Scaling for big MAF

I'm working on calibrating a 2004 Subaru STI with a larger MAF housing using Cobb Access Tuner Race

I have to start out and say that I used the formula taking into account the cross sectional areas of both housings to get starting values and WOW in the lower voltages, it was off by a mile!

I've got it way closer now and have been using Excel to keep track of the changes.

I have been keeping my eye on the MAF sensor graph to maintain its shape, but when I plotted the data in Excel, it was clear that the nice exponential shape was an illusion!

I have included the chart of stock vs my rescaled intake.

I'm pretty new to this, but it seems counter to all that i have learned! Perhaps this is due to the uneven break points in Voltage.

*I'm referring to the change in slope around 4.5v...

Are you using realtime tuning with the engine running or just taking data and making a change in between runs? Also, MAF curve is never going to be fully "pretty" because of pulsation effects and placement of the sensor.

I am using both methods. I am very close to getting the whole range within +/-3%.

I was just thrown by the kink in shape at 4.5v. It is not apparent when you look at the curve in Accesstuner or EcuFlash.

The calculation is only intended to get you in the ballpark and definitely you will require a lot of logging and fine tuning from there. The shape you've got to the MAF calibration certainly isn't usual but if you're getting good control over the AFR then this is what counts. What is important is that the shape is still smooth and consistent. I would be more worried about a MAF calibration which has an erratic shape to it.

I hope you don't take my comment about the formula as me calling you out! I get that it was a starting point, generic formula, just surprised how much work would be left!

Thanks for taking a look at my chart, glad to know it's not something to be too alarmed about. My AFRs are good in that range.

Oh no, not at all! It's very dependent on the particular installation. Iv'e had some situations where the calculation basically got me within a few percent of where it needed to be and other times where it was only enough to get the engine running. As long as your AFR control is good then you're on the right track!