[vasillalov]

Hi Folks,

I've done some logging with the COBB AP recently and I noticed that at idle, the engine barely makes 0.5 psi of vacuum. Normally it sits at 0.0 or 0.01 vacuum.

Is this normal for the N54? ...or do I have a vacuum leak somewhere? If I indeed have a vacuum leak, my guess would be its because of the BSH occ, as I have no other engine mods that could affect the vacuum.

Thanks for your time.

[skim7x]

Uhhh... my boost gauge (and others I've seen) have anywhere from -18 to -21 vacuum at idle... although now that I think about it, -21psi is a LOT and so I'm not really sure what that means... but I've never sat at 0 before...

[cn555ic]

I always see at idle -20 to -21 vacuum.

[bulldog_yyc]

Originally Posted by skim7x

Uhhh... my boost gauge (and others I've seen) have anywhere from -18 to -21 vacuum at idle... although now that I think about it, -21psi is a LOT and so I'm not really sure what that means... but I've never sat at 0 before...

same, -21 at start and settles to -18 after warm up

[vasillalov]

WOW! -18 to -21 at idle! Now I am worried!...

How are you guys measuring the vacuum? I am getting my results from the live engine monitoring from the COBB AP. At idle, I am showing -0.5psi and more often -0.1 psi of vacuum.

[cstmx_ryder]

Are you reading this off your boost gauge?

The ONLY time my gauge reads 0 is only when the car ignition is 'on', engine off......so there's no way that it can read vacuum without the engine running?

As others have said, the vacuum readings should be in the negatives......mine read -17 vacuum.

Originally Posted by vasillalov

Hi Folks,

I've done some logging with the COBB AP recently and I noticed that at idle, the engine barely makes 0.5 psi of vacuum. Normally it sits at 0.0 or 0.01 vacuum.

Is this normal for the N54? ...or do I have a vacuum leak somewhere? If I indeed have a vacuum leak, my guess would be its because of the BSH occ, as I have no other engine mods that could affect the vacuum.

Thanks for your time.

[vasillalov]

Originally Posted by cstmx_ryder

Are you reading this off your boost gauge?.

No boost gauge yet as I am waiting for the P3 v2 gauge since December of last year. I am using the Cobb AP live monitoring to measure boost.

[bulldog_yyc]

Originally Posted by vasillalov

No boost gauge yet as I am waiting for the P3 v2 gauge since December of last year. I am using the Cobb AP live monitoring to measure boost.

I think the measurement point is different between what the ECU/Cobb sees and what the aftermarket gauges report.

[cstmx_ryder]

Originally Posted by vasillalov

No boost gauge yet as I am waiting for the P3 v2 gauge since December of last year. I am using the Cobb AP live monitoring to measure boost.

So that vacuum reading was with the engine 'on' ?

[vasillalov]

Originally Posted by cstmx_ryder

So that vacuum reading was with the engine 'on' ?

Yes! Engine is on at idle...

[vasillalov]

Originally Posted by turkish335

I think the measurement point is different between what the ECU/Cobb sees and what the aftermarket gauges report.

That could be because Cobb gets the CAN bus boost value which is measured before the throttle plate. Most boost gauges will tap into the vacuum lines after the throttle plate. I just wanted to confirm that what I am seeing is normal.

Can someone with a Cobb AP do me a favor: start your car and let it idle? Then tell me what is your boost/vacuum at idle as seen by your Cobb AP unit.

Thanks in advance.

[themyst]

When I had the 135 with mechanical boost gauge, my vacuum read -19 to -21 as well. I think the Cobb reads pre boost; you're fine.

[Joshboody]

you're only seeing map before the throttle, so your lowest reading is baro. Cobb probably takes out standard, average baro in the logging calculation.

[Transport3r]

Make sure you consider units when reading off a gauge. What most of you are seeing is ~18 In of mercury (at 32*F) Which comes out to about 8.5 PSI if I'm not mistaken.

The pressure sensor the COBB is reading sits upstream of the TB, and on idle this sensor will not read the actual manifold pressure, since the TB is closed.
People with boost gauges read off the line from the DV's, which goes to the manifold, readign the true vacuum pressure.

[marv85]

Originally Posted by vasillalov

That could be because Cobb gets the CAN bus boost value which is measured before the throttle plate. Most boost gauges will tap into the vacuum lines after the throttle plate. I just wanted to confirm that what I am seeing is normal.

Can someone with a Cobb AP do me a favor: start your car and let it idle? Then tell me what is your boost/vacuum at idle as seen by your Cobb AP unit.

Thanks in advance.

Hey man,

So I'm sure I read somewhere that it is normal for us not to see negative boost readings with the CObb ap (has to do with how the COBB fetches those values vs a regular boost gauge). So don't worry about negative values.
On the other side, I can let my car idle and check the idle boost for you on COBB AP, after work .

[vasillalov]

Good info guys! Thanks for the clarification. It makes sense. I also watched a demo video of the P3 gauge (v2). It also reads from the CAN bus and it shows almost no vacuum.

[NiVeDh]

Originally Posted by joshuo

Make sure you consider units when reading off a gauge. What most of you are seeing is ~18 In of mercury (at 32*F) Which comes out to about 8.5 PSI if I'm not mistaken.

The pressure sensor the COBB is reading sits upstream of the TB, and on idle this sensor will not read the actual manifold pressure, since the TB is closed.
People with boost gauges read off the line from the DV's, which goes to the manifold, readign the true vacuum pressure.

I normally read about -20 inHg on my P3 gauge. That "PSI" light is misleading.

[ren]

Just to clarify and confirm what a few others already said, the vacuum reading that the P3 gauge shows is in In.Hg but the boost reading is in PSI. To convert In.Hg to PSI, you need to multiply it by 0.4912.

Cobb reads shows the data from the engine's boost sensor that is before the throttle blade and I don't think there is any vacuum in there. Correct me if I'm wrong.

[marv85]

Originally Posted by vasillalov

Good info guys! Thanks for the clarification. It makes sense. I also watched a demo video of the P3 gauge (v2). It also reads from the CAN bus and it shows almost no vacuum.

Hey man,
So I hooked my AP today. I was reading 0.1 PSI when the car is idle.

[vasillalov]

Originally Posted by marv85

Hey man,
So I hooked my AP today. I was reading 0.1 PSI when the car is idle.

Thanks. This makes sense! I appreciate it.

[bimmertt]

what parameter are you monitoring for this?

[Jeffman]

Sorry to revive old thread. Can the Cobb AP read actual vacuum in the engine. Any sensor in there, or do I need to hook up a vacuum gauge like how the P3 is connected? Thanks.