Just terms used in designating engine types by GM.
The 3.6 litre (3564 cc) LLT is a direct injected version based on the earlier LY7 engine. It was first unveiled in May 2006, and the DI version was claimed to have 15 percent greater power, 8 percent greater torque, and 3 percent better fuel economy than its port-injected counterpart. The LLT engine has a compression ratio of 11.4:1, and has been certified by the SAE to produce 302 horsepower (225 kW) at 6300 rpm and 272 lb·ft (369 N·m) of torque at 5200 rpm on regular unleaded (87 octane) gasoline. This engine debuted on the 2008 Cadillac STS and CTS. GM will use a LLT in all 2009 Lambda-derived crossover SUVs to allow class-leading fuel economy in light of the new Corporate Average Fuel Economy (CAFE) standards. In the Lambdas, LLT engine produces 288 horsepower (215 kW) and 270 lb·ft (366 N·m) of torque
The LFX is an enhanced version of the LLT engine. Introduced in the MY2012 Chevrolet Camaro LS, it is 20.5 pounds (9.3 kg) lighter than the LLT, due to a redesigned cylinder head and integrated exhaust manifold, and composite intake manifold. Other components like the fuel injectors, intake valves, and fuel pump have also been updated. Power and torque are up slightly from the LLT. The compression ratio is 11.5:1.