Quoted from EveryMac
Specifically, Intel reports that the applicable Core i3 processors an FCLGA 1156 socket and the applicable Core i5 processor an LGA 1156 socket. It isn't entirely clear how, or if, the FCLGA 1156 socket that the Core i3 supports is different from the LGA 1156 socket that the Core i5 supports, but it is believed that the sockets are the same and just the Core i3 "package" is different.
For the "Sandy Bridge"-powered "Mid-2011" iMac models -- the stock configurations of which are the , , and -- Intel that all have an LGA 1155 socket (also referred to as an "H2 socket"). The custom-configured models -- the and -- as well as the education-only likewise all have the same LGA 1155 (H2) socket.
Processor Upgrade Experiment Results
Knowing how the processor is mounted in a particular Mac only is half the battle. Determining whether or not a processor upgrade will function originally required experimentation by brave souls.
The now defunct was able to successfully upgrade an -- equipped by default with a 2.66 GHz "Core i5" I5-750 processor -- to a 2.93 GHz "Core i7" I7-870 processor. It is worth noting that this model also could be equipped with a 2.8 GHz "Core i7" I7-860 processor at the time of purchase via build-to-order configuration.
Although no details are provided, multiple third-party forum readers also that swapping the stock 3.2 GHz "Core i3" I3-550 processor in the with a 2.93 GHz "Core i7" I7-870 processor worked without a problem. By default, this model was available via build-to-order with a faster 3.6 GHz "Core i5" I5-680 processor.
However, there are limits to the processor upgrade options, as well. In response to a request for reader input, Jason Riley of Sydney, Australia kindly shared the results of his efforts to upgrade a with an LGA 1156-socketed 2.93 GHz "Core i7" (I7-870) processor. He discovered:
The new processor physically fits fine but the computer wont boot with the new chip. I tried swapping the CPUs back and forth a couple of times just in case I missed anything but each time the i7 CPU failed to boot.
I suggest there are one of three possibilities:
Apple have restricted the logicboard firmware to specific CPUs.There is not enough power for the i7 (95W as opposed to 74W).The logicboard requires the graphics capabilities of the FCLGA 1156 Socket chips.