RomRaider

Recently installed Spec C JDM engine complete with harness, ecu and tranny (DCCD too) onto a 2002 Legacy Wagon. All is running well but could be better. The Boost Compensation Atmosphere Multiplier Determination has a value of 0.0979 and the Boost Compensation Atmosphere Multiplier Offset -0.351.
From reading through the stickers I could not get to understand how the actual logged target boost value is calculated. From my logs, this target boost value is lower than the Boost target in the map.
I would appreciate some guidance on how to apply the Atmosphere comp to the map boost target for this particular case. Some earlier work on an STi Forester was more straight forward as the Boost Compensation Atmosphere Multiplier was a positive value and the target boost logged tallied with the compensated target. Thanks all.

It works like this:
Let's say your atmospheric pressure is 12 psi. The multiplier would be determined as follows:
target boost multiplier = (atmos. pressure * atmos comp multiplier) + atmos comp offset = (12 psi * 0.0979) + (-0.378) = 0.7968

Remember, the ECU deals in absolute pressure for boost control, but the target boost table is presented to you in RR as relative pressure with the assumption of sea level (i.e. absolute target - 14.7). So, in this example, if the boost target is 15.3 psig in RR, the actual target as used by the ECU is 15.3 + 14.7 = 30 psia. You always apply the boost compensation to the absolute target. So, in our example:

30 psia * 0.7968 = 23.90 psia

Now, further confusing things is that the target boost extended parameter underyling value is in absolute pressure, but the primary units in the logger are "psi relative sea level" - just like the target boost psig in RR, it is the assumption of sea level. So, to compare to your calculated boost target above, you need to add 14.7 psi OR change the units to psia before logging.

Comparing the boost target example to a boost gauge in relative pressure, your new target would be 23.90 - 12 psi = 11.9 psi. But, remember, the original target of 15.3 psig in RR is only at sea level. The actual relative target would be 15.3 psig + 14.7 psi - 12 psi = 18 psig.

I was using bar for atmospheric pressure hence my multiplier was all wierd! The Forester STi Multiplier (32bit ecu) has to be applied to atmospheric pressure in psi too? Seems to look ok when worked out with bar.


Thanks a lot, you got a blue me breathing again.

For the 16-bit ECU, it looks like I forgot to change the conversion for the metric defs to account for bar rather than psi. For now, you'll need to use atmospheric pressure in psi for your own calculations. The incorrect conversion for the multiplier for bar in the metric defs will not impact anything and will not impact how the ECU calculates the compensation, just how it is presented to you in RR. I'll fix this in the next def update.

It is correct for the 32-bit ECU (i.e. your FSTi), so if you are using the metric defs it will work with bar and for the standard defs psi.

Many thanks again and best regards (gaga) to the little one, have a great weekend.

I have been trying to understand these two tables as well.

Your explanation cleared alot of things up for me, one question i have is why not adjust your values such that you end up with a multiplier that scales down the standard atmospheric pressure(14.7 psi) to whatever your local atmospheric pressure is (13.2 in my case).

For example i would want my multiplier to be 0.8980 such that 14.7 x .8980 = my atmos. pressure = 13.2 psi
I could create this target by either modifying the multiplier or multiplier offset table.

This still would not correlate very well when you try to make the correction to a variable ( x + 14.7psi) that has the wrong atmospheric pressure in it.

Why not just use a scalar correction factor in the first place?

Out of curiousity does anyone have the stock values for these tables?

I'm not sure what you are getting at. The way it is set up now is how the ECU does it. You can't change the underlying logic through a change in the conversion in the ECU defs. The only difference is that I've set it up for psi or bar rather than the ECU's native pressure type (mmHg).

Where does the ECU source this information from?

For example in the ROM i am looking at my offset = 0.25 for all RPMs and my Multiplier input is 0.0528 for all RPMS. Does this value float around over time in the ECU or is it fixed?

I have made an excel chart w/ data logging + calcs that is helping me to understand this, I will post it later on with some questions/comments.

You should look at Airboy's spreadsheet. It does this calculation for you.

No, the way it is calculated is not going to change, only the compensation as it relates to atmospheric pressure, as determined by the ECU, changes. As long as you understand that the multiplier is a multiplier to the current atmos. pressure and the offset is an offset to that calculated multiplier AND that multiplier is limited to between 0 and 1 AND it is applied to the absolute boost target, you should be able to reasonably predict the compensation.

Thanks for the replies Merch and MickeyD

I think i understand how the multipliers are applied now.

Airboys spreadsheet is amazing, what cant it do! haha