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