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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 > Turbo PSI



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      11-05-2013, 11:15 AM   #1
tennis_pr0
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Turbo PSI

I did do a search and could not find a thread on this. I am just curious, when it comes to our twin turbo PSI (n54) is the PSI of both the turbos combined (so if I am pushing 18psi, 9 psi each totaling 18) or is it each one individually? No specific reason I am asking this question, just curious. Thanks.
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      11-05-2013, 11:26 AM   #2
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that is per turbo, it is not combined
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      11-05-2013, 11:28 AM   #3
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So each turbo is making 18 psi on a car pushing 18psi?
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      11-05-2013, 11:29 AM   #4
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Quote:
Originally Posted by tennis_pr0 View Post
So each turbo is making 18 psi on a car pushing 18psi?
each turbo is making 18 however each turbo only serves 3 cylinders each so dont be thinking we are pushing 36psi lol
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      11-05-2013, 11:42 AM   #5
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Too bad the PSI's weren't in kilograms. That'd be near 6" or so.
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      11-05-2013, 11:49 AM   #6
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Quote:
Originally Posted by uberbeast21 View Post
each turbo is making 18 however each turbo only serves 3 cylinders each so dont be thinking we are pushing 36psi lol
So then a tune for the n55 engine would be making only about half the power. Single turbo powering 6 cylinders vs a twin turbo (n54) powering 6 cylinders?
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      11-05-2013, 11:55 AM   #7
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So then a tune for the n55 engine would be making only about half the power. Single turbo powering 6 cylinders vs a twin turbo (n54) powering 6 cylinders?
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      11-05-2013, 11:56 AM   #8
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Seriously? You realize that both turbos merge and feed the same pipe. It's why your intercooler only has one in and one out. Each turbo is fed by 3 cylinders, but then the air compressed by the turbos is merged before the IC and then is fed to the intake manifold. Notice that only has one inlet as well...

Single and twin have nothing to do with it and psi doesn't have that much to do with it either. It's all about flow rate. Ie, a bigger turbo can flow more air at a lower boost level than a smaller turbo. The N55 turbo is bigger than the N54 turbo so it's not a direct comparison.
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      11-05-2013, 12:21 PM   #9
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the pressure is essentially the same everywhere after the turbos. It is not cumulative.
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      11-05-2013, 01:18 PM   #10
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I think people many times confuse boost pressure as a measurement of air flow.
It's simply pressure. Pressure as a result of some restriction. In this case the engine.

Imagine a bicycle tire at 30 PSI and a tractor trailer tire at 30 PSI. Which has more air? (the tractor trailer tire). They are both at 30 PSI though but have different amounts of air.

Both turbo's are not producing the same PSI. They don't produce PSI. They just move air. You can't compare boost pressure from different turbo's let alone different engines. With pressure being equal you still have to account for CFM and flow.
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      11-05-2013, 01:33 PM   #11
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Jeff, if I may, lets tweak your example a bit.

You have 2 tractor tires. Each of them has a hole in it leaking air. One is a pin hole. The other is a 2" gouge. Now imagine that you have air compressors hooked up to the tires and they're sized such that even with the leaks in the tires, each will hold a steady 30 psi.

The compressor hooked up to the tire with the pin hole in it is able to be sized pretty small, because only a small amount of air is flowing out of the pinhole in the tire.

The compressor hooked up to the tire with the 2" gouge in it has got to be huge -air very easily flows out of that big hole so to keep 30 psi in the tire the compressor is enormous.

Both are rated at 30 psi. One is gigantic. As Jeff stated, it's about air flow. That's the biggest reason why running the same boost on a car with downpipes versus one with stock downpipes makes more power; the "hole in the tire" got bigger so more air is flowing to keep the same pressure. It's all about air flow.
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      11-05-2013, 02:00 PM   #12
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Quote:
Originally Posted by C.Pop View Post
Jeff, if I may, lets tweak your example a bit.

You have 2 tractor tires. Each of them has a hole in it leaking air. One is a pin hole. The other is a 2" gouge. Now imagine that you have air compressors hooked up to the tires and they're sized such that even with the leaks in the tires, each will hold a steady 30 psi.

The compressor hooked up to the tire with the pin hole in it is able to be sized pretty small, because only a small amount of air is flowing out of the pinhole in the tire.

The compressor hooked up to the tire with the 2" gouge in it has got to be huge -air very easily flows out of that big hole so to keep 30 psi in the tire the compressor is enormous.

Both are rated at 30 psi. One is gigantic. As Jeff stated, it's about air flow. That's the biggest reason why running the same boost on a car with downpipes versus one with stock downpipes makes more power; the "hole in the tire" got bigger so more air is flowing to keep the same pressure. It's all about air flow.
That's a much better example.
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      11-07-2013, 02:55 PM   #13
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long story short, its the pressure at the CP which is a combined pressure from both turbo's due to a pipe feeding the FMIC (combining turbo outputs into one pipe) ... I did a test once to see if one of the turbo's was weaker than the other (disconnected vaccum line did a run then disconnected the other) .. rear is usually weaker than the front dunno why (cooks more... away from fan ? I dont know)
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      11-23-2013, 02:25 PM   #14
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Quote:
Originally Posted by 007_e350 View Post
long story short, its the pressure at the CP which is a combined pressure from both turbo's due to a pipe feeding the FMIC (combining turbo outputs into one pipe) ... I did a test once to see if one of the turbo's was weaker than the other (disconnected vaccum line did a run then disconnected the other) .. rear is usually weaker than the front dunno why (cooks more... away from fan ? I dont know)
very interesting
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      11-23-2013, 03:49 PM   #15
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Quote:
Originally Posted by 007_e350 View Post
long story short, its the pressure at the CP which is a combined pressure from both turbo's due to a pipe feeding the FMIC (combining turbo outputs into one pipe) ... I did a test once to see if one of the turbo's was weaker than the other (disconnected vaccum line did a run then disconnected the other) .. rear is usually weaker than the front dunno why (cooks more... away from fan ? I dont know)
Probably due to the rear cylinders getting less air.
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      11-25-2013, 11:42 AM   #16
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Probably due to the rear cylinders getting less air.
from intake ? they actually get more air than the front ones, ask me how I know

did walnut blasting and rears were very hard to clean compared to fronts, the CP curves right at the end air probably pushes more into the rears due to that turn .. intake manifold shd be one 'baloon' rather than channeled for each cyl, maybe for fire prevention dunno

Last edited by 007_e350; 11-25-2013 at 11:52 AM.
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      11-25-2013, 12:12 PM   #17
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from intake ? they actually get more air than the front ones, ask me how I know

did walnut blasting and rears were very hard to clean compared to fronts, the CP curves right at the end air probably pushes more into the rears due to that turn .. intake manifold shd be one 'baloon' rather than channeled for each cyl, maybe for fire prevention dunno
Carbon buildup is a resultant of the rear cylinders being closer to the incoming source of pcv gases, not the amount of air being fed into the cylinders via the charge air.
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      11-25-2013, 12:16 PM   #18
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Carbon buildup is a resultant of the rear cylinders being closer to the incoming source of pcv gases, not the amount of air being fed into the cylinders via the charge air.
gotcha! that makes more sense, can you pm me how PCV works pls, thats my 'weak' area haha
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      11-25-2013, 12:54 PM   #19
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Carbon buildup is a resultant of the rear cylinders being closer to the incoming source of pcv gases, not the amount of air being fed into the cylinders via the charge air.
+1
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      11-25-2013, 01:25 PM   #20
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gotcha! that makes more sense, can you pm me how PCV works pls, thats my 'weak' area haha
To be honest, I'm a little rusty on the exact modes for this vehicle. There is an insanely long thread about it that I'll try to remember to find for you later when I'm not on my phone.

The gist, and someone please correct me if I reverse it, is when the vehicle is under vacuum (cruising), the engine sucks air out of the crankcase via the venturis in the valve cover into the intake manifold. When the vehicle is in positive pressure (boosting), air is sucked out of the crankcase by the rear turbo intake tube. The first method is how valves get gunked...the second is why you end up with oil in your intercooler. It's the reason why OCC's don't keep carbon from building up, but ideally help keep your charge plumbing a bit cleaner. Hope I got those matched correctly, and hope it helped. Again, I'll try to find that thread for you and post a link in here later.
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      11-25-2013, 01:45 PM   #21
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Quote:
Originally Posted by pwr hungry
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Originally Posted by 007_e350 View Post
gotcha! that makes more sense, can you pm me how PCV works pls, thats my 'weak' area haha
To be honest, I'm a little rusty on the exact modes for this vehicle. There is an insanely long thread about it that I'll try to remember to find for you later when I'm not on my phone.

The gist, and someone please correct me if I reverse it, is when the vehicle is under vacuum (cruising), the engine sucks air out of the crankcase via the venturis in the valve cover into the intake manifold. When the vehicle is in positive pressure (boosting), air is sucked out of the crankcase by the rear turbo intake tube. The first method is how valves get gunked...the second is why you end up with oil in your intercooler. It's the reason why OCC's don't keep carbon from building up, but ideally help keep your charge plumbing a bit cleaner. Hope I got those matched correctly, and hope it helped. Again, I'll try to find that thread for you and post a link in here later.
Yes you are correct. Even with a catch can on the positive pressure side of the pcv valve the engine still sucks in oil threw the vacuum side via the ports in each runner. Either side of the spectrum the oil separation is the same and gets directed by the pcv check valve. So needless to say a catch can doesn't really do much, you are vacuum more than boost. It will keep turbos and intercooler cleaner but valves will still be gunked up. Best solution is to delete the vacuum ports all together and run the pcv to a constant vacuum source, like the exhaust.
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