OK, I'm old school, and I don't follow all the new stuff, but that back fire is increase fuel of unburn gas. Your stock head may have a fuel trim sensor (usually four wires) that works in conjunction with your O2 sensor (two wires). If your new heads don't allow the fuel sensor, and there is no mod for the car cpu, then I fail to see how the car knows the proper fuel/air ratio. When you first ran the car the cpu stayed at the right ratio, but when it lack the input from the fuel trim it LEARNED a different ratio, hence the back fire. Not only will end up ruin valves but WILL destroy your cat converter if you continue to run the engine..