I'm assuming you're about to put your car on a chassis dyno...don't put your car on it just to see what kind of power it is making. It has been discussed here many times, but the dyno is just a tool. Don't get discouraged if it only says you're making 300 hp, but don't get excited if it says you're making 700 either.
Same car, same day, on 2 different dynos can yield very different results.

CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax