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

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      09-11-2006, 08:25 PM   #1
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BMW 335i turbo boost control information - edit: Dyno added to posting - 277 whp

Some insight into how the boost control logic works in the biturbo 335. (posted by member shiv (shiv@vishnu))

Very early stages of finding a way to add more power.


Hi guys,

I've been playing around with our new 335i and found a few interesting bits. One thing I noticed, after much datalogging, probing and head-scratching is its boost control system. It's pretty unique. I haven't seen anything like this employed before in any turbo application, twin turbo or otherwise.

Unlike other turbo systems (evo included) which have a positive mechanical wastegate pressure (that keeps the wastegate normally shut); the wastegates on the turbo are designed to remain fully extended in its natural state. This means if you disconnected all forms of boost control, the 335i runs 0psi of boost. On the other hand, if you do this to an EVO, it will run 11psi of boost. To get more boost in the EVO, the factory boost control solenoid bleeds off the wastegate pressure so the wastegate stays more shut. If the wastegate signal hose, for some reason, were to pop off or rupture, the wastegate would get no boost signal, and you'd get an engine-grenading overboost condition.

With the BMW, to get more boost, it regulates a *VACUUM* signal to pull *CLOSE* the naturally open wastegate. During normal off-boost driving, the wastegate is fed 10" of vacuum which keeps the wastegate partially open. During WOT, but before full boost is reached, the wastegate is fed 20" of vacuum, which completely closes the wastegates for maximum boost response. Once full boost is reached, the wastegate sees 3-5" of vacuum used to regulate the wastegate opening for sustaining the desired boost. Once fuel cut is reached, the clever folks at BMW decided that boost needs to go to 0psi so as to not result in a big, uncivilized "bang bang bang" against the rev limiter. To do this, it cuts all vacuum to the wastegates and they go fully open (no boost!).

The upside to this approach, as i see it at least, is as follows:

1) During start-up, the wastegates are open and the turbos are effectively bypassed. This means that much of the hot exhaust coming out of the engine doesn't go through the turbine wheel and cool off. Instead, it goes right out the wastegate and right into the catalyst. This gets them up to temp quicker and makes the boys at the EPA happy. This also means that BMW can push the cats back downstream a little bit instead of cramming them right up against the turbo where backpressure effects are most deleterious.

2) There is no way to induce an overboost since the natural state of the wastegate is fully open. In other turbo cars, something as simple as a wastegate line rupture will cause the car to overboost.

3) Its just cool engineering. Leave it to the Germans.

The downside to this approach, as i see it at least, is as follows:

1) It requires the use of a separate vacuum tank that needs to be smuggled under the hood somewhere. This takes up a little bit of space and costs extra money. Not much but we all know how OEMs like to shave off every extra bit of material and cost.

2) It confused me for a good part of the morning.

Edit: Dyno results from vishnu:
Originally Posted by shiv@vishnu
4th gear runs
Dyno actually reads a bit low for a Dynojet. Would probably see 10whp more on other local dynojets.

Last edited by JackMac; 09-12-2006 at 08:06 PM.
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      09-12-2006, 12:43 PM   #2
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That is very interesting information.

I may be incorrect but I assume that also mean that upping boost via ECU control is not going to be possible and the only way to up boost is chaning out the plumbing; waste gates, downpipes etc.

That isn't to say that you can't make some serious upgrades in HP with timing, lift and durations settings.

Am I incorrect?
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      09-13-2006, 12:49 AM   #3
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thats not bad for fi in 90 degree weather
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      09-14-2006, 05:47 AM   #4
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JackMac...Just out of curiousity, what does 277WHP equate to in crank HP?
3**HP? 330-350HP? Also JackMac, from your experience, how much would you guesstimate a cold-air-intake and exhaust setup can increase in HP? I'm going with the aFe CAI and the Rogue exhaust whenever they come out with the dual exhaust mod for the 335i.

BTW...so excited, my 335i-sedan is on its way across the Atlantic to NJ. Just a matter of few more weeks. I hope I can sell my Acura CL-S in 2 wks. Does anyone know of someone that wants to buy a Modified Acura CL type S? Can sell to NY, NJ, MD, DC, and VA folks. Check out "Modified 2001 Acura CL Type S 3.2L Nav" on craigslist.
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      09-14-2006, 05:54 PM   #5
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Can i have the link to Shiv's post? Tia.
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      09-14-2006, 08:33 PM   #6
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Originally Posted by Holygrail
JackMac...Just out of curiousity, what does 277WHP equate to in crank HP?
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