P3 boost gauge readings help

[JHZR2]

Hi,

Bought a new P3 gauge from Turner. Did the install last night, no real hiccups. Did the configuration So it is correct for an n55. No tubing routed because of this. Also verified English units, saved the settings, verified settings were saved.

Took the car out. Remember it is using the digital sensor. Idling, it read -1.1 to --1.3. I was expecting more like -15 to -20.

It almost would be as if it is reading in the SI unit of bar, but going through the settings, it confirms that English units are selected.

So what gives? Any ideas? Something isn't right.

I was driving with the car cold and around town, so I wasn't too heavy on the throttle. But it overall ranged from a reading of -1.6 to 3.0. The other funny thing was that coasting in gear, closed throttle, only gave me a reading of -1.1 consistently. I'd think that when coasting in gear, vacuum would be highest.

Any ideas???

Thanks!

[c_ozanich]

I have a p3 with the same n55 set-up and the same readings. The reason the VAC side of things being -1.3 (roughly) is because that's taken with respect to the normal air pressure. Gauge pressure is zero-ed out with the ambient air pressure, or 14.7psi.

When the turbo spools up and there is a boost in the engine pressure, what the gauge is actually reading is the positive with respect to the zero reference. Which in actuality is 14.7 (with slight fluctuation based on altitude and temp and humidity) You're numbers are correct as far as what the gauge is showing.

Also, 1 bar = 14.5037738 psi--or typical assumption is that 1 bar is equal to given atmospheric pressure of 14.7 psi. It would be impossible to run a vac as strong as -15...let alone -20.

[Pig Farmer]

I can tell you that on the n54, the manifold pressure sits around -22 to -24 psi at idle. That would be pretty close to your figures if the P3 gauge was set to display Bars, but 3 Bars of positive pressure doesn't make sense. Can you run the vacuum line to the diverter valves on the N55? If that is possible, it very easy to route vacuum line through the fire wall.

Also, are you sure it is reading positive 3 and not positive 0.3? That would seem reasonable when driving at a "normal" pace.

[JHZR2]

Yeah, Ive seen the N54 cars showing -2x.y values at idle.

I asked the question, and they responded within minutes, which is great service to me. I was really impressed. They told me the following:

The n55 engine uses valvetronic.... therefore the throttle stays open, and you get minimal engine vacuum. On normal engines, or even the n54, you will see -17 to -21 because the throttle is closed creating the restriction which causes vacuum in the manifold. The readings you saw are perfect for an n55 and exactly what you would see on any gauge, even a vacuum tap mechanical gauge.

Thanks!

-Rick

Driving at a normal pace, it is always negative. On some occasions, like starting to lug the engine under load or some more rapid acceleration, it might go to 0.3 or so.

Under heavy throttle I saw over 7, which would be right. Also, when I put it on metric, the values change, I think I was seeing more like -0.03, IIRC.

[christopherkriens]

The readouts you're getting are normal for the N55 version for the reasons MonsterNtheMaking outlined. I get the same.

[BMW86]

Originally Posted by Pig Farmer

I can tell you that on the n54, the manifold pressure sits around -22 to -24 psi at idle. That would be pretty close to your figures if the P3 gauge was set to display Bars, but 3 Bars of positive pressure doesn't make sense. Can you run the vacuum line to the diverter valves on the N55? If that is possible, it very easy to route vacuum line through the fire wall.

Also, are you sure it is reading positive 3 and not positive 0.3? That would seem reasonable when driving at a "normal" pace.

Sorry to thread jack but my n54 reads -18.XX at idle, after a cold start. Would different charge pipe and DV/BOV have an affect on this?

[froop]

Originally Posted by BMW86

Sorry to thread jack but my n54 reads -18.XX at idle, after a cold start. Would different charge pipe and DV/BOV have an affect on this?

Mine reads 18.xx when idling and I have an ER CP with Tial BOV. Increases to 22-24.xx if I'm coasting at speed.

[JHZR2]

Originally Posted by christopherkriens

The readouts you're getting are normal for the N55 version for the reasons MonsterNtheMaking outlined. I get the same.

It's not quite for those reasons. I researched it and it is due to how P3 described it. Look up how valvetronic works. There is a throttle, but it is only used at startup and if the VANOS system fails. Otherwise we have a vacuum pump (just like my diesels), and the throttle is kept FULL OPEN. The balls timing on the intake valves serves as the throttle, thus no restriction, and no appreciable vacuum.

Between no throttle and DI, the N55 is more like a diesel than a traditional ga$$er engine.

[christopherkriens]

I agree- the readout values are accurate, I stand corrected. There are some good videos on VVL.. learned something new.