Stock Boost Pressure

[Volvoholic]

I had a problem on Monday where my car had an unintended acceleration hiccup. Still not sure what the cause of it was but I'm blaming the ETM for now.

Anyway, Tuesday I had a buddy with an OBDII scanner come for a ride with me. Aside from monitoring some mildly erratic behavior from the TPS that could be caused by the feedback loop in the system or a wearing potentiometer in the ETM, we noticed some cool boost pressures.

First and second gear are a crap shoot because the torque converter is unlocked in these gears, acting as a torque multiplier, putting less load on the motor thus creating less boost, roughly 1-6psi. I noticed a vacuum of 0.1-1psi in some instances under medium throttle. Once the torque converter locked in third we got some interesting readings...

TC locked, low-end, WOT @ 1500-3000RPM, multiple run observed averages;
Third ~20.5 psi
Fourth ~17.8 psi
Fifth ~15.9 psi

Above 3000RPM it dropped about 1 psi until the torque band fell off after 4500RPM and once the power curve passed the torque curve around 5400RPM there was about 3-5psi less.

I thought the information may be useful to some since I couldn't find anything stating real world boost pressures. After being totally surprised with that bit of information. I've been trying to find another turbo motor on the market that makes more boost from the factory and so far there's only a Bugatti Veyron SS which pushes about 21.8psi stock.

I'd imagine the Polestar tune for this 2.5T (B5254T2) motor changes when the torque converter locks to run higher boost pressures sooner.

[Astro14]

Sounds more like the MAP reading than a boost reading. Subtract 14.7 PSI to get your actual boost.

About 6 PSI results, and that's about right for stock. The T5 runs a few PSI more.

[Willy]

Sounds more like the MAP reading than a boost reading. Subtract 14.7 PSI to get your actual boost.

I assume that's because the MAP reading is absolute pressure?
Willy

[Volvoholic]

I assume that's because the MAP reading is absolute pressure?
Willy

Yes, that would be correct.

Astro, I was reading gauge pressure.

[Antherzoll]

I find it interesting that you're able to read boost pressures through OBDII because the MAP sensor only accessible thought the CANBUS next work, simple OBDII readers would not have access to the MAP without a J2534 device and software. Are you sure your numbers are not a calculated value based off of displacement and MAF?

I know Torque Pro uses a similar means of calculating boost pressure, but it's still a rough number at best. Example, with torque pro it states I boost to ~20psig when my MAP show 2157hPa, and my boost gauge shows 16.7psig (which is what I'm tuned for).

[Volvoholic]

Turns out I was a little giddy on this one. The fact a Veyron SS is the only car that pushes more boost than a bone stock base XC70 had me questioning it. Turns out my MAP sensor needed to be slightly recalibrated and the application in use was calculating a reading, not actually reading data. Recalibrating the MAP and opening a port to the compressor outlet pressure sensor (which I do have), I got better information:

(same conditions, taking average)
3rd ~11.2 psi
4th ~10.6 psi
5th ~9.9 psi

In third, the MAP was reading 22.6 inHg and if it is assumed that the flow has minimal losses and the wonderful plastic intercooler pipes don't expand much then the pressure at the compressor outlet would be about 22.8 inHg or 11.2 psi.

[Antherzoll]

On my stock tune with a mechanical gauge connected to the manifold (post throttle) I'd see 7psi in 3rd-5th, 1st and 2nd would be limited by the ECU.

These runs you're doing, are you going WOT from 1st-5th and logging, or you logging runs in each gear separately. The reduction in boost could be atributed to high IATs caused by intercooler heatsoak.