tuning MAT air density table

[kyleconstantine]

Hey, so I've made it this far in my megasquirt tuning and I just realized this table existed. I noticed a problem since it's been ridiculously cold the past couple weeks. The colder it gets the richer my tune got. I've had this table set at the default (125% at fully cold and 75% fully warm).

From what I've been reading I should have set it to 100% everywhere, tuned my VE table, then adjusted the air density table as I encountered different temperatures.

This past fall I've been constantly logging and updating my fuel table. It seemed to always need adjustment.... I'm thinking the different temperatures were throwing me off and I should have left my summer tune alone and adjusted the air density table.

can anyone shed some light?

[downforce22]

air density is always changing based on barometric pressure which is changing due to temperature, altitude, and relative humidity. The base map assumes air is an ideal gas which of course is not true so the base map has some inherent error built in but is roughly close.

Since barometric pressure is always changing your fueling will always be slightly different each day and that is why you have an oxygen sensor. Since STP (standard temp and pressure) is at 14.7 psi and 68 F that would be the ideal conditions to tune your base map fueling (and with MAT correction at 100%, not affecting fueling). I would turn the oxygen sensor feedback authority to 0% and get base fueling where you want it. Obviously make sure afterstart enrichment and WUE are off and the car is fully warmed up. The default mat table should be at 100% at 68 F. With a solid tune there I would revert back to the default MAT table. Since air is not an ideal gas, I too noticed the default graph is not right. Make sure WUE is not the cause of your 'colder it gets, richer the tune gets' problem.

At that point I would warm up the car and as the daily temperature changes, I would adjust the MAT table like you suggest and leave the base fueling alone. I forget off hand how I adjusted mine, but here is a good thread to see what others are using. https://www.miataturbo.net/megasquir...ok-like-89026/

Obviously once your MAT table is good, turn on ego correction to 5-10% and you should be seeing a pretty solid afr all of the time with the oxygen sensor adjusting for the minute differences in temperature, humidity, etc.

As for me, I am at 6500 feet and if I drive an hour to woodland park (9000 feet) or any other passes I can easily hit 10000 feet. With approximately 3% change in air pressure due to altitude per 1000 feet, I could see over 10% change in air pressure simply by driving an hour or two. (this doesn't account for temperature differences, only altitude, but usually it is colder as you go up so that will impact it even more)The oxygen sensor will not correct for that amount if it set below 10%. Megasquirt defaults to an initial reading as the reference for the base pressure, meaning it would read my 6500 ft reading of 12 psi (80 kpa) and only adjust 8 kpa in either direction with a 10% EGO controller authority.

MAP Daddy allows you to have another map to adjust for atmospheric pressure changes but my unit does not have that. The way I solve this is to turn off the unit and power cycle it so it gets a new reference point. This is not the best solution since if I drive up pikes peak to 14000 feet and restart the car after visiting the giftshop, atmospheric pressure is only 8.6 psi (60 kpa). So when I drive down and have 10% authority, my oxygen sensor will only work with that 10% authority when barometric pressure at my house is 80 kpa.

Not the best solution but I can deal it and with restarting the car if I need to and I can live with that. Probably more than you wanted to know, but hope that helps.