Boost Discrepancy + Other Logging Questions

[keithk831]

Currently running a Cobb Stg1 map on my 2005 5MT. I went out today to do some data logging. The AP put me at a max boost peak around 15-16psi, but on the same type of runs using my laptop to data log, it only peaked at 13.3. I tried this a few times with the same results. So which device is lying? Romraider, or the AP?

 

I also took some A/F readings. It gave me a max at 21.13, min at 11.02 (at/around full boost, and around 14.2 at idle. How do these numbers look to you guys?

 

It was about 20 degrees F outside during these runs.

[SubyShop.com]

Were you logging the relative manifold pressure (corrected)? What gear were you in? The Cobb stage 1 map is pig rich like the stock one, so your O2 sensor bottoms out at 11.02. It's probably in the low 10's actually. Your tune is probably ok but never hurts to check for knock during pulls.

-Franz

[keithk831]

Were you logging the relative manifold pressure (corrected)? What gear were you in? The Cobb stage 1 map is pig rich like the stock one, so your O2 sensor bottoms out at 11.02. It's probably in the low 10's actually. Your tune is probably ok but never hurts to check for knock during pulls.

-Franz

 

Yep...I was running relative and not absolute. These runs were mainly in 2nd gear from about 2k to redline. I also did a 1-2 run to redline in both with the same results. This goes for the laptop and the AP.

 

I didn't get any knock at all.

[Spec B]

Boost - what parameter are you logging boost with using RR? I log manifold relative pressure corrected.

 

Because of it's location (placement), the afr reading off of the stock sensor is pretty useless under boost (because of the pressure built up in the pipe). The 2 numbers you describe as min/max are pretty consistent with what the stock sensor shows. At idle and cruise, you should see somewhere around 14.7.

[Spec B]

oops late post.

[keithk831]

Boost - what parameter are you logging boost with using RR? I log manifold relative pressure corrected.

 

Because of it's location (placement), the afr reading off of the stock sensor is pretty useless under boost (because of the pressure built up in the pipe). The 2 numbers you describe as min/max are pretty consistent with what the stock sensor shows. At idle and cruise, you should see somewhere around 14.7.

 

So it sounds to me like my AFR is safe? That's all i'm really concerned about with that.

 

I was running "Manifold Relative Pressure" and not the "corrected one". Think that's the problem? What exactly does it correct?

 

I know the absolute pressure is positive pressure in a vacuum.

[Spec B]

I am not sure..I think it corrects based on atmospheric pressure? I think your altitude has something to do with the discrepancy.

[SubyShop.com]

Your AFR should be safe but we can't say for sure unless you log it during a WOT pull. I always log manifold relative pressure (corrected). I think it compensates for temperature and absolute pressure.

-Franz

[j255c]

stock o2 sensor is not adequate for wot pulls, once there is enough pressure it pegs at 11.02. You can be in mid to high 11's for all it knows.

[keithk831]

Sounds good.

 

So, what do you guys think about the discrepancy? One device says one thing while another says differently.

[NSFW]

What is the ambient pressure in your area? If it's just as many PSI off from sea-level pressure as your two boost readings, then the problem is just that you're looking at a boost parameter that assumes you're at sea level.

[Boostin1657615274]

It's not necessarily a matter of 'lying.' It is a matter of the sample rate of the data.

[NSFW]

OK, here's post #11 in more detail:

 

I am not sure..I think it corrects based on atmospheric pressure? I think your altitude has something to do with the discrepancy.

 

Yes, "manifold relative pressure (corrected)" logs the difference between atmospheric pressure from manifold absolute pressure. In other words, it's the manifold pressure relative to the ambient pressure. With RomRaider, this requires logging both parameters - it's the logger itself that does the math on them. Cobb may be doing exactly the same thing, I wouldn't know.

 

There's another parameter that logs the difference between manifold absolute pressure and sea level pressure. AKA manifold pressure relative to sea level pressure. If you live near sea level, this gives you MRP with one less parameter to log, so you get more log rows written per minute.

 

If Cobb is logging one, and you're logging the other, then the difference between them could be explained by the atmospheric pressure in your area. There's a parameter for atmospheric pressure, you should select it in RomRaider and log it for a couple seconds to see what it is. If it's not the same as sea level pressure, you have your answer.