Other Megasquirt Updates

MS1-Extra 29v Air Density Correction

I recently leaned out my cruise fuel ratios down to around 15-15.5 AFR in a effort to use less fuel (didn't really notice much difference).I carried this out over the winter with ambient temperatures around 10'c with corresponding air inlet temperatures around 20-25'c. This was all very well and good until it was a warm day with ambient's of 20'c and inlet temperatures around 30-40'c (depending on speed etc). The car was feeling rough and I noticed the fuel ratios were down into the 16's. I adjusted the VE table and target AFR ratios to bring it back to around 15 and it was OK again. I then realized that this is a similar problem to when I do a hot restart; High inlet temperatures give a weak mixture

After a hot shutdown, my AIT will shoot up to around 50'c - when you restart it the car will run very rough with a lean mixture (14-15) whereas my normal idle mixture is around 12.5-13 (chevy's like a richer idle). I remember reading things about the Air Density calculations not being quite right - so I decided to examine the function on the stim. Under "Advanced" there is an option for Coolant corrected Air density, but I don't want to correct based on Coolant - I want to correct based on just the Air temp. In other words if the Air temp gets over a certain value - add extra fuel to bring the mixture back to the desired ratio.


First you need to edit your "..\MegaTune2.25\mtCfg\msns-extra.ini" file so I uses Air Temperature not Coolant. I also changed the gauge to Air Fuel Ratio - as that the thing that I'm trying to correct essentially. Edit the file and search for this section and make the highlighted changes:


curve = clt_IATCorrC, "Coolant corrected Air Density, see Settings", 9

topicHelp = airdenHelp

columnLabel = "Temp", "Offset"

xBins = IATCTemp, mat

yBins = IATCorS

gauge = afrGauge

Next from in megatune, from Advanced | Coolant Related Air Density:

For the "reduce correction at RPM", I set these values far past my red line to effectively disable the function. I done this to reduce the amount of variables, if you are just trying to correct a hot restart problem - you may want to set the values to 500 to 3000 rpm, I left it disabled for now - I may revisit these parameters later. (Do one thing at a time!)


here are my settings:

To get the initial values, I took what my AFR ratio would be at normal idle lets say 13AFR and the ratio I get after a hot restart which is 14.5AFR. I divide 13 by 14.5 which gives .89 - which means I need around 10% more fuel at 50'c.

I then done a similar exercise with my cruise afr's at elevated temperatures (15 vs 16) @ 40'c which gave 7%. From there I just made a nice curve starting from around 30'c


After all that was configured and checked on the stim I put it on the car and went out for a drive. I allowed the car to warm up gently to get around 20'c air inlet temperature, I then pulled over and made a note of my idle AFR(~13). I then drove a few hard miles to get the engine bay very hot, then pulled over. Already I could tell the correction was working. My AIT was 40'c but I still had a reasonable mixture and nice idle.

I then shut the engine down and watched the AIT rise to around 50'c through heat soak. I then restarted the engine and loaded up the tuning graph. Essentially all I did was tweak the values in the graph to maintain my desired AFR in the window. As the inlet temp slowly dropped down from 50'c I just kept tweaking the values.

Back out on the road, I notice my cruise AIT temps were slightly richer than desired, so I adjusted the correction down a little. This is where you would use the RPM reduction function to start reducing the amount of correction once you are away from idle.

The car now restarts and idles nicely after a hot shutdown, which is a problem I have put up with for years.

Information from this MS Extra Thread.