I read some stuff from CarReview about the 5Zigen Fireball....sound very good to me...
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Summary:
This cat-back has in my opinion the best sound of anything on the market. Nice low purr, and when under stress at higher rpms sounds even better. Takes about 2 weeks for gaskets to seal and get the full sound out of it.
Installation is straightforward and made easy by the design of the cat-back itself. The 5zigen sits nicely under the rear section of the car, angled slightly for an agressive look
Strengths:
Noticeable increase in power. (esp. in 2nd, 3rd gear)
Great sound, very low.
Weaknesses:
None yet.