lolachampcar

This thread is about Long Term Fuel Trim and how it is built over time. LTFT is important because it affects how you choose mixture values for performance tuning. If you get mixture just right but your car is building negative trim, you can end up running too lean. This is why I put it in the Performance Modification section.

Before I get beat up again, this thread is not about reflashing tools.

I have attached two screen shots of rips with the same car with an initial 5.5% positive LTFT and then after the LTFT was cleared (by pulling the magic fuse – bottom one of the vertical stack by your left foot). There is a significant difference. I’ve also included a screen shot of the same car building LTFT after the trims had been cleared. Lastly, the data file is attached as well for your viewing pleasure. Just go to www.efidude.com and download the ProLogger application (for free). Do not bother to install the drivers as you only need them when you want to download data from the logger. Once installed, go to file import car data and import the attached file (after you have unzipped it).

What I would like to do is work with a good number of cars to collect data about LTFT. Specifically, I am interested in what is normal or to be expected in stock cars as they age. I’ve put a call into Mazda North America to ask them what they expect LTFT to do as an engine ages as well and will post the results of that conversation. I am also interested in how aftermarket air inlet modifications affect the calibration of the mass air flow meter as this will affect LTFT.

LTFT is built when the car is running in closed loop and steady state (like driving down a long road at 40 mph and 50 mph as shown in the plot and attached data). The fuel control has a target fuel mixture that it wants. The mass air flow meter is measuring the air coming in which determines the fuel to provide via the air to fuel target. When the actual air/fuel ratio measured by the O2 sensor does not equal the target, the fuel control “trims” the “how much fuel to squirt” function so that actual measured air/fuel equals the target.

If the above is accurate, then a little error in the MAF system will cause trim to build. If the MAF system reports a little more air mass than is actually going in the engine then the fuel control will squirt a little too much fuel. The exhaust mixture will then be rich and the LTFT will go negative to correct it. Likewise, if the MAF reports less air mass than is actually getting in the engine, the fuel control will provide too little fuel, the exhaust gas will show lean of target and the fuel control will build positive LTFT.

Once we have a handle on what LTFT does in an unmodified car, we can then see if aftermarket air intake systems change the MAF calibration (and thus cause LTFT to build). There are no worries if this is the case as we can always recalibrate those portions of the MAF system that are out and causing trim to build.

If anyone out there is interested in helping, please log some data as your car is now and clear your trims and try the 40 mph and 50 mph steady state five mile run to see how your car is building trim. If you do not have a logger, I can try to send mine around to people for a day loan to get some data. I just want to make sure I get it back. It would be really good to get data from someone before and after they add a new intake modification especially if they are going from stock to first intake modification. It would also be helpful to get a few bone stock results as well.

Thanks!

CnnmnSchnpps

iTrader: (1)

Once I get the EMU installed I'll be taking lots of logs.. I don't have an aftermarket intake but I removed those little partitions from the air box and want to try removing the resonance camber attached to the accordeon hose. I'll see what effect that has on trims..