Q8 GooGles

Hello.

I bought an '05 Mustang GT around a year ago and it had no cats + Flowmaster mufflers.

Where I live they are very strict about car modifications so I decided to just return the car back to stock.

I asked several people about this and mostly what I got was it's not recommended to put the cats back on a car where they were removed before because it will affect the car's performance. How right is this statement? And would you guys recommend I put the cats back on + stock mufflers or just get the stock mufflers?

Also, when I say "affect the car's performance" I mean it would drop to below what it was before when the car originally had the cats on.

99GTvert

Do you know if the car has the stock tune on its computer? If it was tuned for the no cats exhaust then putting cats on it again with the no cat tune might make the car's performance worse than when it was stock but otherwise if the tune is the original, from the factory tune then I don't understand how it would be worse.

Q8 GooGles

Thanks for your answer

Nuke

If it has no cats and isn't throwing a code then I have to assume that the previous owner more than likely had a custom tune to turn off the O2 sensors. Putting the cats back on won't hurt anything but you'll lose a few minor RWHP and also quiet the exhaust slightly. However, the O2 sensors will still (presumably) be turned off. Depending on how emission testing is done in your area, that may still cause a problem.