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LS3 is used in manual version SS models. L99 used in automatic version SS models.
LS3 produces 426hp/408lb-ft tq compared to L99's 400hp/395lb-ft tq
L99 camshaft is different -- having a little less duration and a little less lift, both of which give the engine a smoother idle.
L99 has a cam phaser and, to get the additional piston-to-valve clearance required when cam phasing changes, the pistons are different parts having valve reliefs machined into their tops. The reliefs lower the compression ratio to 10.4:1.
L99 PCV system is different having its oil separators in the valve covers rather than down in the valley like the LS3.
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