If I tune my car w/ MP, then change to HF cat, will everything be ok? I have the same A/F readings coming out of engine, but what will HF cat do?
I figure to do it this way to get my A/F down before HF cat, which will lean it out(if using a sensor at end of exhaust), right?

Steve

 

I think you'll be fine. The HF is more restictive so if anything you should be richer. Keep aware that your spot of peak power will be lower than with the MP so you may wish to pay more attention to the lower/mid sections which *may* be more troublesome with the HF than with a MP.

Usually people are worried with higher boost/higher RPMS so in that instance I think you'd be definitley safe in that area if you tuned with MP first.