Does anyone run HHO in a late 90s Chevy/GMC 5.7L (350cu) motor? What mileage gain do you get and HHO per minute? Do u run it before or after the MAP sensor?

I have bought the equipment to add HHO, and want to know what I can expect.

PS. I get a code that the knock sensor goes off, but I just fill it with premium every 10 fill ups. Will the HHO fix this?

Thanks
Todd