P3 gauge - boost from OBD2 port

[mmilkov]

I purchased the Turner Motorsports edition of the P3 gauge and installed it in my 2008 BMW 135i. I just want to let you know that you can, in fact, read the boost from the OBD2 port. You just have to configure the P3 gauge to use digital (d) instead of analog (A) input for boost. There is a third setting (C), but it doesn't work for N54 engine. By doing this you don't need to run a vacuum line through the firewall to the P3 gauge.
Your results may be different if you have a piggyback, i.e. I don't know if you will be reading the correct boost. I am running Cobb Accessport, which is not a piggyback and does not try to trick the ECU. The boost readings I get with the P3 gauge are consistent with what I get using the Cobb Accessport, i.e. about 14 psi peak using the Sport map.

[rjwojcik]

Lol, I too discovered the boost can be read from digital info from the OBD2, but only after spending a lot of time feeding the vacuum tube through. I've an N54, btw.

And yes, using the OBD2 will not be accurate if you use a piggy back solution as these intercept the signals to the ECU and trick it. So, in these situations you'll need to use the analogue solution via the vacuum tube.

[mmilkov]

Originally Posted by rjwojcik

Lol, I too discovered the boost can be read from digital info from the OBD2, but only after spending a lot of time feeding the vacuum tube through. I've an N54, btw.

This is why I created this small thread. I didn't feel like feding tubes through the firewall. I tried to find definite information on the forums about whether or not it is possible to read boost through OBD2 for the N54 - and I couldn't. So I decided to give it a try and share my experience. Note: I believe that the digital readout of boost is only possible with the version 2 of the P3 gauge (the only version sold now). The earlier version couldn't do it.

[Harlequin]

The OBD2 data is boost pressure BEFORE throttle body. If you tap into the DV lines, you will get true manifold pressure.

[mmilkov]

Originally Posted by Harlequin

The OBD2 data is boost pressure BEFORE throttle body. If you tap into the DV lines, you will get true manifold pressure.

Interesting... But then why do I read vacuum (-14 psi) when I let off the gas? If the pressure measurement were done before the throttle body, shouldn't I be getting atmosphering pressure (0 psi boost)?

[boostedtwin]

Good info, so OBD2 works on N54 for boost measurement. So only thing to do is to swtich to (d) on the configuration?

[jkoral]

Did you ask RiXst3r (inventor of the gauge)? He specifically told me the OBD2 value on the N54 is not accurate. I deleted his PM, but a quick google turned up a few comments from others:

From http://www.e90post.com/forums/showthread.php?t=581156

"The p3 gauge can read boost over canbus on the n54, I just choose not to because it's pre-throttle and makes peak recall useless with all the 20psi spikes whenever you let off the gas. Manifold boost is better, vacuum lines are easy on the n54."



FWIW, my N54 with the vacuum tube reads around -24 psi at idle, which is consistent with most others.

[yawn]

Originally Posted by jkoral

Did you ask RiXst3r (inventor of the gauge)? He specifically told me the OBD2 value on the N54 is not accurate. I deleted his PM, but a quick google turned up a few comments from others:

From http://www.e90post.com/forums/showthread.php?t=581156

"The p3 gauge can read boost over canbus on the n54, I just choose not to because it's pre-throttle and makes peak recall useless with all the 20psi spikes whenever you let off the gas. Manifold boost is better, vacuum lines are easy on the n54."



FWIW, my N54 with the vacuum tube reads around -24 psi at idle, which is consistent with most others.

Hmm I don't know what normal is anymore. I thought it was between -18 to -22psi? What does it mean if its higher or lower? Say if I had a reading of -15 or -30?