Improper cam timing. in another words, the valves are open when the piston gets to top dead center or too much lift via the cam or rocker arm ratio. If you do not have a "High Lift" cam and are using stock rocker arm ratio then you need to pull tha timing cover off and check that the timing marks are properly aligned.
You may want to also check your connecting rods to ensure that they are not bent. I had a customer install a HP timeing set incorrectly and destroyed the lower end because of a bent rod.