Baro Rise duing WOT pull

[cadillactech]

Reviewing some logs a customer sent me before I start changing anything after being at another shop.

I noticed his barometric pressure sensor reading rising with RPM during a WOT.

It reads correctly for our near sea level elevation at idle, 100kpa, but over a pull at the drag strip, it peaks at 108kpa. I thought at first maybe the air intake set up he has that was pulling air from the front grille was actually pressurizing the airbox but the baro peaks line up with RPM peak and not vehicle speed so that rules that out.

P2227 is disabled in the current calibration, possibly because it was throwing this code on the dyno while being tuned.

Basically p ratio drops as RPMs increase, which in a car with a steady baro reading and a intake set up that normally drops MAP as RPMs rise due to restriction, P ratio drops just for a different reason.

Don't think it is causing an issue at this point but the car is currently in MAF only mode but I am going to calibrate the VVE so just making sure I don't have any issues there.

Set up is an LT1 with TSP CNC heads, TSP VVT-3 cam with a phaser lockout kit, TSP long tubes, LT2 intake and LT5 TB, Rotofab intake. Customer did all the work himself.

Thanks in advance!

drag 2.hpl7.9.23 MAF Adjust WOT3.hpt

[Alvin]

That is around a pound of pressure. I'd be really surprised if the airbox did that but I don't see how else that could happen.

What speed did you see the baro go up by that much?

I've done a test to see the pressure recovery of a front mounted scoop. I drove the car up pretty fast like 100 mph, keyed it off and looked for manifold pressure with the throttle body open. Hardly scientific but just to see the pressure drop as the car slowed down. I think with that we were happy to see 1-3 kPa. not 8.

[Alvin]

just looked at log carefully.. This is a weird one.

[cadillactech]

just looked at log carefully.. This is a weird one.

Yes, weird for sure.

As soon as the throttle closes, the baro drops, so it isn't the speed of the "ram" air scoop. If we were getting 8kpa from a ram scoop that would be great!

I guess it really doesn't matter as far as where the VVE P ratio is referencing because .84 is the highest p ratio the table references but if I wanted to rescale that axis to get a more accurate table then it might present an issue. Lower RPMs it has got 100kpa manifold pressure so its close to a 1 p ratio.

I think I will put it on the dyno and put a manometer on it and see if pressure really is rising in the intake tube or its some kind of sensor glitch.

[cadillactech]

Could the air be stacking up in front of the throttle and pressurizing the intake? Seems like a stretch but just had a thought.

[TriPinTaZ]

Could just be the wrong PID for that vehicle?

[cadillactech]

Could just be the wrong PID for that vehicle?

Ya, that is possible. I will have to see if there is another baro option.

[cadillactech]

Just as an update, I logged both the broadcast barometric pressure sensor parameter(which is what I typically log) and the polled parameter and they both match, pressure rises about 1lbs from idle to WOT.

Will be investigating further with my manometer and the dyno when the customer comes to me so I will update after that.

[TriPinTaZ]

Certainly interesting, I've never run into this issue before. My only other thoughts are if the MAP sensor is the correct one for the vehicle, if its damaged or faulty or if the MAP scalar was changed.

[djyankees31]

Just as an update, I logged both the broadcast barometric pressure sensor parameter(which is what I typically log) and the polled parameter and they both match, pressure rises about 1lbs from idle to WOT.

Will be investigating further with my manometer and the dyno when the customer comes to me so I will update after that.

My 2015 C7 with the LT4 Supercharger conversion and stage 2 BTR cam has always, 5 years, had the barometric pressure increase during WOT. I thought it was strange too but always and still occurs.

[blindsquirrel]

Is there a separate BARO on these having this problem, or do they infer BARO from MAP?

[Alvin]

Is there a separate BARO on these having this problem, or do they infer BARO from MAP?

Its part of the MAF sensor.. Its MAF IAT Baro now.

[blindsquirrel]

Well, then... would it be a useful diagnostic to put it in SD (these can still do that, right?), and run it with the air duct removed altogether?

[Alvin]

A lot will.. Some won't run in strictly SD.

The problem is speed density / VE uses the pressure ratio between what's outside of throttle body and what is inside the intake manifold. That's why breaking the baro out is important on centrifugally supercharged or turbo cars. Baro isn't real important on Gen 4 and older. Pretty important for gen 5

[blindsquirrel]

Wouldn't the integrated BARO still read barometric pressure with the tube removed? Everything electrical still plugged in of course. If you could do that, and it still showed the same rise even when not in the airstream then it at least rules out some weird reflected pressure wave somethingorother. Leaves electrical or software as the only possibilities.

[cadillactech]

Wouldn't the integrated BARO still read barometric pressure with the tube removed? Everything electrical still plugged in of course. If you could do that, and it still showed the same rise even when not in the airstream then it at least rules out some weird reflected pressure wave somethingorother. Leaves electrical or software as the only possibilities.

Yes, this was going to be part of my diagnostic, I have a baro breakout harness for a centrifugal supercharger setup that is used to specially keep this from happening when the MAF sensor is pressurized so was going to install this and see what happens when I get the car on the dyno.

Its a weird one for sure.

[GHuggins]

From everything I've seen from these if there are any codes at all set for the barometric pressure sensor it will use the map. It also has baro rationality tables in the cal where it compares the two to one another and seems to believe the map more so over the baro. Then for blower cars there is a baro smoothing factor which im not entirely sure what it does. Another person informed me of that one. Fwiw