Why Does Chevy use the 6.2L rather than the 7.0L in the Corvette ZR1?

[Jay Leno74]

The ZR1 uses a supercharged 6.2 and the ZO6 uses a 7.0L. The ZO6 uses a 7.0 and when supercharged it's faster than the ZR1. Why don't they use the 7.0L Supercharged as the ZR1 to make a faster car?