I had a tune done by Ion about 2 months ago, and now I want to replace my Granatelli MAF with a stock unit. Will this affect anything negatively since I had it in my list of mods when the tune was done? I am supposedly running a little lean right now. from 3500 rpms up to 6000 it runs at 13.5-13.1 slowly getting richer as the rpms go up, but never goes lower than 13.1. I want to be around 12.8 right?

 

You want the A/F ratio where it makes the most power and torque..... that's the advantage of a real "dyno" tune, or tuning it based on track performance - reading the plugs and looking at the ET and MPH. There is no one "best" number.

The Granatelli has an intentionally screwed up calibration curve, causing it to under report mass air flow under high flow conditions - that's how it tries to lean out the A/F ratio with a stock tune. I would assume the tuner reflected that curve when he set up the MAF frequency vs. grams/second calibration table in your PCM.

Ask him directly what he recommends.