I'm about to swap inlets on my car as it goes back together and I went from the stock 65mm to a 3.5 inch (89mm). I remember a little bit about this a long time ago, was buried in a thread and can't find it again. With MAF size changes happening more and more, it's time for a dedicated thread.

Common intakes this will be needed for

Z1 M-Spec Race
Limp mode tuning
FuRookie intake

 

Well slowly figuring this out, took it out for a drive, was running a bit lean. New calculations, learning about flow seem to be a 110% more flow vs the 46% different in area I first set the ECU for.

Brain still hurts

 

Because of the 'every car is different' thing there is no set number you can just change depending on the intake.

I can tell you that with an increase in MAF tubing diameter, you will end up increasing the K factor in UpRev because the air flow velocity (what the MAF measures) will be lower for the same air flow volume.

You could probably get a rough value to multiply the K factor by by calculating the difference in velocity from the original tube size to the new size at a set air flow volume. This would be just an estimation though, because the change in distance of where the MAF is measuring from the center of the tube will affect the air flow velocity as well, this considerably more complex calculation.

At the end of the day, use the velocities to find a rough K factor multiplier. Then datalog and adjust the k factor more precisely. Then you may even have to make adjustments to the MAF table itself once the K factor is dialed in. If your tune is decent to begin with I don't think you'd need to touch the fuel comp tables or timing tables. Just the K Factor and maybe MAF table.

 

To get it close the calculation for bigger maf is

Current K value x (new maf size/old maf size) = New K Value

 

I'll build a map with that tonight. Can't test and bodywork is kinda torn off

 

It should at least get you in the ballpark and you can fine tune idle AFR from there.