Apple has announced new Sandy Bridge Core i5 and Core i7 dual-core CPUs in the new MacBook Airs. But, as usual, they’ve glossed over some details. Here are the additional specs that will help you select the right model.

We have come to expect quad-core CPUs in our 15- and 17-inch MacBook Pros and desktops. However, likely due to space and heat considerations, the 13-inch MacBook Pro and the 11- & 13-inch MacBook Air line announced today still use dual-core processors.

Of course, a dual-core Core i5 will be a lot faster than the old Core 2 Duo (C2D), and Apple is advertising up to 2.5x as fast.

MacBook Air (July 2011)

When you drop back to a dual-core processor, you worry about having enough (executable CPU) threads to improve performance. Apple typically doesn’t supply Intel part numbers, so from what I’ve been able to dig up, it appears that the 1.6 GHz Core i5 in the MBA is Intel’s 2467M, the 1.7 GHz i5 is the 2557M, and the 1.8 GHz Core i7 is Intel’s 2667M. (Click on these links to see the spec pages for each. The label at the top if the i7 page appears to be an error and says 1.7 GHz, but the spec sheet below that says 1.8 GHz.)

Here’s the Wikipedia chart of part numbers, and you can check for yourself. Scroll down to “Mobile Processors.”

All three of these CPUs are rated at 17 watts output, and the chart above shows that faster i7s would be too hot for the MacBook Air. The first two i5s only have 3 MB of L3 Cache while the i7 has 4 MB. Most important: all three are capable of Hyper-Threading. As a result, each CPU can execute four virtual threads simultaneously. That’s not quite as good as having four real hardware cores, but it’s a good substitute for mobile operations.  (That’s something to note because, if I recall correctly, on other models, in the past, the jump from i5 to i7 added Hyper-Threading.)

Of course, the apps you run have to be properly threaded to take advantage of Hyper-Threading, and that’s not always the case. You may well see more significant speed improvement simply in the jump from the C2D to the Core i5/7. Also, from what I can tell from the specs, the only advantage in moving to the i7 is the increased clock speed and increased L3 Cache, but here may be other nuances.

Apple doesn’t like to delve into details on these Intel part numbers because most customers just don’t need to worry about it. But if you’re considering a new MacBook Air, you’d probably like to know what compromises had to be made, compared to the MacBook Pro line, to allow the MBA to be so slim sand light. Intel part numbers are usually the key to that analysis. For example, if you have some heavy duty work to do on the move, a 2.2 GHz quad-core MacBook Pro with Hyper-Threading* and 8 GB of RAM may be more appropriate.

Finally, note on the Intel specs page that all of these processors are capable of accessing 8 GB of RAM, but, so far, Apple restricts us to 4 GB. The reason is uncertain.

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Hyper-Threading is standard on all MacBook Pros now.