I bought my 06 Mustang GT used. The previous owner installed Flowmaster American Thunder 40 series. I noticed that these mufflers have a 2 1/4" inlet and outlet. Just wondering if the slightly smaller inlet/outlet is restricting flow thereby hurting performance? I can say I am overall pleased with the sound.

 

It shouldn't make much difference either way you go. Axle-back mufflers increase your performance so little, if any.

 

The 40 series aren't axle backs - they are the SN95 mufflers.

Are you saying that the previous owner installed SN95 mufflers (this does happen) in your 'Stang?

 

It appears so. I checked the part number on summitracing.com

 

It shouldn't hurt performance much at all as officespace said. It was probably done for slightly better sound than the American Thunder muffler for the '05+.

 

All of you guys must be blind, you didnt even answer his question

The stock pipes are 2.5" so if you're feeding them into a 2.25" inlet you have a restriction there, not good. You're probably losing at least 5hp

 

If the pipes are smaller there is a restriction for sure. I'd agree with the previous poster that you're loosing power. A 5 hp or maybe greater loss would not be unreasonable.

 

Perhaps it is time for new mufflers.

 

I ran the car on a Dyno Dynamics at JBA racing. Only mods are Steeda UDP. No Cai and no tune.
RWHP=291.4 @ 5965 RPM
TQ=299 @ 4412 RPM
So maybe I don't need new mufflers.

 

Those numbers are way off for just having mufflers and udp's.