So I checked the voltage outputs on my stock MAP sensor using a Mityvac and the PFC Commander. I had asked about them in another thread in the 3rd Gen forum but didn't really get much of a response. So I thought I'd bring it up here with an additional question regarding Scale and Offset.

I found a spreadsheet created by Wargasm a while back. That spreadsheet is a calibration tool and deals with MAP sensor voltages at various pressures. Turns out that according to that spreadsheet I should input values of 32,000 for Scale and 10,500 for offset in my PowerFC.

Does this seem OK? Anyone here mess with these values? I have done a search but can't really find a lot of information on Scale and Offset or what it is used for. Does it have anything to do with air density as a result of altitude?

Again, thanks for any information you guys can give me.

 

The only reason to recalibrate the stock map sensor is to have the PFC boost values match whatever your boost gauge is reading. You have to tune everything anyways, so if the pfc doesn't exactly match your gauge it doesn't matter. It's kinda like having a clock that's 5 mins fast, it doesn't matter that it's not exactly the right time, because you know it's 5 mins fast and can plan around that. So if your P rows are all 1 psi lower then what your boost gauge is reading just tune accordingly.

If the spreadsheet you're referring to is the same one I used to calibrate my greddy 4 bar map sensor, then I believe you have to input which datalogit box you're running. Either that or which software version. I can't remember for sure. But the values displayed will be way off if you have it selected wrong.

In the following diagram you can see the scale and offset of my greddy map sensor compared to the stock map sensor. As you can see both are 5v sensors but the stock sensor reads 32 psi at 5 volts, while the greddy 4 bar is off the map and would probably be somewhere around 64 psi at 5 volts.

If I hadn't changed the scale and offset, then the greddy sensor would be expressing voltages that the ecu interprets as a much much lower manifold pressure then what the sensor is reading. As an example the greddy sensor reads a voltage of about 1.5v for atmospheric pressure (14.7psi)(or 0 on your boost gauge) while the stock sensor, and stock scaling would be reading way below atmospheric pressure, at 6 psi (or 12in/hg vacuum on your boost gauge) As you can see that presents major tuning problems. Not sure how much help that is but it's as simple as I can make it.

 

Thank you for the reply. That explained a lot.

I guess I am trying to understand how my elevation and the resulting air density needs to be accounted for (or not).

I am at 6700 to 7000 feet above sea level. We have to de-rate all of our mechanical equipment (where I work) to compensate for 25% less air density and pressure.

It seems like the base map in the PFC runs very rich. So I wasn't sure if our air pressure affected anything.

I reran it with the other selection. Different Scale but Offset the same. Are the choices to be made regarding the black or beige box or whether you run the Universal or FD versions of the software?

 

Here's the one I use. And you have to select if you're using universal or the fd3S version of FC edit.

Looks like its the same one you're using. Except mine has different PSI values entered in the aqua colored area. Most people are now using the universal software, the excel file defaults to the FD version. You don't need to calibrate your map sensor you just need to get the car tuned. If you want to calibrate it you can, but the car should still be tuned either way.

It also looks like you're using the file wrong. You have to factor in atmospheric pressure. You should not be seeing 2.24 volts at 0 psi absolute. 0 psi on the boost gauge (relative pressure) is actually 14.7 absolute pressure at sea level. So you should be entering close to 2.24 at 14.7 psi in the map calibration file. You appear to be entering relative pressure, not absolute(ABS). For instance 4.88 ABS is 20 in/hg on you mity vac

 

As previously established absolute Atmospheric pressure at sea level is 14.7. I found a table that shows 5000 ft altitude = 12.2 PSI Absolute. Or 2.5 psi lower than sea level. At 10,000 ft it's 10.1 psi Absolute (4.6 from sea level)

Your 7000 ft would fall somewhere between those values. So for you, 0 psi on your boost gauge is something like 11.7 psi absolute. (as a guess) So all your measurements need to factor in that 11.7 psi. (or whatever it would be) If we use my guess, 5 Psi on you boost gauge would be 16.7 psi absolute, 10 psi is 21.7 psi absolute.

Vacuum readings are the same thing, you have to factor in the atmospheric pressure to get absolute pressure, except with vacuum you'll have to change the measurements from psi to in/hg that the mity vac displays.

In reality none of this matters because your car uses a MAP sensor. MAP stands for manifold ABSOLUTE pressure. So all it's measurements are done in absolute pressure. Not relative pressure (gauge pressure). The fueling would basically be the same for 30 psi absolute pressure. Regardless if it's 15 lbs of atmospheric pressure and 15 lbs of turbo pressure, or 10 lbs atmospheric and 20 lbs turbo pressure.

The difference is what your boost gauge is reading relative to the PFC. My P row at 0 psi gauge pressure is P10, your P row at 0 Psi gauge pressure would be maybe P9 or P8. This also means on the stock ecu you wouldn't hit fuel cut at 10 psi. It would be more like 13 psi. To make the same amount of power of a similar setup you would need to raise your boost pressure.

Most people use gauge pressure to express the limits and expectations of different setups. Usually this is referenced close to sea level. For example 15 psi is often used as the limit on 93 pump fuel. Or 15* lead advance at 15 psi. Or 400 hp from a gt35 at 15 psi. This wouldn't apply to high altitudes since the amount of air is what's important. Higher boost pressures would need to be used to make the same power, 93 pump fuel can be used at boost levels over 15 psi, leading timing can be advanced more at higher boost pressures, and on the same token the turbo charger is working harder to make the same amount of air as at sea level, so there is more heat, more wear and less power potential on a given setup. Those at high altitudes need to think more in absolute pressures rather then gauge pressures.

In the end, your setup still needs to be tuned, and it's not the altitude or the sensor calibration that is making the car run rich. Map pressure is the same amount of air regardless of where you are.

 

Thank you for the information Brent. It looks there is no compensation for me to make. Just tune it!

Would you expect though my car would idle in a different P row if I went down to your elevation than where it idles at mine?

 

Yes i would expect so.. Because there is more air available for the engine to consume.

 

I've always thought that this calibration spreadsheet is more complex than necessary and hinders understanding of the calibration process.

The reason that there is so much confusion about this issues is that the PFC manual is terrible when it comes to defining these two parameters and the Datalogit and its manual causes yet more confusion with arbitrary multipliers. Neither define clearly the units being used. Also, the stock calibration with the PFC is nearly always wrong with the stock MAP sensor.

The PIM value reported by the PFC is absolute pressure measured in kg/m2. Gauge pressure reported by the PFC (called 'Boost') is measure in kg/cm2 and is calculated directly from the PIM value by subtracting standard atmospheric pressure (10332 kg/m2) and scaling:

Gauge pressure (in kg/cm2) = (PIM - 10332)/1000

(To add another source of confusion, the value indicated in the the Datalog 'Boost' column is inexplicably multiplied by 100.)

For the PFC, scale is the response of a MAP sensor in kg/m2/V and describes how the sensor reacts to pressure change. That is, it indicates the change in pressure (measured in kg/m2) required to change a sensor's indicated output by 1 volt.

So, determining scale (in kg/m2) involves finding two PIM values 1 volt apart (say, 2 and 3 volts) and subtracting the two values. Several pairs of voltages should be used to account for noise and these voltages should probably be non edge values because most sensors are not linear near the edges of their range.

(And to add yet another source of confusion, earlier versions of the Datalogit required that the scale be multiplied by 6.666. Later versions do not require this stupid multiplier. This random constant I think has caused most of the confusion over the years.)

Offset (also measured in kg/m2) is simply the number required to zero the gauge at local atmospheric pressure. Offset will vary with altitude (and with local pressure changes, though these are typically minor). Scale is a characteristic of the device and will not change with atmospheric pressure.

Once these numbers have been determined, the final calculation used by the PFC to calculate absolute pressure from the output voltage of the MAP sensor (PIMV) is:

PIM = scale * PIMV + offset

As mentioned, indicated PFC boost (aka 'gauge pressure') in kg/cm2 is calculated directly from the PIM value:

(PIM - 10332)/1000

There is no magic here - just two sloppy manuals, and some sloppy display calculation software (and, for earlier PFCs, a random constant multiplier).

 

As an additional data point...

I checked my MAP sensor by using a calibration gauge, then used the spreadsheet to best match the curves.

The resulting scale and offset were 5400 and 3900, very close to the PFC default values (5263 and 3947).

- Sandro