Barometric correction in Percent Baro mode/Turbocharging

[RallyRat]

I just got my turbocharged engine running on MS3. I'm running in Percent Baro mode, with MS reading its barometric pressure from my exhaust MAP sensor (EMAP). In this way, the bins on my main VE table correspond to engine pressure ratio vs. RPM which is a more precise model of volumetric efficiency when back pressure is variable.

I noticed there must be a problem with my set up when the values in higher RPM bins were getting very high during initial tuning (200+). I then went through my data logs in MegaLogViewer to estimate how MS was calculating pulse widths. The published equation is of course, "PW = REQ_FUEL * VE * MAP * E + accel + Injector_open_time" ("Multiply MAP" is turned on in my general settings). However, I found that it is much more likely that MS is using something like PW = REQ_FUEL * VE * MAP/Baro * E + accel + Injector_open_time. Note that the equation I came up with contains the term, "MAP/Baro." Is this a bug in Megasquirt 3?

To correct the problem, the linear barometric parameters were set to 0% at total vacuum, and 100% per 100kPa (default values are 147 and -47, respectively). These settings are found in the Tools > Calibrate MAP/Baro dialog in TunerStudio. What this does, in effect, is multiply the MAP/Baro term in the fuel equation by Baro; ie, MAP/Baro*Baro = MAP. After re-tuning, I am finding that my VE values are now converging around 100 or less across the whole VE table, as expected.

Has anyone else tried something similar? I think other tuners running N/A engines on Percent Baro can be pretty confident with this method of barometric correction (while measuring actual barometric pressure), as it looks like MS is compensating pretty well as "Barometric pressure" is varying continuously from about 82kPa up to over 260kPa in my application. On turbocharged engines, this method should compensate for altitude changes as well as changes to the turbocharger, the reasons I decided to try the pressure ratio vs. RPM method in the first place.

For completeness, here is how I discovered the MAP/Baro term in the fuel equation: The output of the custom field, ([Gve]*[MAP]/[EMAP]*6.7*Gair/100+.5)/[PW], in MegaLogViewer was always within 20% of 1. On the other hand the custom field, ([Gve]*[MAP]/100*6.7*Gair/100+.5)/[PW], diverged greatly from 1 as exhaust pressure grew. Req_fuel is 6.7 ms and the injector opening time is 0.5ms in this case. It should be noted that for these tests most of the gamma corrections were turned off.

-Will

[techsalvager]

Thanks for this very useful post, I was wondering myself is this was possible to setup.