I'm street tuning my B18C with Blox A cams, and I feel like I've had to pull a lot of fuel everywhere on the map to get my AFRs where I want them.

I'm at a little over 3000 feet, and my question is: does tuning for altitude usually involve changing the fuel map? Or does altitude only affect where I will be on the fuel map for any given throttle position?

For example at WOT I max out at about 880-900 mbar. Will my fuel value at 900 mbar be the same as a car at sea level, or will it be lower? If not, I need to figure out what's going on...

 

You lose ~3% per 1000' due to the decreased oxygen content. So 900 mbar of air at 3000' has ~9% less oxygen than 900 mbar of air at sea level.

This is what a baro. sensor is intended to correct.

With a speed density system, if a baro sensor is present and the table for it is correct, you could drive from Denver to New York with consistent A/F ratios without adjusting the primary fuel map.

Hope that helps.