I have one video (I'll embed it below) using the AWFUL internal mic and then a cheap $2 ebay mic. The Hero2 external mic will ONLY work with unpowered microphones, and you cannot split the signal with a splitter (I've tried).
Email me if you have any other questions. I've had a lot of time to mess around with it. I'd HIGHLY suggest getting the $80 LCD screen (it comes with a new case back too).
External mic in the engine bay:
Internal mic vs external:
All internal mic.. (terrible quality mic)
Z's: 1990 TTZ RB26DETT now running! 636 RWHP/504 FT/lb @24psi @ 200' ALT Nov. 2011 (retuned from original Dyno) 505 RWHP/483 FT/lb @18psi @ 380' ALT Oct. 2008 457 RWHP/441 FT/lb @18psi @ 5800' ALT Nov. 2007 436 RWHP/433 FT/lb @17psi @ 5300' ALT June 2006