Couple of quick ? about C6's...
What do C6's put down new as far as power (6 speed auto) and what do they run in the 1/4? How do they respond to mods? Anyone wish they wouldn't have bought theirs?
as far as mods, LSx engines love mods. Cobras love mods as well. a smaller pully and a chip swap seem to make a hell of a difference.
check dyno sheet in sig.
I'm not really a mod-a-holic, so I'll let the others expand in that arena.
I would recommend you/he test drive both, and I bet the vette comes home with you.
I don't mean to sound rude but a Blown Mustang is still just a Mustang and everyone knows that the vette gets em wet!
I'd take a stock C6 over a heavily modded Mustang anyday....
That's like comparing an F-body (modded or not) to a C6... there's just no comparison in the overall package.
Hell, I'd take a stock C5 over a blown Mustang...... no contest.

