Originally Posted by Modshack

On the MAF: The diameter of the tubing at the MAF determines the velocity of the Air flow over the MAF. The MAF controls the fuel signal with the 02 sensor sending corrective info to the ECU. Since The stock Z runs rich, I am constantly seeing a -9 to 10% correction on the long term (Partial throttle) fuel trims. You can open up that tube "slightly", drop the velocity therefore reducing the MAF signal while upping the volume, and therefore bring the fuel trims closer to 0% where the car will make more power. It's a balancing act to do it right. Trust me, I've done a LOT of experimenting on this with the Audi's and have Hundreds of thousands of test miles on the Tubes I build for them. . Very careful tuning of the pipe diameter can result in real gains as long as you don't go to the extreme of needing to alter the fueling. If you go too big you will need to. Lean cars make more power than fat cars from the fueling perspective. Ideal A/F ratios are around 12.5:1. Z's with a full compliment of exhaust and intake mods still run richer than that per Semtex's dynos (11.8:1 or so). This is why tuning gets more power...It brings the car up to a better A/F ratio for power. Corrections will happen automatically with regular ECU adaptation. The stock MAF tubes are 2.34" in ID. As best as I can determine the Stillens are around 2.37" which is a 2.5% difference in surface area! More air + less fuel = more power. If you want to study up on some of the reasoning, check out my page here:
Modshack | Stage 2 MOFO (BAMM)
My initial Guestimate of a 2.5" ID seems pretty close. Just went out and ran the car for 40 miles and the long term fuel trims seem to average out to 0, + or - a few % points of correction. (BTW, The long term fuel trims or Partial throttle adjustment sets at the 20 mile mark after an ECU reset, then continues to adjust based on input from there on). 0% correction means the ECU is happy with the fuel mix and doesn't have to alter it to bring it into spec. A + number is a lean correction, a Minus is a rich correction. I need to put a few more miles on to see if this LTF % holds in that range. If it goes lean, I'll just cut some tubes a tad smaller.. It's all an experiment at this point...