How to Convert your 1HZ to 1HDT
Toyota 1HZ is an indirect injection, which means they have a pre combustion chamber in the cylinder head. The injector injects directly into the pre combustion chamber (this is why it is common for 1HZ cylinder heads to be cracked.)
Toyota 1HDT is direct injection, which means the injector squirts directly in the bowl, if the piston is shown below and an explosion occurs in the piston bowl, preventing the cylinder head from getting hot and reduces the cylinder head from cracking.
In the direct injection 1HDT , the combustion happens in the piston.
In the indirect injection 1HZ, combustion happens in the combustion chamber in the cylinder head.