They are a closed chamber head, like the boss 302 and 4v heads (4v= barrell carb., 2v= 2 barrell). The 4v heads' ports are too big for any N/A application under 600hp though. The aussie head has the smaller 2v ports which promote mid-range intake velocity for better street use. I have a 4 barrell intake manifold now that will work with these heads. I bought a reconditioned set on ebay for a good price, and I'm probably getting the headers next week. My compression ratio is probably between 7-8:1, and these should bump it to 9 or more, depending on my clearances. I am expecting to see well over 300 to the wheels with the cam setup I am doing. If everything works just right 350-400 is feasible.