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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 > Where is Boost measured by the ECU



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      09-27-2009, 02:53 PM   #1
e.n335
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Where is Boost measured by the ECU

Hi,

Measuring my boost via OBD II ( Auterra Dyno Scan 8.0.9 ) I get 17.4 psi = 1.2 bar boost. My vent gauge ( post FMIC pressure ) shows < 15 psi always. Is the ECU OBD II boost sensor located before the IC and / or what kind of data is the ECU OBD II sensor delivering ? Is it the REAL boost the turbos have to provide, respecting altitude compensation and FMIC pressure loss ? What's going on here ?

Thank you in advance,
Eugen
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      09-27-2009, 02:58 PM   #2
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+1 good question
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      09-27-2009, 03:48 PM   #3
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Quote:
Originally Posted by e.n335 View Post
Hi,

Measuring my boost via OBD II ( Auterra Dyno Scan 8.0.9 ) I get 17.4 psi = 1.2 bar boost. My vent gauge ( post FMIC pressure ) shows < 15 psi always. Is the ECU OBD II boost sensor located before the IC and / or what kind of data is the ECU OBD II sensor delivering ? Is it the REAL boost the turbos have to provide, respecting altitude compensation and FMIC pressure loss ? What's going on here ?

Thank you in advance,
Eugen
Pretty sure there are two such sensors....one MAP sensor before the throttle plate on the intake charge pipe and a TMAP sensor located on the intake manifold itself.....so neither one is before the intercooler.

Obviously the ECU reads both sensors or otherwise they would not be there

So the question is which one is used primarily to report boost pressure to your OBD device.

Your boost gauge measures "actual" boost that the engine sees from the intake manifold so the 15 psi reading makes sense.

The +17 psi reading is probably one of two things:

1) The boost spike seen by the sensor before the throttle body
or
2) You need to subtract atmospheric pressure from your reading as normal atmospheric pressure is about 1 bar. Boost pressure is expressed as being above this value, but some sensors don't do the math and just give you an absolute pressure.

BTW, just because an aftermarket intercooler has less resistance to boost pressure (hence the term "pressure drop") doesn't mean that your DME still won't hit the same boost target as before.

There is some boost drop due to cooler intake temps associated with a better intercooler, but for the same intake temps, an intercooler will just allow your turbos to hit the stated boost target with a little less compressor speed since they are fighting less resistance to pressure from the intercooler.

Clear as mud?
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      09-27-2009, 04:00 PM   #4
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Quote:
Originally Posted by Ilma View Post
Pretty sure there are two such sensors....one MAP sensor before the throttle plate on the intake charge pipe and a TMAP sensor located on the intake manifold itself.....so neither one is before the intercooler.

Obviously the ECU reads both sensors or otherwise they would not be there

So the question is which one is used primarily to report boost pressure to your OBD device.

Your boost gauge measures "actual" boost that the engine sees from the intake manifold so the 15 psi reading makes sense.

The +17 psi reading is probably one of two things:

1) The boost spike seen by the sensor before the throttle body
or
2) You need to subtract atmospheric pressure from your reading as normal atmospheric pressure is about 1 bar. Boost pressure is expressed as being above this value, but some sensors don't do the math and just give you an absolute pressure.

BTW, just because an aftermarket intercooler has less resistance to boost pressure (hence the term "pressure drop") doesn't mean that your DME still won't hit the same boost target as before.

There is some boost drop due to cooler intake temps associated with a better intercooler, but for the same intake temps, an intercooler will just allow your turbos to hit the stated boost target with a little less compressor speed since they are fighting less resistance to pressure from the intercooler.

Clear as mud?
Well, parts of it . The 17.4 psi are no spikes, it's constant boost. Thanks
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      09-27-2009, 04:09 PM   #5
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Quote:
Originally Posted by e.n335 View Post
Well, parts of it . The 17.4 psi are no spikes, it's constant boost. Thanks
The T-map sensor and the manifold boost tap where the gauge is are both after the IC... the thing that is making the difference is that your boost gauge is reading from AFTER the throttle, and your dynoscan software is probably using the t-map sensor BEFORE the throttle.

If you tune is getting throttle closure, it will cause constant boost to be higher pre-throttle (where the t-map is), and lower in your intake manifold (where your vent gauge reads from). I have seen tunes with really bad throttle closure keep the throttle at 50% even though the pedal is to the floor and boost is reading 15psi in the manifold... this will cause pressure to build up slightly more in the intercooler pipe, pre-throttle.

What is really important is what boost makes it into the manifold, which is what you see on your vent gauge, but at the same time, if you do have throttle closure, that is working your turbos harder than need be.

Can you log actual throttle position compared to pedal position to see if you are getting throttle closure?

-Rick

Last edited by RiXst3r; 09-27-2009 at 09:28 PM.
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      09-27-2009, 04:12 PM   #6
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+1

I can and will do.

Thank you Rick !
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      09-27-2009, 04:29 PM   #7
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Hmmm ...

it's simply physics that the turbos have to provide more boost pressure than you see in the intake manifold. This is due to piping, FMIC and temperature drops. Altitude compensation as well. So the question I have is:

Do I see the REAL turbo boost using OBD II ?

Thanks,
Eugen
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      09-27-2009, 04:44 PM   #8
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Quote:
Originally Posted by e.n335 View Post
Hmmm ...

it's simply physics that the turbos have to provide more boost pressure than you see in the intake manifold. This is due to piping, FMIC and temperature drops. Altitude compensation as well. So the question I have is:

Do I see the REAL turbo boost using OBD II ?

Thanks,
Eugen
Keep in mind, your OBDII (EDIT: With a BT Tool) value is reading from the T-Map sensor, which is just about a foot or so BEFORE the throttle... so it is after all the piping, FMIC, etc which you speak of... but movement of the throttle causes it to spike or raise up... this is why you have DV's... to release this excess pressure and keep the turbos from "pumping up" your intake piping... but they do not open until a set pressure differential between the intake piping and the manifold is reached.

OBDII boost is not "real" boost... it is an ABSOLUTE value, and therefore needs calibrations depending on your ambient conditions. Take the reading while the engine is off, and figure it from there. (that will be your zero)

Last edited by RiXst3r; 09-27-2009 at 09:28 PM.
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      09-27-2009, 05:13 PM   #9
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Quote:
Originally Posted by RiXst3r View Post
Keep in mind, your OBDII value is reading from the T-Map sensor, which is just about a foot or so BEFORE the throttle... so it is after all the piping, FMIC, etc which you speak of... but movement of the throttle causes it to spike or raise up... this is why you have DV's... to release this excess pressure and keep the turbos from "pumping up" your intake piping... but they do not open until a set pressure differential between the intake piping and the manifold is reached.

OBDII boost is not "real" boost... it is an ABSOLUTE value, and therefore needs calibrations depending on your ambient conditions. Take the reading while the engine is off, and figure it from there. (that will be your zero)
Great. Thank you.
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      09-27-2009, 05:51 PM   #10
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Pick up the BT interface and then subtract ambient pressure from intake tract pressure to get a reading. But even then, this does not account for the drop across the TB. The manifold sensor is primarily meant for vacuum only and does not measure significant boost. The TMAP will measure boost but little vacuum, as there is none prior to the TB.
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      09-27-2009, 07:29 PM   #11
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Quote:
Originally Posted by e.n335 View Post
Hi,

Measuring my boost via OBD II ( Auterra Dyno Scan 8.0.9 ) I get 17.4 psi = 1.2 bar boost. My vent gauge ( post FMIC pressure ) shows < 15 psi always. Is the ECU OBD II boost sensor located before the IC and / or what kind of data is the ECU OBD II sensor delivering ? Is it the REAL boost the turbos have to provide, respecting altitude compensation and FMIC pressure loss ? What's going on here ?

Thank you in advance,
Eugen
You are reading from the manifold sensor, which will max out at around 3psi, 17psi less atmos as it is absolute. You would need a BT to read from the TMAP sensor. You can order one from my signature.

Mike
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      09-27-2009, 07:39 PM   #12
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Or he can use mine for free. I dont need it at the moment.
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